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FAQ


Questions and Answers about Combat Robotics
from Team Run Amok


The Ask Aaron Archives: Radio and Electrical
This page is one of several archives of older 'Ask Aaron' questions and answers categorized by topic. To see the most recent questions or to ask a new question, go to the Ask Aaron Home Page
Click on a green category button to browse thousands of previously answered questions, or use the search box.

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Caution
Even small combat robots can be dangerous! Learn proper construction and safety techniques before attempting to build and operate a combat robot. Do not operate combat robots without proper safeguards.


Radio and Electrical

Q: hello sir,i am building a thirty kg drum bot i am deciding to use radio system for drive the four locomtion motor and a weapon motor.But still now i have no idea about the radio system ,as i am a eee student i have good knowledge on circuit.so now tell me where exactly i have to start and learn and do it? [India]

A: [Mark J.] Read thru the next six or eight posts in this archive. In those posts you will find links to:

  • our R/C Radio Reception guide;
  • an article on How R/C Radios Work;
  • our Combat Radio Function Guide;
  • an R/C robot circuit diagram in our FAQ;
  • our Radio Transmitter Programming Guide. and
  • our Combat Robot R/C Gyro Guide.
It's not like I'm trying to hide this stuff.
 ___________

Update: I put a single reference to all of the Run Amok radio guides in the Frequently Asked Questions -- FAQ #20.



Q: Hey Mark, I've been planing a new beetle weight after my first competition (An 8"x8" plate of steel with chunky lite flites) and I was planing on using modded VEXtrollers with the kitbots 1000RPM motors. The problem is that VEXtrollers don't have BEC built in, and I really don't have much money to buy a dedicated BEC of reasonable quality, so I was wondering if I could an off the self Radio Shack voltage regulator. If I can, what should I look for, but if I can't, why? There must be a reason why, because if they worked easily, RC guys would be using them. [Woodburn, Oregon]

A: [Mark J.] If all you're powering at 5 volts is your receiver, you can certainly stop by Radio Shack and buy yourself a 7805 linear voltage regulator IC and wire it as shown in the diagram to power your receiver. The capacitors (35 volt rated) are optional, but will give cleaner power to your receiver.

So why don't R/C guys do this? Linear voltage regulators are inefficient. They waste a lot of power to do the job and lose capacity as the input voltage rises. That makes them a poor choice if you have multiple servos plus your receiver drawing power.

A 'dedicated BEC of reasonable quality' like you are trying to avoid buying is a switching power regulator. More complex but more efficient, a switching regulator also doesn't lose capacity at increased voltage inputs. Take your pick.

7805 voltage regulator circuit



Q: VestedMadScientist here again, this time with a more technical question. I have a Spektrum AR6210 DSMX receiver and a lightweight BattleKit with F30-150s (I also have a Vantec RFDR36R, but that's still in the mail), and I'm a bit concerned with whether or not the aluminum chassis will interfere with the radio signal. Is there an optimal place to mount the receiver, or does it really matter? [North Carolina]

A: [Mark J.] The Spektrum AR6210 manual provides advice on mounting and orientation of the main and remote receivers (yes, a two-piece receiver). Keep the receiver away from electrically 'noisy' devices like the ESC and motors, and don't tape the wire antennas down directly to the aluminum -- space them up on a foam strip. As long as the receivers aren't enclosed on all sides by conductive material (metal, carbon fiber...) you'll get a good signal.

For more radio help, see our R/C Radio Reception Guide: Radio Reception Guide



Q: can u please tell me how to control a servo wirelessly? [West Bengal, India]

A: [Mark J.] The simple way is to use a standard R/C radio system. See: How R/C Radios Work.

If you're looking to wirelessly control with Arduino, try this tutorial: Wireless Servo Control.

A web search for "wireless servo controller" will give you other options.



Q: Dear Aaron, I am new to this field of robowar but I wanted know what type of transmitter and reciever can be used in robowar (6 or 8channel?) ?
I wish I could make robo like Sewer Snake. [New Delhi, India]

A: [Mark J.] Have a look at my Combat Robot Radio Function Guide. A typical combat robot will only use three channels, but you'll probably want a five or six channel radio for the useful extra features that come with them. The guide will help you sort thru the many available features to pick out the right radio system.



Q: I read old posts from Battlebots forum and it says Vladiator was extremely hard to control. Gage has to aim it, not drive it to attack others. Was that common for extremely powerful ramming bots? [Chinese Forum]

A: [Mark J.] That was a common control problem, and it still is. A high-power ramming 'bot will wildly spin its wheels all the way across the arena and can be difficult to keep pointed in a straight line, let alone make fine turning control motions. A peizo gyro can help with the straight line issue, but properly modulating power for a controlled turn under full throttle is pretty much beyond the physics of the situation. You really can't do much more than point, pull the trigger, and hope.



Q: Hello sir my name is DpD me & my team make a robo for robo war it consist 2 high tork moter & 4 side veg bot so,what is a circuit diagram for robot & Rimot control? [Maharashtra, India]

A: [Mark J.] See Frequently Asked Questions #19. Your 'bot won't have a 'Weapon ESC' or 'Weapon Motor', but everything else is correct.

For R/C transmitter programming help, see the Run Amok R/C Transmitter Programming Guide.

Q: How to make a non weapon simple 4 side wedge robot with non wire less rimot control ?

A: I've never built a non-wireless combat 'bot, and I don't know what the Indian rules allow. Other Indian builders would have experience with wired remotes, so I suggest asking for guidance at the 'Combat Robotics India' Facebook page.

'Combat Robotics India' is a closed group that requires an invitation to join. If you need an invitation, write back to me with your Facebook account name and I'll invite you.



Q: Hey, i am from india. I have two wolfpack lipo's 8000mah 35c 3s... yesterday during their usage the voltage dropped slightly below 9v (went to 8.9v)... the batteries developed a slight puff... i use a charger with output current of 700ma... when im charging them they dont go beyond 11.1v overall... any comments on their state what could it be ? should i use them anyway? [Maharashtra, India]

A: [Mark J.] Do NOT attempt to use a 'puffy' lithium battery. Do NOT even attempt to recharge a 'puffy' lithium battery. It's dangerous!

Lithium batteries are succeptable to damage from excessive current draw or excessive discharge. When damaged, the battery releases hydrogen gas that inflates the battery covering and causes it to puff outward. A puffy lithium battery has reduced capacity and is in danger of bursting into flame with continued use. It should be properly disposed of immediately.

How to dispose of a damaged lithium battery:
  • In a plastic container, dissolve 1/2 cup of salt in 1 gallon of cold water.
  • Drop the battery into the salt water and leave it there for at least two weeks.
  • Remove the battery, wrap it in paper, and throw it in the trash.

A good tech inspection at any combat event should always examine lithium batteries for signs of damage and should immediately refuse entry of a 'bot using such a battery -- no exceptions. Why be so cautious about damaged lithium batteries? Here's what happens when a LiPoly ignites.



Q: Hello!

I am building an antweight with a Sabertooth 5xRC, Spektrum 4 channel receiver (AR400 I believe), and I am controlling it with a DX6i. I have Delta mixing turned on in the TX with dual rate on and the Aileron rate set fairly low (~25%). For the most part, it handles really well, but when I take it to full speed I find that the turning is too responsive. If I turn the aileron rate down any lower, turning becomes sluggish at low speeds.

Is there any way to make the robot turn slower at high speed without slowing down the turning response at lower speeds?

Thank you, Kit Buckley [USA]

A: [Mark J.] Hi, Kit.

That's a fairly common handling problem, and there are a few things to try. Different drivers like different solutions, so you may want to try them all - individually and in combination - to see what works best for you.

  • Exponential Response Setting: On the same menu screen as the Dual Rate, you can enter a value for the Exponential Response curve for the Aileron channel. A negative value for the EXPO (change from INH% to about -50% for a start) will decrease the sensitivity of the steering near the center of the stick throw, making the 'bot less 'twitchy' at speed while still giving full turning response at full stick deflection. I'd try this first.

  • Aileron Dual Rate Switch: You mentioned that you had dual rate turned on. The DX6i offers dual rate control options on the aileron, elevator, and rudder channels. You can set two aileron rates and quickly change between them with the flip of a switch. It's a little awkward to use the standard aileron D/R switch if you're doing Delta mixing since its on the same side of the transmitter as the right joystick (in mode 2) -- but you can use the D/R COMBI Switch Assignment function (page 46 in the manual) to move the activation switch to the GEAR or EVEVATOR D/R over on the other side of the transmitter to give your left hand something to do.

  • Add a Piezo Gyro: If the transmitter programing options aren't giving you the control feel you're looking for, you can add a solid-state piezo gyro to offer stability and control at all speeds. Piezo gyros are not compatable with transmitter mixing -- you'd need to turn off transmitter mixing and use the on-board mix option on your Sabertooth ESC. Consult the Run Amok combat robot gyro guide for specific help with gyro setup.
You might want to browse the Run Amok transmitter programming guide for some additional hints on addressing handling problems. The guide is written for Futaba transmitters, but most of the information translates well to the Spektrum radios.

Write back if you need clarification on my recommendations -- I went thru them pretty fast.



Q: HEY....I AM USING AMPFLOW E 30 -400 MOTOR AS MY WEAPON DRIVE .I have calculated it in spinner spreadsheet that my ampflow will consume 1 ah per match .I am using automobiles lead acid battery for that but the problem I face is it gets drained quickly. And over here its wired robot..so suggest some solution for light weight and high power lead acid battery [Chennai, Tamil Nadu, India]

A: [Mark J.] Lead acid batteries are very old technology. Given the list of things you want:

  • light weight;
  • high power;
  • lead acid;
...you can have any two items from that list -- but not all three! Modern batteries have been developed specifically to improve on the poor power to weight ratio of lead acid chemistry, so the solution is a more modern technology.

That said, even a small lead acid motorcycle battery is capable of providing much more than the one amp-hour of current that your weapon requires. If you aren't getting a full match worth of power from your battery, I would suspect that your battery and/or charger is faulty.

Q: I'm planning to use motorcycle battery (Lead-Acid) which has an rating of 12 Volts 35 Ampere-hour, can that battery discharge a burst current of 200 Amps which my weapon motor requires? If Not, which batteries can supply me that amount of Amps ?

A: First, your weapon motor does not require 200 amps. It can use up to 200 amps at stall to produce maximum torque, but if that much amperage is not available it will cope with the current it can get. In a typical spinner application, the weapon motor will only pull very high amps for an instant at start-up. Amperage consumption will drop rapidly with increasing weapon speed and will level off at just a few amps at max RPM.

A battery that can supply full stall amperage to the weapon motor will shorten your spin-up time by a bit, but isn't required from lead-acid batteries that are not sensitive to high drain rates. Some newer battery types (E.G. - lithium) can be damaged by high current draws and must be carefully selected to avoid trouble.

To your question: the maximum burst current of lead-acid batteries varies greatly with the style and quality of manufacture. I suspect your 35 amp-hour cycle battery likely can deliver 200 burst amps, but you would need to check with the manufacturer to make sure. I think you'll probably be fine using this battery, but a properly selected Lithium Polymer battery could supply your weapon with ample power and burst amperage at a fraction of the weight of your lead-acid battery.

You are using two of the lead-acid batteries to power your 24-volt motor, right?



It greatly saddens me to announce that my son, Aaron Joerger, died very suddenly on the afternoon of October 18th, 2013 of an apparent pulmonary embolism. He was 22 years old. Aaron's obituary.

The 'Ask Aaron' project was important to Aaron, and I have decided to continue the site in his memory. Thank you for the many kind messages of sympathy and support that have found their way to me.

- Mark Joerger, Team Run Amok




Q: can we get two throttle in one remote and at least 3 channel........i tried my level best.....and what if two opposite side wheels are rotating at different speeds....would it rotate on one wheels or the wheel with more rpm will pull the other......and we have brushed motors....... [India]

A: These are VERY basic questions. You'd be well advised to find a book on robot construction to gain a base understanding of robot design and operation.

A robot that maneuvers by 'differential steering' turns by powering the wheels on one side of the robot at a different speed or in a different direction than the wheels on the other side. A small difference in speed will give a gentle turn, while greater speed difference will make for a sharper turn. If the wheels are spinning in different directions, the robot will rotate in place.

You can use any or all of the proportional channels on an R/C system for throttle control. Just because a channel is labeled 'aileron' or 'elevator' doesn't mean you can't use it for a throttle; the type of output from the receiver is the same for all of the proportional channels. Most robot drivers use 'channel mixing' to control the two drivetrain throttles -- see the Team Run Amok Radio Feature Guide for details on mixing and for help in selecting other radio features you will need.

Q: i am asking about two throttle sticks on one remote to control the speed of two different wheels?????though by connecting it to other channels i can control it but not to much extent

Futaba T6XAs control layout A: That's the way almost all combat robots are controlled -- two channels controlling opposite sides of the robot drive train.

Using one stick for each side of the robot is sometimes called 'tank steer' because of its similarity to the way armored tanks were once driven with two control sticks. Assigning the left transmitter stick (usually channel 3 - throttle) to the left side speed controller in the 'bot, and the right transmitter stick (usually channel 2 - elevator) to the right side speed controller will provide you with basic tank steer control of the robot. Push both sticks up to go forward, both sticks down to reverse, and sticks in opposite directions to spin in place.

Most drivers prefer to use special features available on many R/C systems to 'mix' two stick channels for better control. The popular 'mix' is known as 'elevon mixing' where channel 1 (aileron) and channel 2 (elevator) are electronically combined so that moving just the elevator stick up/down controls both sides of the robot to move forward/reverse, while moving the aileron stick left/right will alter the speed of the two sides of the drivetrain to turn the robot left/right. This control style is more natural, and movement of the two sticks can be coordinated for smooth turns and precise control.

More information on mixing is available in the link I gave above to the Team Run Amok Radio Feature Guide.

Given that you have told me nothing about your radio, speed controllers, robot design, or the specific control problems you are having, I really can't give further advice.

Robot haiku:
Find some robot books
To answer basic questions
Before you design.



Q: hi aaron i bought this rc switch to control my weapon motor with fly sky 6ch transmitter (2.4ghz) but im new to the rc department and i dont know how to configure the transmitter to that switch i connected the switch with my reciever (CH3), when i turned on the (CH3 ) in my transmitter status light starts blinking and rly light is started glowing in that switch but the motor is not running. please help me........ [Maharashtra, India]

A: There is no on-line manual for this switch, and the instructions given on the web page are very sketchy. Since the power light is on and the status light blinks when you switch on channel 3, it sounds like the R/C link is correct and the switch is receiving the proper command from the receiver. So far so good.

The web page does warn that the R/C transmitter MUST be turned on before the Whiplash Switch is powered up. The reason is unspecified, but it may be some sort of fail-safe function. Make certain that you follow the proscribed start sequence.

The only remaining thing to check is that the motor is correctly wired to the switch. See the diagram at right for the correct method.

That's all I can think of. I'd suggest contacting the manufacturer if you still have trouble. Note that 30 amps is not a lot of capacity for a weapon switch. You haven't given me details of your weapon but, in spite of what the product webpage says, a large weapon motor can very easily overload and destroy a 30 amp relay.

Robot haiku:
Hard to diagnose
An electrical problem
Half a world away.

R/C switch wiring diagram



Q: Hi Aaron,
i am using ampflow two A28-150 motors for drive system and a A28-400 for weapon system. my weapon weight is around 17-22 lbs. And the total weight of my bot is around 120 lbs. i want to run my bot 24V. Please tell me what type of batteries should i use ??? will one LiPo 22.2V, 5000mAh, 65C be sufficient to run the whole bot for 3-4 mins ??
Please help me. [Maharashtra, India]

A: Ask Aaron provides access to tools that allow you to estimate battery capacity requirements.

  • The Tentacle Drivetrain Calculator provides an estimate of the battery capacity needed to operate the robot propulsion drive train for 3 and 5 minute matches. In addition to the information you have given, the calculator also requres the gear reduction ratio and the diameter of the driven wheels. Assuming 4" diameter wheels and a 7:1 gear reduction (about 10 MPH top speed), the Tentacle calculator estimates the drive train will consume 4.4 amp hours of current in a 4 minute match. You'll want to check that figure for your robot's gearing and wheel size.

  • The Team Run Amok Excel Spinner Spreadsheet provides an estimate of current consumption for spinning weapons. The spreadsheet requires much more information than you have provided. Critical factors missing include the shape and exact dimensions of the weapon, the material used to construct the weapon, and the gear reduction from the motor to the weapon. As a wild blind guess I'd say about 1.0 amp hour of current might be required for a match, but you will want to run the calculations yourself with your weapon's specific data.
As a guess I'd say about 6000 mAh would offer a reasonable capacity with a small (10%) reserve for your robot, but you REALLY should run the exact numbers for youself.

Robot haiku:
Heavy motor load
Pulls more current than light load.
Learn to use the tools.



Q: hai Aaron,,, i just whnt to know,,,
1. what mean "C" in lipo battry,,?? thare a lot type of "C"
2. are the mAh of lipo battry can give power full to motor,,??

thnks Aaron =) [Melaka, Malaysia]

A: [Mark J.] "C" refers to the maximum safe continuous amperage discharge rate for the lipo battery. A '20C' battery with 1500 mAh capacity can safely provide a continuous current of 20 times 1500 milliamps = 30,000 milliamps = 30 amps. Discharging at a higher rate may permanently damage the battery and create a fire hazard.

The mAh rating refers to the total amount of current the battery can provide before it goes 'dead'. A 1500 mAh battery can provide 150 milliamps of current for 10 hours.



Q: Sir, my bot weighs 132lbs. Run down by two A28-150 motors attached with TWM3M gearbox with 6 inch wheels and a my weapon weighs around 16kg producing 6600 joules at 6000 rpm powered my A28-400 motor through a v-belt. Voltage supply is 24V. arena is wooden. I want to know how many Amps battery will i require to drive my bot and to use the weapon for a period of 3 mins match. Including the weapon gets stalled for about 3-4 times in a match. [Mumbai, India]

A: [Mark J.] I'm getting a lot of battery capacity questions lately, and this is not the first time Ask Aaron has answered this question for you. The tools you need to answer this type of question are available here at Ask Aaron. We supply the tools -- you do the calculations:

  • The Tentacle Torque Calculator can provide an estimate of the amp-hour capacity needed to operate your drivetrain. All robot builders should become adept in the use of this tool. Entering the drivetrain parameters you have provided gives an estimate of 5.325 amp-hours consumed in a 3 minute match. Try it yourself!

  • The Run Amok Spinner Excel Spreadsheet can provide an estimate of the amp-hour capacity needed to operate your weapon. Unfortunately, the information you have provided for your weapon is both incomplete and somewhat nonsensical. Spinning a 16 kilo weapon at 6000 RPM is both a poor idea (TOO FAST!) and should store a great deal more than 6600 joules of energy. I suggest you run your calculations again.

In general I can estimate that a 6000 joule weapon spinning up 4 times in a 3 minute match might use somewhere around 0.75 amp hours of current at 24 volts. That would put your recommended battery capacity (including a 20% reserve) at just under 7.5 amp-hours. I would STRONGLY suggest that you run thru these numbers yourself after you sort out the weapon calculations.



Q: Hi Mark, I have this confusion regarding the DC power supply for a Lipo charger. I have a 6 cell lipo, 22.2v. Now the I/P voltage to the charger, according to their specifications should lie between 11~18V DC. Now to charge the battery the potential difference should obviously be higher. Does the charger step-up the DC voltage??

I have chosen this power supply. Will it suffice my need for charging something which exceeds 14V?? Please explain.

Oh and btw I'm not from Las Vegas, Nevada, I'm from mumbai Maharashtra. There seems to be some problem with your tracking system. [Las Vegas, Nevada]

A: [Mark J.] Hobby battery chargers routinely have voltage step-up capability so that they can operate from an automotive 12 volt battery 'in the field'. Pop the hood on your car, connect to the battery, and charge up your R/C plane/copter/buggy/boat. If the charger you have chosen says it can charge a 6 cell LiPo on 16 amps @ 14 volts you should be fine with the power supply you have selected.

So you claim you're from Mumbai, do you? Then why do you type with a Nevada accent? Nice try, but I can hear the slot machine jackpots going off in the background.



[Cross-posted from the Weapon archive]
Q: Sir, I am making a bot. I have used a A28-400 motor for weapon to drive a 16 kg hollow drum using a pulley belt mechanism. Voltage provided is 24V. I have to use the weapon for about 5 mins. total no. of spin ups will be around 10-15. How many Amps will it discharge?

I have a confusion of buying a Lipo and want to know which battery should i opt for- should i use a 6S 5000mAh 65C-130C battery or a 6S 5000mAh 45C-90C battery. Which one will be efficient considering the cost too? [Mumbai, India]

A: [Mark J.] You've given me too little information to answer your questions.

The amount of energy storage in a spinning weapon depends on:

  • The mass of the weapon;
  • The speed of the weapon (RPM); and
  • The shape and dimensions of the weapon.
Without a full description of the weapon dimensions and the speed at which it spins, the energy needed to spin the weapon up to speed cannot be calculated. Everything else being the same, a long but small diameter drum will store MUCH less energy than a short but large diameter drum. Example -- a hollow steel drum with a 1/2" thick wall spinning at 2400 RPM:
  • a drum 4" in diameter and 24" long will weigh 16 kilos and will store 920 joules of energy; but
  • a drum 9" in diameter and 10" long will still weigh 16 kilos but will store 4800 joules of energy -- more than five times as much!
The Run Amok Excel Spinner Spreadsheet can perform all of the calculations needed to determine the energy storage of your spinner weapon, and can estimate the battery capacity requirement. I strongly suggest that you make use of this powerful tool.

As to your battery selection: the 5000mAh capacity you propose is MUCH larger than just your weapon might use -- I assume you will use it to power the robot drivetrain as well. You have given me no information about your drivetrain, so I cannot comment on the overall suitability of the battery.





Q: Hello, we are building a 3 kg robot for Robosumo. For making it wireless which module we should use which would be best for us and also protect us frm jaming ? [Poona, Maharashtra, India]

A: Jamming? Please tell me that intentional radio jamming is not allowed in the competition!

If you are genuinely worried about intentional signal interference, your radio of choice would be a DSMX based radio system such as the Spektrum DX5e. All 2.4 GHz DSM R/C systems are resistant to interference, but the DSMX systems can switch to alternate frequencies if they encounter interference on the frequencies they selected at start-up. Radios employing the DMSX protocol are effectively immune from narrow-band jamming.

An alternate frequency switching protocol called FHSS is offered on some Futaba radios, such as 6J. FHSS provides similar security from interference, and the Futaba 6J radio has many desirable features for robot control.

Check with the event rules to assure that the failsafe behavior of a specific radio system meets the event requirements before you purchase!

You may also be interested in reading thru our Radio Function Guide for help selecting a radio with the features best suited to your needs.

Robot haiku:
Radio jamming?
Truly awful sportsmanship.
Should not be allowed.



Q: hi aaron,i am working on my battle bot .i want some tips on how to avoid radio frequency interference during the match. [Uttar Pradesh, India]

A: We have a page devoted to Radio Reception Problems in Combat Robots.

Robot haiku:
Good reception starts
With quality R/C gear.
Then follow our tips.

Radio Reception Guide



Q: In one of your older questions you explained how to overvolt a servo for a drivetrain, am I correct in my thinking that this is the same way you would overvolt a servo for a lifting arm?

A: Yes, you are correct.

There is a question that specifically asks about bypassing the battery eliminator circuit to run a lifter servo at full battery voltage -- search this archive for 'compact antweight lifter'. The diagram from that question is shown at right.

Mark J. here: for more radical overvolting beyond the normal capacity of the servo controller board, take a look at the servo hacks detailed on the Team Kiss website.

Robot haiku:
Servo overvolt
Can deliver more power,
But may toast servo.

Battery Eliminator Circuit bypass for a servo.



Q: Hi Aaron, its VA engineer again. After school finally being over I'm back to making my battle bot, but I'm facing problems. Some friends came by and messed with the pin setup for the speed controller (Sabertooth 25A) and the programming on the Spektrum DX5e transmitter.

A: A fine group of friends! Drag out the manuals for the ESC and radio and wade thru the set-up process again. This time, get a notebook and write down all of your switch settings and transmitter programming to avoid future problems like this. Documenting your 'bot can be a life saver.

Robot haiku:
Document your 'bot!
A few minutes spent today
Might save your bacon.



Q: Dear Aaron,

People commonly have four wheeled robots with four motors, each with its own ESC (I think). However, on a six channel receiver (or any I would think) there is only one channel port for the left and one for the right. This would mean that in a four wheeled robot, the ESCs have a "Y" connector of sorts to all left ESCs to get the signal from channel one and likewise for the right? I hope the Hamburger is good here.

Thank you,
New York

A: There are several ways to handle a four-motor robot, New York.

  • You can, as you describe, have four motors and four ESCs with two ESCs on one side of the 'bot connected to a single receiver port with a Y-adapter cable. Four ESCs -- two ports.

  • A computerized transmitter can avoid the special cable by 'slaving' an unused port to the main control channel for each side. The same signal will come from the slaved port as the original, and the second ESC many be plugged in there. See our Transmitter Programming for Combat Robots Guide and your transmitter manual for info on mixing and coupling receiver channels. Four ESCs -- four ports.

  • Two or more motors on one side of the 'bot may be controlled by a single ESC -- if that ESC has the current capacity to handle the combined load for all the motors. This only works for brushed motors. Two ESCs -- two ports.
Take your pick, they all work fine.

Robot haiku:
Single E.S.C.
Can control two brushed motors
Driving the same side.

Q: Aaron, are you certain that the Y adaptor you included in the link above is the correct one? I would think it should be two female to one male. I am having trouble finding one that is in this version, and I do not think RMP makes one.

Thanks, New York

A: The Y-adaptor link is correct. RMP botched the description, but the photo is right. Male/female correctly refers to the shape of the actual electrical connectors, NOT the shape of the insulators -- but some hobby R/C sources confuse this and get it wrong.

The metal pins in a receiver port are recessed 'male' connectors. The single female connector - with metal ports to accept the pins - plugs into the receiver, and the twin recessed male connectors on the other end of the cable are available for the ESC female plugs. I don't think anyone makes the type with opposite gender connectors -- there's no application for it.

If you are concerned, support your local hobby shop and buy one there that you can examine.

Robot haiku:
Split one signal source
To control two devices.
Reverse makes no sense.



Q: Which company transmitters nd recievers are cheap nd best for robowar and I'm using brushed motors.. [Haryana, India]

A: Repeat after me -- 'Cheap' and 'Best' don't go together!

Modern computerized radio control systems have a wide range of functions that can make the difference between an uncontrollable beast of a robot and a robot with precise and predictable behavior. Some functions and attributes are required by combat regulation, so check carefully with the event(s) you plan to enter for special requirements to make sure you get what you need.

Go take a look at our guide to 'Combat Robot Radio Systems - what functions do you actually need?' It lists common R/C functions of value to combat robots and ranks them by importance. You'll at least know what you may have to give up if you go with a cheap radio. There are also many posts on radio gear selection elsewhere in this archive.

If you really have to go cheap on your radio gear, stick with a radio that is being used by other combat builders so you have a base of users to ask for help if you run into trouble. Currently, the popular cheap full-featured system is the HobbyKing Orange T-SIX transmitter and the OrangeRx R620 receiver. Read our comments on Orange R/C gear before you decide to go with this radio.

The radio gear does not care what type of motors (brushed/brushless) you run -- motor control is the job of the Electronic Speed Controller (ESC). An ESC is a separate device that plugs into the radio receiver. Many posts about ESC selection are in the Motors & Controllers archive.

Q: Thanks for helping me in deciding radio control .I'm new to this r.c department so I decided to go for cheap one and In hobby king website I found turnigy,hobby king and orange rcs out of these three which is best for the begginers and easily programmable

A: Mark J. here: being 'new' to R/C is an awful reason to go 'for a cheap one'. To the contrary, it is experienced users who have the knowledge required to sort thru the poor user manuals and difficult programing procedures that come with cheap radio gear -- a beginner will be best rewarded by the features of a better radio.

A quick scan of the HK site shows 11 Turnigy, 6 Hobby King, and 2 Orange transmitter models -- each quite different and most with multiple receiver options. I don't have time to review each combination for you, and I don't know the specific radio requirements for your event. I can repeat the advice Aaron gave above: the popular HobbyKing option amongst combat robot builders is the Orange T-SIX transmitter with the Orange R620 receiver. This is a full-featured radio that should meet the requirements of any robot combat event.

Personally, I don't consider any of the HK 'house brand' radios to be either good for beginners or easy to set-up. The user manuals are truely horrible, quality control is poor, and programming can be very frustrating.

Futaba 6J transmitter My suggestions:

  • Do your homework and find out what minimum requirements your event has for R/C gear.

  • Read ALL the links Aaron gave in the post above.

  • Weigh the additional cost of a name-brand R/C system against the frustration and uncertainty of working with a 'cheap' radio.
My personal choice for a beginner-friendly, full-featured, easy to use combat robot R/C system is the Futaba 6J. If you have questions about the suitability of a specific radio system, send me a list of your minimum radio feature needs and I'll do my best to comment on how well that radio meets those needs.

Robot haiku:
A cheap radio
Is a one-way ticket to
Frustration and loss.



Q: Hi, i bought a E-Flite HP6DSM Spektrum, the transmiter when i turn it on makes 3 sound signals, shows the voltage but the red led doesnt blink and stay fix, why? [Aveiro, Portugal]

A: You might benefit from reading this post on RCGroups.com. If that doesn't help, your closest support center is Horizon Hobby Limited in the UK:

"Please call +44 (0) 1279 641 097 or e-mail us at sales@horizonhobby.co.uk with any questions or concerns regarding this product or warranty."

Robot haiku:
Transmitter made for
Blade SR Helicopter.
Not good for much else.



Q: Hi Aaron, I'm currently building a 15lb wedge pushy robot. However, after prototyping with two RS 775s at 19.2 Volts, the robot is a bit too uncontrollable. It accelerates incredibly quickly! Wheel diameter is 3 inches and gear ratio is 16:1. What can I do with my Spektrum Dx6i to give me a little more sensitivity while driving instead of immediately slamming into walls and making wild uncontrollable turns?

I also notice that it pulls right a little bit when it's supposed to go straight. Could you recommend anything to help me balance the wheels out? Thank you.

A: Mark J. here: overvolted RS-775's in a 15 pound pusher? That's somewhere close to two horsepower! I would guess that it does accelerate 'incredibly quickly'. Fortunately, your Spektrum transmitter offers some features that will help tame an overpowered beast like you've built.

I'm going to assume that you have the DX6i transmitter set-up for elevon mixing. If not, write back.

  • Find the Elevator and Aileron Dual Rate switches at the upper left and upper right corners on the front of your transmitter. Set them both in the down ('1') position and leave them there.

  • Turn to page 62 in the DX6i manual for instructions on accessing the Dual Rate and Exponential (D/R&EXPO) menu and roller your way over to it.

  • The Aileron is the turn channel for your robot, and you really don't need to spin at full throttle! Roll to the Aileron (AILE) setting, click to the Dual Rate currently set to 100%, and change it to 50% with the roller.

  • The Elevator is the throttle channel for your robot. You don't want to reduce full power for your pushybot, so we'll handle the throttle response differently than the turning response. Roll down to the Elevator (ELEV) setting and click over to the Exponential setting that currently reads 'INH%' and roll it to +100%. This will make turning response near the center point of stick travel less sensitive without reducing the maximum throttle available.

That should get you back in control of the beast. You can play with the values if it doesn't test to your liking.

Getting the 'bot to track straight is a different type of problem. The RS-775s spin a little faster in one direction than the other, and that translates into a pull to one side. Some radios have receiver-level 'Adjustable Travel Volume' (ATV) adjustment that can dial that out, but Spektrum transmitters don't have that feature. If it really bothers you, read thru our Guide to Combat Robot Gyros for a possible solution.

Afterthought: I kept thinking about the straight tracking problem and came up with something that might help, but it's a little complicated. Turn to page 70 of the DX6i manual and roller over to the Programmable Mix 1 function:
  • Select ELEV as the master channel, and AILE as the slave.
  • Input mix values of -5% down and 5% up -- you'll fine tune these when testing.
  • Set the switch value to 'ON' and the trim setting to 'INH'
Return to the main screen, fire up the 'bot, and give it a try:
  • If it pulls even worse in the same direction, reverse the signs on the mix values: 5% down and -5% up.

  • If it's better but still pulls the same direction, increase the absolute values on the mix: more negative and more positive like -7% down and 7% up.

  • If it pulls the other way, decrease the absolute values on the mix: less negative and less positive like -3% down and 3% up.
Keep adjusting 'til it's right. I don't have a DX6i handy to try this on, so let me know if it's useful.

Robot haiku:
Great big horsepower
Can solve some problems, and cause
Many, many more.



Q: Hey there,
I'm using a Spektrum Dx5e transmitter with an AR600 receiver. I have Victor 883's controlling my motors. I bought PWM signal driver cables to make my victors compatible with the receiver, but no matter what I try, they still blink orange indicating that no PWM signal is found. What could I be doing wrong? The weirdest thing is, over the weekend, they were working fine. It seems like after I replaced my receiver battery and re-bound the system, the PWM drivers no longer work. [Philadelphia, Pennsylvania]

A: Everything was fine 'til you replaced the receiver battery and re-bound the receiver, and now neither of the 883s have signal? OK, let's start there:

  1. Simple things first: I'll assume you charged the receiver battery, but check the voltage anyway.

  2. The AR600 receiver outputs no PWM signal until it connects with the transmitter, so make sure the transmitter is on.

  3. My question is, why'd you try to re-bind the receiver? It's not uncommon for a receiver re-bind to fail, and I'm guessing that's your problem. Try it again, and follow the instructions carefully.
The 'troubleshooting' section on page 3 of the Victor 883 manual has some further suggestions, but I suspect your receiver simply didn't bind.

Robot haiku:
PWM output?
Must connect to transmitter.
If not bound, no joy.

Q: Radio problems guy again!
Radio connection is fine, I plugged some servos in to test and everything is good. Victors alone seem to be running okay. I think my problem is in my PWM driver cables. I tried plugging them between my receiver and servos, and they would not relay the signal. The servo would let a quick twitch, and after that, would not respond to my radio commands. The black shrink-wrapped part of the PWM cable got very warm, however.

Have you ever had this problem? Is it possible to blow a driver cable? My receiver battery is 6V. What else can I do? Thanks

A: It's very possible to blow an IFI signal booster cable, but I didn't think it was likely that you blew both of them at once -- until you mentioned the 6 volt receiver battery...

Under that black plastic wrap is a Fairchild MM74HCT14 chip that has a max input voltage of 5.5 volts. A nice fresh charge on your 6 volt receiver battery has it up around 6.5 volts. Yea, you blew the PWM boosters.

  • You can try running the Victors without boosters. Some people get that to work and some don't. Worth a try.

  • You can go back to a 4.8 volt receiver battery and buy a new set of signal boosters.
You'd think IFI would make a big deal out of telling you not to exceed 5.5 volts, wouldn't you?

Robot haiku:
Six-volt battery?
Too much for signal booster.
IFI should warn.



Q: Hey- I have two identical 24v Nimh battery packs to power my motors. I was going to plug them into their respective ESC's separately, but after re-reading the rules, I realize I [must have] only one manual disconnect to cut all power. Does this mean I have to wire the two batteries in parallel briefly, just to they can be fed into the same power switch, only be split up again and plugged into the ESCs? Or is there something else I can do? Thanks.

A: Wiring the packs in parallel has a few benefits, but if you want to keep the supply to each motor discrete there are a couple ways to do it:
  • Use a double-pole disconnect switch (hard to find); equivalent to two SPST switches controlled by a single mechanism; or

  • Use a single-pole disconnect switch or link to disconnect a common ground wire (see diagram).
Robot haiku:
With a common ground
Each battery still powers
Only one motor.
Common ground circuit.



Q: Hi Aaron, first i wanna say that this site you have here is VERY useful indeed, will definitely be coming back here again for references. My question right now is of uncertainty. I have GWS 75Mhz 4-channel Pico Receiver - Vertical Pins, a Futaba style, a GWS GWT-4A 75Mhz 4-channel Radio System, and a JR 700MAH 4.8V NiCad Receiver Pack - Flat. I will be building a battle bot spin bot class 30 lb with a max RPM of 400. I wanna know what kind of motor and battery i will need that will not cause my receiver to burn up or keep move my bot at a very slow pace. Also will i need a ESC for this project or is it possible to make due without one? If i do, then what type will i need?
-VA Engineer

A: First thing to check is the radio rules for the event you will be entering. A 30-pound robot will a spinning weapon generally requires an R/C system that will 'fail safe' and shut off all power to the weapon and drive train when the radio signal is lost -- something your GWS radio does not do. Some events may also require more advanced 'spread spectrum' 2.4 gHz radios to avoid radio interference. Check with the organizers before you continue with your current radio.

Your radio receiver cannot directly control your drive and weapon motors. The output from your receiver is a very low power pulse-coded signal that must be interpreted by an Electronic Speed Controller (ESC) that will handle the high-power demands of your motors. You will need an ESC for each motor you want to independently control. Servos that plug directly into the receiver have a built-in low-power ESC as part of their circuitry.

Which ESC to use depends on the voltage and power consumption requirements of the motors you select. See FAQ #21. There are many posts in the Motors & Controllers and Robot Weapons archives about ESC selection that will get you started. You'll probably be interested in reading our Optimum Robot Drivetrains Gearing Guide as well.

Robot haiku:
R/C receiver
Has a low-power output.
Will not drive motor.

Q: Hi Aaron, thanks for responding so quickly, thankfully my card was declined before the unfit receiver and controller was processed. Instead i will be ordering the Tactic TTX404 2.4GHz 4-channel radio system with a Tactic TR624 6 channel 2.4 GHz receiver along with the sabertooth RC dual motor speed controller. the motors i have are ungeared similar to the small johnson motors with speed of 18720 rpm at 12v. Did not finalize order yet but this is where my group and i stand right now. Would like to know your opinion on the matter before we process everything.

A: You're getting closer.

The Tactic TTX404 radio [manual] does not provide an adequate fail-safe response on loss of signal. Your robot drivetrain and weapon must both stop under these conditions, but the TTX404 will only return channel 3 to an 'off' condition on signal loss. The other channels will 'freeze' in the positions they were in just before signal loss -- and that could be at full throttle! This does not comply with the fail-safe provisions of the rules and is not allowed for robots of your weight class and weapon type. Suggest you read thru this archive for guidance in selecting a suitable radio system. Something like the Spektrum DX5e transmitter paired with the Spektrum AR6115e receiver might be a reasonable low-budget choice.

I can't really comment on unspecified motors 'similar to' the Small Johnson motor. If you were using the actual Small Johnson, it would be suitable for use in a 30-pound robot if geared down about 25:1 with 3" diameter wheels. How do you plan to gear down the motors? You can't use them to directly drive your wheels!

The motors, geared down as specified, would be expected to break tire traction and pull around 15 to 20 amps each when pushing against an imoveable object at full throttle. If your Sabertooth Controller is the Dual 25 amp model you should be OK.

Mark J. here: Spektrum radios are popular in the robot community, but Team Run Amok has always used Futaba radio systems. The Futaba 6J six-channel 2.4 gHz system is my current favorite radio system for combat robots. The transmitter is full featured, set-up is simple and intuitive, the receiver will fail-safe to user-determined positions on all channels, construction quality is excellent, the manual is well written and clear, and the receiver is pre-linked (bound) to the transmitter. Current pricing is less than a comparable 6-channel Spektrum system, which makes the 6J a bargain. A good radio can make a big difference in the driveability and operation of a combat robot. If you have a few extra dollars in your budget, spend it on a radio that won't frustrate you.

Robot haiku:
If transmitter fails,
All robot motors must stop
Immediately!



Q: I heard somewhere that it's possible to "over-charge" A123 batteries to get more mAh capacity, but it kills the battery in the long term. Is this true, or is over-charging always a bad thing to do?

A: Mark J. here: it's possible to get both a little more voltage and capacity out of most types of rechargeable batteries by using destructive charging techniques, but it involves more than simple 'overcharging'. The gain is quite small, and it trashes the battery immediately -- the pack will lose significant capacity on the next charge cycle. R/C racers have known about this for a long time, and factory sponsored teams routinely did this on NiCad and NiMH packs for the small advantage it gave them.

My advice: stay safe. NEVER overcharge any type of lithium battery!

Robot haiku:
Do not overcharge.
Gain is slight and it costs both
Money and safety.



Q: Hey there,
The wires manufactured with my motors and batteries are 12 AWG. With my gear ratio I'm expecting the motors to pull approx. 45A continuously, but obviously they can peak higher. Discharge rates on the batteries go up to 70A. When I look up current limits on the wire size, I see that 12AWG maxes out around 40A. Does that mean my wires are going to limit the amount of current my motors draw from the batteries? I read somewhere online that those values are a conservative recommendation for home wiring applications, where the wire length is expected to be several feet.

I'm even more concerned about my weapon motor which may draw current much higher, during startup.. in excess of 100A. Again, the motor is outfitted with 12AWG. I don't want to solder on thicker wire if I don't have to.. how concerned should I be?

Thanks

A: Mark J. here: there are many different power ratings for wire that vary according to the planned use. Greater current flow adds up to higher operating temperature, and it is that temperature that impacts the safety factor and amp rating. You don't want a wire bundle heating up to dangerous levels in the wall of your home!

AWG 12 gauge copper wire has a resistance of only 0.0016 ohm per foot of length, so it is not going to significantly restrict your motor's current draw. It's possible to draw 235 amps through 12 guage copper for about 10 seconds before the wire itself starts to melt, but the insulation will burn away a bit quicker than that.

I think you'll be fine with the 12 gauge, but I could be more certain of that if you had mentioned the actual motors you'll be using.

Note: the rated 'discharge rate' of a lithium battery is NOT the maximum amperage it can provide; it is the maximum amperage it can provide safely. If placed in an environment where the battery is asked to provide more than the rated amperage it will do so and will overheat and damage itself. In extreme cases the battery may catch fire. If your lithium battery is rated for a maximum 70 amp discharge, it is suitable ONLY for applications where the maximum amperage draw does not exceed that value for any significant time.

Robot haiku:
A wire too small
Gets hot, but motor still fine.
Battery is toast.

Q: Hey Mark-
12 gauge wire guy again. My Motors are Ampflow E30-150's, and my batteries are pre-assembled packs manufactured from robotcombat.com, 24v and 4-Ah. The largest wire diameter they could provide was 12 gauge. I'm going to press onward with the wire I've got, unless the new information raises any concerns?

Forgot to add: the 24v packs are NiMH, not lithium.
Thanks.

A: Is this for a 120-pound middleweight? Running some calculations on the AmpFlow motors and the amperage draw you expect works out most reasonably with a middleweight: break traction at 45 amps with a 5:1 gear reduction and 3" wheels. If I'm correct about the weight class, you might consider a bit greater gear reduction to decrease the maximum current draw and give better acceleration. The gearing isn't bad, but top speed is of no use if you can't reach it in the confines of the arena.

I think you're fine with the 12 AWG wire. Run a few stress tests when the robot is complete and inspect the wires for signs of overheating. I suspect that the battery pack and/or the motor controllers will be the amp-limiting elements in your power circuit.

Robot haiku:
No lithium here.
Nickel Metal Hydride cells
Handle high amps very well.



Q: Hello Aaron what do you think about the T-six 2.4gh transmitter from hobby king for use with insect class combat robots with and without active weapons? [San Benito, Texas]

A: In general, we aren't fans of Hobby King products for robot combat. There are reasons why their products are inexpensive, and those reasons don't match well with combat robots that place a lot of stress on components. Magnets come unstuck in their motors, unhardened shafts bend under stress, and an impact can scramble their electronics. Quality control appears to be a major problem -- read thru the comments section on their products pages. Maybe you get a 'good one' and maybe you don't.

Download the user manual for the Orange radio and give it a good read. Can you make sense of the poor translation from chinese? Does the manual answer critical questions like "Does this radio fail-safe?", "On which channels?", and "How?" [hint - the manual says nothing at all about fail-safe]. The compatable Orange 620 receiver claims full failsafe response, but good luck reading the manual to find out how -- plus the 620 receivers tend to be out of stock.

There are posts in this archive about selecting an appropriate radio for your robot. If you've used both cheap and quality radio equipment you'll understand why we avoid cheap. My advice is to pay a little more and avoid the problems that come with cheap components. Save a few bucks and you may go home early from the tournament.

Q: RE: Hobby King 2.4 GHz tx: FYI, Pete Smith just did a review of the HK Orange Tx in the March issue of Servo Magazine. He mentioned the manual was confusing, but the menus on the tx itself were pretty straightforward and it failsafes properly. I've had no issues using the Orange Rx's in my robots. In addition, even the "trusted" brands such as Spektrum aren't very good about documenting their receiver's failsafe behavior. In fact, there can be different bind methods to get different behavior depending on how you want the failsafes set.

I'm all for quality parts, but just because something is cheap doesn't mean it is of poor quality and just because something is expensive doesn't mean it is of good quality. Always, always test the crap out of any component, cheap or expensive, to be sure it will handle the abuse. [Columbus, Ohio]

A: Mark J. here: pay your money and you take your choice. Here are my thoughts:

  • If you have as much experience with R/C transmitters as Pete Smith has, you can most certainly feel your way thru the menus without a readable manual. If you're a novice, you need a decent manual or you're toast.

  • Here's the failsafe operation and function description from a good manual (Futaba 6EX):

    F/S Fail Safe (Throttle channel only)

    The Fail Safe function is recommended to use for safety reasons in the event of radio interference. In this menu, you may select from one of two options. The NOR(normal) setting holds the servo in its last commanded position, while the F/S(Fail Safe) function moves the servo to a predetermined position. The setting is set to F/S as a default.

    To set the Fail Safe Function:

    1. Enter the programming mode. Access the F/S screen with the MODE key.

    2. Press DATA Input lever downward when you need to set Fail Safe. The arrow moves to F/S side. This means that the F/S function is activated. Then move the throttle stick to the position where you want the servo to move when F/S function works and press DATA INPUT lever downward for about two seconds. A figure in percentage will be shown with a beeping sound. Press DATA INPUT lever upward if you want to set NOR. The arrow moves to NOR side and then the NOR function is selected. The F/S value is set to 20% of the full throttle for F/S function as a default.

    3. Verify if the F/S function has been correctly set. Turn off the transmitter, then check if the throttle servo moves to the position that you set.

    That's complete, accurate, and a whole lot better than saying absolutely nothing intelligable in a broken chinese translation. Team Run Amok has always used Futaba radios; their documentation and reliability are primary elements in that choice.

  • You can only do so much testing. If you test to failure, you throw the smoking result in the trash. At some point you have to rely on quality construction and the reputation of the manufacturer -- particularly in electronics.

  • If I pay $60 for a receiver and it craps out, I blame the manufacturer for selling me a bad product. If I pay $6 for a receiver and it craps out, I blame myself for trying to get away with paying $6 for a receiver. I'd much rather blame the manufacturer than myself.



Q: This is an RC boat question but thought maybe you might have an answer. I am somewhat confused about the info regarding receivers, bec's, and esc's. What would happen if I replace the current traxxas 2015 bec receiver (2 channel digital proportional radio system) that has a BEC with another receiver that does not have a BEC? The unit does have a separate esc and one 20T 540 stinger motor. Any info you could provide would be appreciated. Thank You.

A: A Battery Eliminator Circuit (BEC) takes the full battery voltage from the main battery pack, reduces that voltage to 5 volts, and feeds that voltage to the radio receiver. This eliminates the need for a separate dedicated battery pack for the receiver.

The BEC is typically incorporated into the Electronic Speed Controller (ESC), as that device is connected to both the main battery pack and the receiver. Since the receiver is not typically connected directly to the main battery pack, a 'BEC receiver' does not make much sense -- but the Traxxas 2015 manual does claim that the receiver has a BEC. Puzzling!

The manual also claims that Traxxas ESCs have a BEC. As long as your ESC has a BEC, any receiver you choose will have the power to operate correctly.



Q: I am using a Tactic 2.4 Ghz radio system, a 7.4 volt 2 cell Li-poly battery, and two tiny V 2.3 ESC's from fingertech. One of my motors constantly turns and no input effects it. The ESC that drives it gets warm. I read through what little of a manual there was after many soldering attempts. It said that I should pull the red wire on one of my ESC's, the problem is that there are three red wires! One to the battery, one to the receiver, and one to the motor, could you tell me which wire to pull? Or if you think the turning wheel is caused by something else?

A: Each tinyESC has a battery eliminator circuit (BEC) that provides 5 volts DC to power to the R/C receiver thru the red wire of the yellow/red/brown flat cable from the ESC to the receiver. If you have two tinyESCs both attempting to power the same receiver there will be a feedback problem that will cause one ESC (sometimes both) to malfunction. This may very well be causing your problem.

Pull the red pin out of one of the plugs from an ESC to the receiver and tape it back out of the way. The pin is still electrically 'hot', so insulate it.

Wiring diagram for Fingertech tinyESC showing power wire to receiver.

Q: I am the two ESC guy. My right motor continues to spin no matter what I do on the transmitter. So I was wondering if not removing the extra BEC wire could cause some permanent damage?

A: Highly unlikely. The problem may be with the radio, or it may simply be a bad ESC. Try swapping the receiver ports that the ESCs are plugged into:

  • If the right motor responds to transmitter input after the swap, the problem is with the radio.

  • If the right motor still just spins, the ESC is bad.



Q: Hi Aaron!! I want to know whether the twin stick RC system can be used in conjunction with relays to operate a robot. Thanks. [Mumbai, India]

A: Yes, but there are several drawbacks.
  • You will require a special R/C interface between the receiver and the relay to translate the R/C signal into an on/off current to control the each relay -- you can't just plug the relays into the receiver.

  • If this is a fairly large robot, the relays needed to control the high current levels the motors require are expensive, heavy, and bulky.

  • A standard tank-steer robot will require at least four relays and interfaces to provide forward/reverse/off/left/right control.

  • Relays do not provide speed control. A robot controled by relays will be difficult to maneuver precisely and will be frustrating to operate.
Although there are commercially available dual relay boards with a built-in R/C interface that can be used to control a single motor forward/off/reverse, a dual channel electronic speed controller is more compact, lighter, more reliable, provides better control, and costs less than two relay boards of the same capacity.
Two DPST solenoids wired to provide forward/off/reverse control of an electric motor

Forward / off / reverse
motor control via relays.



Q: I'm working on a Beetleweight that is 2 wheel drive and has a vertical spinning blade. I've already got my drive train, RX and using a sabertooth speed controller. I'm going to be getting another ESC for the blade's motor. how should I wire this all together since the additional ESC needs power too, correct?

A: See the diagram in FAQ #19.



Q: Can I use an SLA battery for powering an E30-150 motor? The exact specifications of the battery is not specified but its a 12v, 5Ah motor bike battery. I will be using two of them in series. Apart from low performance what are the other drawbacks of this kind of batteries?

A: First, check the rules for your competition. There may be restrictions that require specific safety features for SLA batteries. Two 5 AH batteries in series should be more than adequate for an E30-150 weapon motor.

Sealed Lead Acid (SLA) batteries were very common in early combat robots, and they have many desireable properties. They can be charged with an inexpensive 'dumb' charger, are inexpensive, and are not damaged by high discharge rates. Their major drawback is that they are bulky and heavy for their output capacity.



Q: Hi aaron, it's the three lb drum spinner guy with sabertooth problems again. So I replaced the small receiver with a larger failsafe capable one and it worked great. For a time. I was testing it because I have a compition this weekend. Well, it decided that it doesn't want to forward/ reverse anymore. Channel 2 works fine and so does the flip. I have a spare, but I won't get it till next Friday . Do you think something burnt out or that maybe one of the signal wires came loose? I looked at the wires and the solders all looked intact. I feel like just maybe I should just replace it. Thank you for your time/help.

A: So, your 'bot will spin left/right but won't run forward/reverse? It certainly could be the receiver, but let's check out a few other things too. I recall that you are running a Sabertooth ESC, and I'm assuming that you're using the Sabertooth's channel mixing.

  • First, check the DIP switches on the ESC to make sure none of them got bumped to a different setting.

  • Next, unplug the 'Channel 2' (left/right) lead from the receiver and plug the 'Channel 1' (forward/reverse) lead into whichever receiver port you had 'Channel 2' plugged into. Power up and give the 'bot some left/right input from the transmitter. If you get forward/reverse, the ESC is OK and the receiver is suspect. If you get nothing the receiver is OK and the ESC may be the problem.

  • If the receiver is suspect and you still have your AR6115 receiver, plug it back in to verify that a different receiver cures the problem. If it does, replace the receiver.

  • If the ESC looks like the problem, check your transmitter to make certain that it is on the correct 'model memory' and that transmitter mixing is turned off.
Write back and tell me what the testing shows.

Q: I went down the check list of tests that you gave and nothing produced any response from the forward/reverse channel of the motor controller. I checked the DIP switches on the ESC to make sure none of them got bumped. I unplugged the 'Channel 2' (left/right) lead from the receiver and plugged the 'Channel 1' (forward/reverse) lead into the other port I had 'Channel 2' plugged into. I powered up and gave him some left/right input from the transmitter. nothing. I then gave forward/reverse input and my robot decided to try a new career as a thwak bot. I checked my transmitter to make certain that it is on the correct model memory and I confirmed that transmitter mixing is turned off.

I checked the solder joints for the wire to the board as well. I'm starting to think that something might have gone wrong with the board. The board's failsafe function doesn't seem to want to work either, which is why I made the change in receiver. I have a spare, but I am thinking of switching to a different brand eventually as this is not the first bot I've built that had one fail.

A: Mark J. here: OK, no panic. It sounds like the ESC is the problem, but the trouble could be entirely in the mixing section. Try switching over to transmitter mixing:

  • Turn OFF the Sabertooth on-board mixing -- DIP switch #1, I think;

  • Plug the CH1 and CH2 ESC leads into channel 1 (elev) and channel 2 (aile) on the receiver;

  • Turn ON the transmitter ELEVON mixing.
Power up and see if you can get forward/reverse out of some type of movement on the right transmitter stick. If you can get forward/reverse, follow the instructions in APPENDIX A of the Team Run Amok Radio guide to get the mix working correctly and you're home free!

If no joy, scrap the ESC and go with a back-up plan. Best luck!



Q: What exactly is a microcontroller, and can it used as an alternative to an antweight speed controller?

A: A microcontroller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals (Wikipedia article on microcontrollers).

A properly programmed microcontroller can be used to send commands to a speed controller in order to autonomously operate a robot, but a microcontroller does not have the current capacity to directly control an antweight motor.



Q: Aaron, I am writing in distress. I received my lipos for [my antweight] 'Hangar 11' and attached them after I properly charged them. The trim setting was off when I put in the lipo, the bot moved, and I looked to adjust it on the controller. The other nine volt [battery] connector still had current in it and I think it made contact with both motors, and I saw one of the motors spark.

When I give the robot command from the receiver, the ESCs show that it is receiving, the [indicator] lights go solid when I give 100% throttle, but there is absolutely nothing from the motors. I took off the lipo and put on a nine volt (I have adaptors for the 9v snaps and lipos) and the same thing happened. I already have two spare motors, luckily, but the competition begins at 7:00 on saturday and I am going to Philadelphia that night.

Do you have any recommendations? Thank you, a very distraught New York

A: I don't see how you could damage the motors by 'sparking' a hot battery connector to one or both of them. Try wiring a 9 volt battery direct to each motor to check them, but they should be fine.

You know the radio is working properly -- the ESC indicator lights are showing correct response to radio command input. If the motors are OK and everything up to the power output from the ESCs is good, then the problem must be either the power output stage from the ESCs or something in the wiring.

  • Carefully check the wiring. A high amperage surge may have burned thru an undersized wire or connector.

  • If the wiring all checks out, your power surge likely blew out the main power chips in the ESCs. The R/C interface could still be operating and showing correct response from the indicator lights, but with the power chips dead you'd get no output.
You've already figured out that it's a VERY bad idea to have loose, uninsulated, 'hot' wires flopping around your robot. A wrap of electrical tape could have prevented this accident. I'm also concerned about your adaptors for the 9v snap connectors. That type of connector is not well designed to carry high amperage loading, and including it in your wiring will eventually cause trouble.



Q: Will the Spektrum AR6255X fail safe on all channels, even with DSMX?

A: Yes, the Spektrum AR6255 receiver will failsafe to preset points on all six channels with either DSM2 or DSMX transmitters. All DSM2 receivers are upwardly compatible with DSMX transmitters -- the failsafe functions of the receiver will not change. Note that DSM2 and DSMX receivers are not compatible with the first generation DSM transmitters (e.g. DX6).

See the Spektrum DSMX User Guide for more info.



Q: I'm confused by all the different features available on R/C gear. I can't even tell what a lot of them are for. I know you've recommended the Spektrum DX6i, but it seems to have a lot of extra features I don't know how to use. Can you tell me what I actually need to control a fighting robot?

A: It's not your fault that you're confused. Only a very small percent of radio gear is sold to robot builders -- the majority of sales are to model aircraft fliers, and the terminology used to describe a system's features is all in airplane/helicopter lingo. A lot of the features found on good radio equipment don't have any use at all on a combat robot, but it's kinda like cable TV: you have to buy a package with a lot of stuff you don't want in order to get the stuff you need.

We've just finished a guide to help combat robot builders sort thru the unfamilliar aircraft language and find the radio features they need to obtain the best possible control of their machines with the least frustration. This should help: Team Run Amok R/C Feature Selection Guide.



Q: Dear Aaron,
I want to start building combat robots, but before I do, I want to get some hands-on experience with the basics, and I plan on doing that with a project: turning a Battlebots Metal Mechanics Backlash toy into a remote-controlled vehicle. I want to go very cheap on this project and have most of the stuff picked out already, but I'd like to know what kind of radio receiver/controller you'd recommend for something like this (keep in mind: it won't be fighting anything other than old pieces of cardboard and plastic).
Thanks. -Lightning

A: I don't claim any particular expertise in 'very cheap' radio systems. I can tell you that once you use a good radio you'll understand very well why we recommend staying away from these cheap systems.

If you're willing to put up with very limited adjustability, a poor manual, and bare-minimum robot functionality - you could try the HobbyKing HK6S for your small, low-power backyard fighter. It includes mixing for single-stick throttle and steering, and has a simple setup that does not require a computer connection. Lack of failsafe capability makes it unsuitable for competition, but it could be OK for your purpose. Best luck.

Mark J: given the possible choices I like Aaron's radio recommendation, but I think he is being overly kind when he mentions the 'poor manual'. I'd say the manuals you get from Hobby King / Turnigy are completely awful. Here are a couple of entirely typical passages from an actual Turnigy R/C manual -- no kidding:

Press +/- key for short and regulate the parameter (when showing for F/S XXX% for parameter, Press MENU key for short and see that reads the output of the corresponding passway, regard value read as the establishing value)

The time-recorder is used calculating comparable bo stipulated time unexpectedly, or the possible time of flight under the state that the fuel fill it up with, it is very convenient. The pattern of the timer-recorder is the count-down. Pour time-recorder from set for time is it is it count to change, show surplus time at interface to begin.

If you don't already know what you're doing, that gibberish certainly won't help. Novice builders don't need the world of frustration that comes with a cheap Chinese radio system. If you really don't have the budget for a good radio, your best option is to go with the simplest cheap system that will suit your purpose and hope you can figure it out without the manual.



Q: I just competed in a competition and lost every battle. My robot put up a decent fight every time but was dominated in general. I consider myself as a winner though because I learned more in that competition than anywhere else.

My electronics came out alive but my ESC is acting weirder than usual. I'm using a Sabertooth 5 RC and after I drive my robot around a bit a red light flashes. I checked to see if it was overheating and I checked my battery and everything checked out fine. I know my current isn't too high as this didn't happen in battle and the system I'm using is identical to the one I used at the tournament. Do you have any idea as to what happened?

A: That's a great attitude! Failure is a much better teacher than success if you're willing to accept the lesson.

You've got a battery problem. As I recall you're running a 9.6 volt NiMHd battery. If I'm mistaken, write back -- Lithium batteries requireire a different diagnostic process.

A flashing red error LED on the Sabertooth indicates a SEVERELY depleated battery that's dropping well below 6 volts under load. Fully charge the battery and try again. If you still don't get reasonable run time, one or more of the cells are damaged or dead. I don't know the history of your battery, but if it's an old hand-me-down pack from an R/C toy car it's time to upgrade.



Q: Dear Aaron, does the fingertech tiny esc V-2 have lipo cutoff protection? from anthony

A: The short answer is 'no'.

Lithium Polymer batteries should not be discharged below ~3.0 volts per cell to avoid damage to the battery. Many ESCs have a 'LiPoly cutoff' mode that monitors the battery voltage and cuts off power when the battery reaches the critical voltage. The FingerTECH tinyESC description says it has 'undervoltage' protection, but it wasn't clear to me exactly what that ment. I wrote to FingerTech for clarification and received this explanation:

"The undervoltage protection is for the tinyESC only. In controllers that do not have this, supplying less than the rated voltage will damage the electronics. This controller turns itself off before any damage can occur. It is not a lipoly cut-off. A lipoly voltage detector can be found for a few dollars if you want the extra protection." So the tinyESC does not have a true LiPoly battery protection voltage cutoff built-in, but the ESC will shut itself off somewhere below its 6.5 volt minimum voltage rating. For a 2-cell LiPoly, that may be close to the same thing. The undervoltage protection cannot be turned off. I've never heard of low voltage damaging an ESC, but I suppose it's possible.

I don't recommend going into combat with a 'hard' LiPoly voltage cutoff that shuts down your robot. That's fine for practice, but losing a match to save a battery is not a good trade-off in my book. If your ESC has a LiPoly cutoff, turn it off for a tournament. If you're worried about the battery you can use a LiPoly voltage alarm buzzer rather than a cutoff -- it gives you the option of finishing the match (maybe winning) or shutting down (losing).



Q: I built a 21 motor robot using battery powered screwdriver motors.What is the best way to control my robot?If I use R C transmitters how many chanels would I need?Will I need to use an E C B ?

A: You want me to believe that you built a 21-motor robot before you decided to ask around to find out how to control it? Bullpucky!

The Robo-One competitors have experience with control of large numbers of servo motors -- see if the crowd over at the Robosavvy forum will believe your story.



Q: I'm running a Sabertooth 5 rc in my robot, and it works okay. But whenever I test it out it follows my commands but will suddenly do something else. Do have any idea as to what is going on?

A: Maybe, but I need more information:

What does it do when it does 'something else'? Will it do 'something else' when just sitting still, or does it only happen when moving? What motors are you controlling with the Sabertooth? Is there also a weapon motor (controlled by another ESC)? What is your battery type and capacity? Are you SURE the battery fully charged? What R/C system are you using?

Q: 'Something else' is sudden backing up, turning, full power forward, and so on. It will do 'something else' just sitting there and while moving. I'm controlling two FingerTech Spark 35:1 Gearmotors with my Sabertooth. My power supply is a Ni-MH, 9.6 volt, 600 mAH battery. I'm using a simple pistol grip two channel [AM] radio system without a weapon system. Also, as a side-note, my antenna was dammaged and I have repaired it by soldering a 'like' wire to it to lengthen it. My antenna is enclosed in an aluminum body.

A: OK, I remember you asking questions earlier about this 'bot. I don't think you have an ESC problem, you have a radio problem. Your AM radio is particularly sensitive to radio frequency noise. If your radio reception is weak, all the receiver 'hears' is the noise. A few things to check to improve the signal strength and reduce the noise:

  • The length of the receiver antenna is important. The total length of your repaired 27 mHz Traxxas antenna should be ~22 inches.

  • Keep the receiver antenna away from the motors and ESC. Motors and controllers may be electrically 'noisy', particularly to AM radios.

  • Solder a 0.1 microfarad ceramic capacitor across the brush leads on each of the drive motors. See Team Delta's Application Note #1 - Reducing Motor Noise for a diagram and explanation.

  • Do not lay the antenna wire along your aluminum bodywork or other conductive surfaces. Keep some distance.

  • Do not 'bunch up' the antenna. You can wind the first foot or so of the antenna neatly around a plastic soda straw, but leave a few inches at the end as straight as possible, away from conductive surfaces, and outside the metallic armor of your robot. You can't receive a radio signal inside a sealed metal box!



Q: My antweight 'Hangar 11' uses dual alkaline 9v batteries for it power, as that is the standard for the Viper kit. Can I/how should I switch to a Lipo or another battery, which could save me a huge amount of weight? I saw the battery that 'Little Scoop' used, and it was a tiny Lipo that could provide more than enough power for the bot.

Thanks, New York

A: Your 'bot uses two 9-volt alkaline batteries in parallel to provide extra amperage at 9-volts. Since you have no active weapon you could simply remove one of the batteries to save weight. Give that a try and see if you still have good pushing power. See the PowerStream 9-volt battery discharge tests for guidance on the best alkaline batteries for use in high-drain applications.

Switching to a LiPoly battery will save more weight and is simple -- if a little expensive. You'd need a 2 or 3-cell LiPoly of suitable capacity, a LiPoly charger, some suitable multi-strand wire, good quality plug connectors for the battery/'bot/charger, and a couple minutes with a soldering iron. There are many prior posts on LiPolys and connectors in this archive.

Q: Aaron, thanks for the advice on Hangar 11. I tried using one 9v battery, and the performance is suboptimal. I would be happy to purchase a Lipo, and I have some for model helicopters. I doubt they are the right ones.

How much weight would I save with a Lipo? With that weight I save, could I try upgrading the motors to something with more torque and/or a little more speed (I know you cannot usually get both), or a basic lifter like the ones in your diagram that could give me an edge against wedge bots, and the remove it when I fight a spinner? For the lifter, I think the most basic mechanism would be the best for this bot. This all, of course, depends on the saved weight. What are your thoughts? I hope I am not steering away from what worked, but every time that I fought a wedge bot I was losing for some of it until I found a way to manage to pit them. Another option is to make a better passive weapon.

Thanks, New York

A: Your two 9 volt alkaline batteries have a combined weight of about 3.25 ounces. Under load they supply only about 7 volts to the motors due to their high internal resistance, at a combined capacity of about 400 mAH -- see the PowerStream 9-volt battery discharge tests. A two-cell LiPoly battery of about 400 mAH capacity has much lower internal resistance and can provide at least that much true voltage at a weight of about 1 ounce. I'm assuming that you get several matches from your 9-volt alkalines, so you could reduce the capacity (and weight) of the LiPoly battery if you were willing to recharge after every match.

You CAN get both greater speed and greater torque from a motor upgrade. You can also keep your current motors and overvolt them to gain both speed and torque. Many competitors run the motors you have at 11.1 volts. Consider -- do you really need more power and speed?

Exactly what you do for your upgrade is your choice. Consider what weakness your 'bot currently has, then upgrade with something that will strengthen that weakness and blend with your driving style.



Q: What exactly would happen if I used a single conversion crystal in a [dual-conversion] receiver?

A: Mark J. here: a dual-conversion receiver requires a dual-conversion crystal. Installing a single-conversion crystal will do no harm, but the receiver will simply not operate.



Q: I am building an rc lawn mower useing a sabertooth 2x25 motor controller along with a futaba t6ex transmitter and a futaba r617fs receiver. I followed the instructions reguading hooking everything together but the motor controller doesn't respond. Help please.

A: Mark J. here: I'd guess that you have not succesfully bound the R617FS receiver to the T6EX transmitter. There are reported problems with this transmitter/receiver pairing. As a first step I'd suggest you consult the manual and confirm that the receiver is properly binding to the transmitter.



Q: I am building a small robot that runs on modified S3003 servos and is mostly built from Meccano. I have just recently purchased a DE BattleSwitch to operate a small spinner motor (a standard 6v Meccano motor). I followed the diagram exactly, by attaching it to the motor and the 200mAh 8.4v battery, but every time I turn the switch on, it misbehaves like it's going through some sort of radio interference, before flashing its LED and taking away my control. Is there something else I need to get, or something I need to change, to make it work properly?

A: Mark J. here: does that 8.4 volt 200 mAh battery also provide power to your receiver and servos? If so, there may be a couple of problems:

  • The flashing LED indicates a loss of signal from the receiver. I suspect that your tiny 200 mAh battery may not supply enough current to simultaneously supply the receiver, switch, and spinner motor without a serious voltage drop that causes a receiver 'drop out'. Fully charge the battery and try again. If no improvement, try a battery with greater capacity.

  • The BattleSwitch is rated for a maximum 5.5 volts power from the servo pigtail lead. If you're powering the receiver at 8.4 volts that same voltage is passing on to the Battleswitch; you may have fried it, or it may just be malfunctioning at the higher voltage. See the post on overvolting servos for an explanation of how to run your receiver at one voltage and your servos (and spinner motor) at another. You will need to add a small Battery Eliminator Circuit or a second battery to pull this off.
I'm assuming that you're using the Dimension Engineering 'BattleSwitch' 10A relay and not the 'PicoSwitch' solid state relay. The PicoSwitch requires an antiparallel 'flyback' diode when used with an inductive load, such as a motor. Search this archive for 'antiparallel' to find an example diagram.

Q: The 8.4v 200mAh battery is only connected to the Meccano motor. It doesn't provide power to the receiver and servos. I already have a 6v battery pack powering the radio gear. When the motor system is not connected, the robot moves about fine, with no problems whatsoever. It's only when I try to turn the motor on with the BattleSwitch that it starts jigging about out of control, and the motor tends to operate intermittantly. All I want is to turn the motor on and off normally, AND have the drive system functioning without interference.

A: OK, first have a look at the Ask Aaron Radio Reception Problems in Combat Robots page for general tips on radio interference.

Next, if that Meccano motor is new you should break it in to contour the brushes to the commutator. Run it unloaded at about 3 volts for five minutes. Properly countoured brushes will reduce radio interference.

I'm still concerned about the battery that powers your receiver. The BattleSwitch relay adds only a small drain, but if you're running a marginal battery it could be enough to glitch your radio. Charge it, and try another battery if the problem continues.

Q: I have changed the receiver battery from 6v to 4.8v, so it stays within the BattleSwitch's input limit, but it still glitches out of control. I have tried keeping the wires as far away from each other as possible, but the same problem keeps occurring. However, I have just tried using the BattleSwitch on an LED light instead of the motor, and it seems to work fine. Maybe the problem could be the motor? Will it need a diode?


Q: Don't worry about it. The problem is solved! It turns out it WAS the motor that caused it. I just switched it with a newer Meccano motor, and it finally works the way I want it. I'm thinking the new motor probably has a diode attached inside its plastic case.

Either way, thank you so much for your help and all your suggestions. Maybe you could humour me with one last question. How fast does a Meccano motor spin when powered by an 8.4v battery?

A: Happy to hear you found the solution! For the benefit of other readers: Meccano 6v motor

  • The old motor likely had worn out brushes; worn brushes create a LOT of radio 'noise'.
  • The BattleSwitch 10 amp relay does not require a flyback diode when used with a motor.
  • It's very unlikely that a small DC hobby motor would come with a flyback diode. It would cause a short circuit if the motor polarity was reversed.

Meccano kits are not very popular in the U.S. and I'm not familliar with the motors. There seem to be several varieties. The most common is a small high-speed low-torque plastic-cased motor with a square flange. This motor would have an unloaded speed near 15,000 RPM at 8.4 volts. Actual speed under load will be much less, depending on what you're spinning.



Q: To build an articulated design like Flexi Flyer would the wiring be the same as Hammerhead's?

A: Mark J. here: as I recall, there was no wiring passing between the two halves [left/right] of 'Flexi Flyer'. I believe that each side of the articulated robot had its own battery, receiver, and ESC -- it was essentially two robots held together with a pivot hinge.

The two articulated halves [front/rear] of 'Hammerhead' were most likely set up the same way, but I can't confirm that. Running power cables or other critical wiring thru a flex-junction is best avoided.

'Flexi Flyer' combat robot.'Hammerhead' combat robot.
Articulated robots 'Flexy Flier' and 'Hammerhead'



Q: My Sabertooth 5 RC Dual Motor Speed Controller [was] working fine, I used a DIP switch wizard to make sure I have everything set right. When I turn my ESC on a dim blue light appears, when I turn on my transmitter it brightens. When I turn the throttle on my motors run for a second, stop, and a blue light flashes rapidly. What's going on?

A: Mark J. here: have you considered reading the manual?

I'm guessing that you have the ESC in 'Lithium Mode' and that the battery needs to be charged. Read the one-page manual; it will tell you what the lights are about. Charge the battery, and try again. If you still have questions after you read the manual, write back and tell me how you have the option switches set and what type of battery you have.

Note: I recommend turning off 'Lithium Mode' when actually competing. It shuts down your robot to protect your lithium battery from discharging below 3 volts per cell. That's very handy when practicing, but I'd much rather fry the battery than have my robot shut down near the end of a match!

Q: I'm the Sabertooth guy, I'm reading through the manual, flipping the DIP switches the way I think they should, and my robot goes wild. It dosn't do anything without the remote on but as soon as I switch it on my robot does nothing I want it to. I'm using a non-lithium, pistol grip transmitter, I want the remote to be linear, could you tell me how to set it up? I'm sorry for begging but I'm confused to no end.

A: That's why we're here.

First, make sure the ESC connectors are plugged into the correct receiver ports - the ports on the Traxxas receiver are reversed! Channel 1 controls steering and channel 2 controls throttle -- plug the 'channel 1' ESC lead into channel 2 on the receiver, and plug the 'channel 2' ESC lead into channel 1.

I make the Sabertooth switch set-up like this: switch 1 down, switches 2 thru 6 up. See the photo.

Power up and adjust the trim settings until the motors stop. Now try forward throttle:

  • If the 'bot backs up, flip the throttle reversing switch on the transmitter.
  • If the 'bot spins to the right, reverse the motor power leads for the left-side motor.
  • If the 'bot spins to the left, reverse the motor power leads for the right-side motor.
  • If the 'bot goes forward, move on to the steering test.
Next, try a right turn:
  • If the 'bot spins to the left, flip the steering reversing switch on the transmitter.
  • If the 'bot spins to the right, you're done.
'Sabertooth ESC set-up switches.



Q: I have a 12v 700mah battery and a charger with the same output can it charge it safely?

A: Way too little information.

A battery charger must be correctly matched to the battery. A charger designed for NiCad batteries cannot, for example, be safely used to charge lithium batteries. Assuming that your charger is the correct type for your specific battery, most batteries can be charged at a rate that will fully charge them in one hour. Your 700 mAH battery can likely be charged at a 700 mA rate with an appropriate charger, but check with the battery manufacturer for a battery spec sheet to find the maximum charge rate.

NEVER charge a battery with a charger not specifically intended for a battery of that type, voltage, and capacity!



Q: In the Wikipedia article on robot combat it says lasers above 1 milliwatt are prohibited. Does that mean lasers under that rating can be used on a robot because I have never seen it done before?

A: I don't know where that 'wikifact' came from. Early Battlebots rules said:

Lights that are bright enough to obstruct an Official, Contestant, or Judge's vision shall be forbidden. This includes, but is not limited to the following:
  • Lasers over 5mW output.
  • Any Strobe Light
  • Flood type lights

Later BattleBots rules were modified to explicitly restrict lasers to "Class IIIa or below".

Current RFL rules forbid any lights that impair the view of entrants, judges, or the audience. Lasers above Class I - which are very low power - are specifically forbidden.

So, yes you can use very low power, eye-safe lasers as long as they don't interfere with anyone viewing the robots.



helicopter Q: I want to make an RC Helicopter so i want all relevant details apart from it i want to know about servo motors , 4 channel transistor and reciever. What all things would be requires and i also want some knowledge about transistor and reciever ?

A: Mark J. here: we give short answers to questions about combat robots. Maybe you could find a website called 'Ask Some Guy: R/C Helicopters Q&A' and hope that he has time to write a few dozen pages to answer your very general questions. If I ever write a book about R/C helicopters I'll be sure to send you a copy.



Q: Is it possible for me to use Futaba 6EX to control a robot? The robot has motor to control the wheel, a lifter and a gripper. [Malaysia]

A: The Futaba 6EX is a 6-channel 2.4 GHz radio system, but only the throttle channel will failsafe on loss of signal. Under US combat rules a combat robot with active weaponry must failsafe all drivetrain and weapon channels on signal loss. The Futaba 6EX does not meet that requirement.

Check the rules governing your competition to see what failsafe functions are required.



Q: I have a Traxxas digital proportional two channel radio, whenever I turn on the radio one of my motors starts two run while the other stays put until I change direction, then it turns just as fast as the other motor. Do you know how to fix this?

A: What speed controller are you using? A simple pistol grip radio like the Traxxas requires an ESC with on-board mixing to control a differential steering robot, and this sounds like an ESC set-up problem. Check your ESC documentation.

Q: I was using old servo ESC's, does this mean I'll have to get a new ESC?

A: It might be a good idea, but you can use an inexpensive plug-in channel mixer (or a fancier version) between the receiver and the ESCs.

Your transmitter throttle trigger sends out commands to only one channel of the receiver, and your steering knob sends commands to the other channel. If you have an ESC plugged into each receiver channel, only one will respond to throttle commands -- the other will respond only to steering input. A channel mixer will take the throttle and steering information from the receiver and send appropriate signals to the two ESCs so that both motors will respond to throttle and steering commands correctly.

  1. Plug the two leads from the mixer into the output ports on your receiver.

  2. Plug the ESCs into the output ports on the mixer.

  3. Power up and adjust the throttle and steering trims to get the motors to stop. You may need to re-adjust the trims as the set-up process goes on. Servo ESCs are very sensitive to transmitter input -- just a little input will push them to full throttle. Your transmitter has no sensitivity adjustment, so it may be a little touchy to drive.

  4. Throttle test: pull the trigger back (forward command):
    • If the robot turns either left or right, swap the positions of the two mixer leads plugged into the receiver.
    • If the robot moves backward, flip the throttle reverse switch on the transmitter.
    • If the robot moves forward, move on to the turning test.

  5. Turning test: rotate the steering knob to the right (right turn command):
    • If the robot turns left, flip the steering reverse switch on the transmitter.
    • If the robot turns right, you're done. Go kick butt.
I hope you're not trying to pull much current thru those old servo ESCs -- they won't take much abuse!

Q: I was looking at the diagram on FAQ #19, and I was wondering, my receiver has a battery port, in the diagram it appeared as if there was no battery port. Could you explain this?

A: Some receivers have a separate battery port and some do not.

  • Most robot Electronic Speed Controllers (ESC) have a Battery Eliminator Circuit (BEC) that will feed 5 volts to the receiver thru the receiver cable(s), eliminating the need for a separate receiver battery.

  • If there is no BEC and your receiver has no battery port, a receiver battery connector may be plugged into any unused port since the (+) and (-) lead pins for each port access a common power bus.
The diagram for FAQ #19 assumes that the Drive ESC has a Battery Eliminator Circuit, as alluded to in the text.

Q: My battery port has 3 leads, how do I run a battery through it then? Could you just give me a diagram of a BEC-less circuit?

A: There is a wiring diagram on page 2 of the Traxxas manual. Battery + is red wire, battery - is black wire, the white 'signal' wire is not used in the battery hook-up. Exceed 6 volts at your own risk!

Q: So just to make sure I'm right; the BEC-less circuit is just like the BEC circuit except that it has a small battery running through the receiver?

A: Since you're using hacked servo ESCs that draw power for their motors from the receiver cable, this 'small battery' will be the only battery in your 'bot -- so maybe it isn't so small. Your circuit will look like the Traxxas manual diagram, but with the V-tail mixer in between the receiver and the servos.

Do not exceed the max voltage rating of the receiver.

Q: I'm switching to sturdier ESC. Does this mean I'll be using two batteries now?

A: Good plan. If you're switching to a real ESC it will probably have a battery eliminator circuit. If so, you need only a single battery and you will wire like the diagram in FAQ #19. Many ESCs also have a built-in mixer, eliminating the need for the stand-alone V-tail mixer.



antweight robot 'Rat Amok' Q: Can you give me a diagram of how you wired Rat Amok?

A: The wireing for our infamous rat trap antweight 'Rat Amok' is very simple -- no diagram required. The drive motors are hacked servos; they simply plug into receiver outputs 1 and 2 for Elevon mixing. The rat trap release is controlled by a very small servo plugged into receiver output 5 and controlled by the landing gear switch of the Futaba transmitter. A small 6 volt NiMHd battery pack plugs into the battery port of the receiver and powers everything.

Q: How do directly wire the battery to the receiver?

A: First, check the voltage tollerance range for the receiver and servos. Exceeding their maximum voltage can very quickly fry the receiver. Solder a servo cable to the battery terminals: red wire to the '+' terminal, black wire (sometimes brown) to the '-' terminal. The third wire is not used. Plug the cable into any unused port in the receiver and you've got power for the receiver and servos. You can splice a switch in the '+' wire, or just unplug the battery to turn off the power.

This method does NOT work for more conventional robots with a discrete Electronic Speed Controller. Most ESCs have a 'Battery Eliminator Circuit' (BEC) that feeds a steady 5 volts to the receiver thru the receiver connections. See FAQ #19 for a wireing diagram that includes drive and weapon ESCs.



Q: Hi Aaron, i'm using two one channel speed controllers (like victors), and i'm asking, how do i wire it to the receiver? FYI, i use the Mode 2 Spektrum DX6i, the AR6210 Reciver, and i want controls on the right stick.

A: Thank you for providing full information on your radio and stick preference -- it helps a lot. The wireing is easy, but the transmitter setup takes a little work. For right stick throttle and right stick steering on a Mode 2 transmitter:

  1. Select WINGTAILMIX from the SETUP LIST on your DX6i transmitter and activate the ELEVON option. Check the DX6i manual for detailed instructions.

  2. Plug your left ESC into the channel 1 (AILE) receiver port.

  3. Plug your right ESC into the channel 2 (ELEV) receiver port.
Throttle Test: power up the robot and push the right stick forward a little (forward throttle command):
  • If the robot moves forward, good! Go directly to the Turning Test.
  • If the robot moves backward, select REVERSE from the transmitter SETUP LIST and reverse both the ELEV and AILE channels.
  • If the robot spins to the right, reverse only the ELEV channel.
  • If the robot spins to the left, reverse only the AILE channel.
Turning Test: move the right stick a little to the right (spin right command):
  • If the robot spins right, your basic setup is done.
  • If the robot spins left, swap the ports your ESCs are plugged into (left into ELEV, right into AILE) and repeat the Throttle and Turning tests.
For other options on transmitter stick assignments and modes, search this archive for 'the usual aileron'.

Q: After i tested it, because both of the single channel speed controllers have a BEC, I've cut the red wire that leads from the ESC to the receiver, however, the motors that is connected to the ESC that has the cutted red wire won't move even after i moved the forward throttle(only the motor that is connected to the ESC that still have the uncutted red wire moves). What should i do?

A: You haven't told me what speed controllers you're using. You mentioned that they were 'like' Victors, but Victors have no battery eliminator circuit (BEC). It might help if I knew.

Cutting the red (power +) wire from the ESC to the receiver should not cause a problem with the speed controller. The receiver uses only the ground and signal wires to communicate with the ESC. My guess is that you damaged one of the other wires while cutting the red power wire in the center of the flat receiver cable. Examine and test the remaining wires. If all else fails, patch the red wire to confirm that the ESC still does work with all three wires in place.



Q: Is a frequency clip the same thing as a frequency crystal?

A: No, two different things.

  • Frequency crystals are parts of R/C transmitter and receiver circuitry that control the frequency with which the radio communication takes place. FM radio systems typically have crystals that can be replaced to change the frequency to avoid interference with other robots.

  • Frequency clips are a set of labels maintained by the robot combat event organizer. If you want to turn on your transmitter for testing or to enter combat, you must first obtain the frequency clip that matches the frequency of your radio from the EO, attach ('clip') it to your transmitter antenna, and return it when you are done. This system helps prevent inadvertant interference from multiple transmitter operating at the same time. At some events there was additional frequency security -- transmitters themselves were impounded and had to be checked out.
The newer smart 'spread spectrum' R/C systems will automatically select an unused frequency and the receivers will reject the signals from any transmitter that has not been 'bound' to it. These systems do not require either replaceable frequency crystals or frequency clips to avoid interference.



Q: Hi Aaron, i knew that you can wire 2 motors(parallely) to a 1 channel ESC if the amps requirements still below what the ESC can handle, but, how do i wire it so that i can move forward-backward-left-right? do i need 2 RC cables? I use the Spektrum DX6i transmitter and the AR6210 reciver(don't nag me about the reciver, i got it with the box that has the transmitter, and besides, the event organiser dosen't know what failsafe is#. And, this is for a hobby weight Horizontal spinner#can be a wedge if the weapon system isn't ready in time for the competition).

A: To control one drive motor forward/backward you need:

  • a single-channel bi-directional (reverseable) speed controller connected to one output channel on your receiver.
To independently control two drive motors for forward/backward left/right differential control you need either:
  • two single-channel bi-directional speed controllers connected to two output channels on your receiver, or
  • one two-channel bi-directional speed controller connected to two output channels on your receiver.
The Sabertooth 12 ESC you've previously asked about is a two-channel bi-directional speed controller. Take a look at FAQ #19 for a diagram that shows how it might wire into your robot.

Which receiver output channels you will plug your speed controller leads into depends on which transmitter sticks you want to use for control and whether you want simple 'tank steer' or the more convenient and adjustable 'channel mixing'. Search this archive for 'create proportional drive mixing'. There is also useful information in our transmitter programming guide. The guide is written for Futaba transmitters, but a lot of the information applies to all radio systems.

Mark J. here: for the record, the Spektrum AR6210 is not intended for use in a combat robot. If the transmitter signal is lost, channel 3 (throttle) will failsafe to a set position, but all other channels will lock in the position they last received from the transmitter. Whether or not your event organizer cares about your safety, I can't recommend that you use this receiver.



Q: What's a failsafe?

A: Combat robots are dangerous. Combat tournament rules require that robot propulsion and weapon systems shut down if the radio control signal is lost; the robot control systems must safely respond to a radio 'fail' condition. The specific requirements of the failsafe response depend on the weight class and weapon type -- see FAQ #18 for specific requirements.

From the RFL ruleset:

4.4.1. Radio systems that stop all motion in the robot (drive and weapons), when the transmitter loses power or signal, are required for all robots with active weapons or any robot over 12lbs. This may be inherent in the robots electrical system or be part of programmed fail-safes in the radio. Robots 1 lb and less [do not require] drive fail-safes. The final call on correct failsafe response comes from the event organizer -- check with them if you need specific guidance.

Q: What's a master kill switch?

A: A master kill switch is another safety device. In case of short circuit, fire, or a runaway condition caused by equipment failure, tournament rules require a single manually operated master power switch that will safely cut power to the major electrical systems of the robot.

From the RFL ruleset:

6.3. All electrical power to weapons and drive systems (systems that could cause potential human bodily injury) must have a manual disconnect that can be activated within 15 seconds without endangering the person turning it off. (E.g. No body parts in the way of weapons or pinch points.) Shut down must include a manually operated mechanical method of disconnecting the main battery power, such as a switch (Hella, Wyachi, etc) or removable link. Relays may be used to control power, but there must also be a mechanical disconnect. Please note that complete shut down time is specified in section 1.6.



Q: Hi Aaron, i've 2 questions for you about the Anderson PowerPoles:
1.instead of crimping it, can i solder the cable to the PowerPole? and, how do it?
2.and, if i can't solder the cable the the Powerpole, how do i crimp it using a standard crimping tool?(the one you said that it costs $8)

A: Instructions for crimping and soldering PowerPole connectors are at the Anderson Power Products website.



Q: Is it possible to roll 3 cables to 1, and then crimp it into an Anderson PowerPlug?

A: How many wires you can crimp into an Anderson PowerPole connector depends on which model of the connector you use and how big the wires are. The 45 amp PowerPoles will crimp a single 10 guage wire -- three 14 guage wires should just about cram in there. I solder the connection after crimping just to make sure everything stays put.



Q: Hi Aaron, in the wiring diagram [FAQ #19], there are some links which connected the battery power from the battery to the motors. what can i use for the links? can i use gator clips?

A: The electrical connectors must handle high amps and survive severe mechanical shock loading -- so no alligator clips, please. We like Deans Ultra plugs for small robots and Anderson PowerPole connectors for larger applications.



Q: Aaron, I'm working on designing a combat robot to control with the spektrum Dx6i controller with an IFI VEX Pro Victor 885 speed controller for the weapon. The design calls for the ability to run the weapon forward as well as backwards. The drive system is a two wheel system driven using tank steering. We can't use a simple toggle button on the controller to control the weapon anymore because there are 3 states, forward, off, and reverse. Is there some way we could use that push button, but have an option up on screen to reverse that direction?

Would that be the best way to program this, or do you have any suggestions for possible solutions?

A: Mark J. here: as the radio specialist for Team Run Amok I'm going to intercept this question.

You haven't mentioned if this weapon is a spinner, a lifter, or something else entirely. Without knowing what the weapon is I can't offer a 'best' control option. For example: that 'Throttle Cut' push button on the DX6i is not a 'toggle', it's a momentary contact switch that is active only so long as you continue to hold it down. That could become inconvenient and tiresome for some types of weaponry. You also wouldn't want to abruptly reverse a spinner from full-speed; you'd want a gradual 'spin down' to avoid killing the motor and ESC.

The simple generic solution is to assign the weapon to the ratcheted throttle channel (or rudder channel if a spring-center is more convenient) for full proportional forward/reverse weapon control from the left stick, and use Delta-Wing (Elevon) mixing to control the robot drive system from the right stick.

If you really want to use the transmitter switches for on/off and forward/reverse control the weapon, send me more information about the weapon itself and I'll see what I can recommend.



Q: I been looking for A123 battery packs, but I'm having no luck. HillTop, if I heard correctly, shut down for small consumers, and they were the only source of A123 packs I could find, and RobotMarket place doesn't carry any A123 packs and they stopped building any new packs. Should I build my own pack from loose cells, or do you know any other suppliers?

A: You didn't tell me what size packs you're looking for. A quick web search ('A123 battery packs') turns up several sources (Electricwingman, Esprit Model...) and a selection of small packs ARE available thru Robot Marketplace. Take another look around.



Q: Hi Aaron, does the Spektrum AR6255 DSM2 6-Channel Robot Receiver have BEC? if not, i'm going to buy the Park BEC for the receiver's BEC. I know that this BEC isn't really cheap, but it'll save me some money rather than buying a new battery and finding a cable which fits it (i live in Indonesia, nobody knows combat robotics here, so finding the right wire will be a pain). One more thing, do i have to use a battery, or can i use this BEC when i'm doing the binding process with my R/C?

A: I don't think you understand what a Battery Eliminator Circuit (BEC) does.

An R/C receiver typically requires a lower voltage power supply than the main battery for the vehicle. A BEC is a voltage regulator that connects to the main system battery and provides a stable 5 volts to power the receiver via a standard receiver cable. This eliminates the need for a separate small battery to power the receiver. Search for 'how do battery eliminator circuits work' in this archive for more info.

Receivers do not have BECs -- but Electronic Speed Controllers (ESCs) often have a built-in BEC to power the receiver. This makes sense as the ESC will be attached to the main battery and already has a cable going to the receiver that can carry the power. Check the documentaion for the ESC you plan to use to see if it has a BEC.

If main battery power is available to the BEC and the BEC is connected to your receiver, the receiver will be active and can be bound.

Note: the Spektrum AR6255 receiver has a relatively broad input voltage tolerance: 3.5 to 9.6 volts. If your main battery is within this range you can power this receiver directly from the main battery without a BEC via a standard receiver cable.



Q: Aaron, I would like to have two separate battery 24v power sources for my bot. Lets say one source gets low, I would like to switch to disconnect the primary source and connect the secondary source.

I was think I would like to use two high amp Mosfets. Is this possible? If so, what would the wiring look like? Simular like battery isolator, but totally disconnect the primary battery.

A: If the two battery sources are identical, just wire them in parallel to double the capacity. No need to complicate things with MosFETs and switching. Keep it simple.



Q: Aaron, I have a Spektrum DX6i transmitter. I'm trying to use my switches (channels) such as "Rudo", "Mix/ThrottldHold", "flap/Gyro". I managed to get my Aux channel using the Gear switch successfully, but I can not get the others to work. When I did try to use the "flap" switch, it worked but it also made the robot go forward which I did not want the robot to go forward. How do I use the switches without affecting my forward, reverse, right and left?

A: Mark J. here: most of the switches on your DX6i aren't 'channels'. Your radio is a six channel transmitter designed primarilly for aircraft, so some of the functions don't do anything at all useful for a combat robot.

  • The first four channels (THRO) (AILE) (ELEV) and (RUDO) are proportionally controlled by left/right and up/down for the two control sticks.

  • Channel 5 (GEAR) is by default controlled by the 'Gear' switch and can provide two position instruction to a device plugged into the [GER] port of the receiver with the flip of that switch. Useful for activating a flipper, gyro, or weapon ESC.

  • Channel 6 (FLAP) is by default controlled by the 'Flap' switch -- but it effects multiple receiver output ports. Activation of this channel mixes in a set amount of response to both the [AILE] and [AUX] receiver outputs, plus optional response from the [ELEV] output. This makes perfect sense for an airplane, and no sense at all for a robot.

    If you really need to use the [AUX] output and control it with the Flap switch, you can either move the robot steering off the [AILE] output channel or set a 'programmable mix' to offset the effect of the (FLAP) input on the [AILE] output. Both are fairly advanced tweeks -- leave it alone unless you really need that sixth channel.

The other switches on the DX6i do not control specific channels. They turn on/off dual rate functions, programmable mixes, throttle override, and receiver binding. These functions are well covered in the DX6i manual. Give the manual a good read -- you'll be amazed at how much flexibility that radio has.

Tip: aileron dual rates are VERY useful in combat robot control. See if you can figure out why and how.



Q: Hello Aaron. I created a homemade robot that works with my Spektrum Dx6i transmitter/receiver fine. I have two NPC T64 motors on 24volts (four batteries 2 in series, 2 in parallel). I use a Sabertooth 2x25 controller. (also have Vantec RDFR36E Speed Controller and RoboClaw 2x25 that I test one at a time). My question is that I would like to control it with a Stamp BS2 MCU. I'm having a hard time getting started with using the PULSOUT command to determine forward, reverse, right, left, slow and fast. I'm aware that the Sabertooth has a MCU mode. Do you have any sample commands I can use to achieve this? Or a better way? The reason I would like to do this is to have an autonomous mode and switch back to transmitter mode. Thanks (in advance).

A: Sorry, we don't work with BASIC Stamp programming. There are a couple autonomous robot builders on the Western Allied Robotics forum that may be able to help -- try a post there.



Q: Hi Aaron, i'm seeking some good R/Cs, and i found this Futaba 4YF 2.4GHz FHSS Radio. Is this R/C good, adequate or sucks? give me your opinions.

A: Futaba is a major R/C manufacturer that builds quality equipment. However, the 4YF FHSS is a really basic 'entry level' radio with very limited features. Most importantly the 4YF FHSS system does not failsafe, which makes it generally unacceptable for combat robot use.

We have posts covering our recommendations for combat robot R/C gear in this archive. I suggest you start there rather than asking our opinion on every inexpensive radio you find.



Q: I know this is a very basic question, but must the battery voltage be no less than all of the component (ESCs, Motors, Recivers) voltage needs combined?

A: Mark J. here: no, the electrical components of a typical combat robot are wired as a parallel circuit. Each of the components will be able to access the full voltage of the battery. The voltage needs of the components are NOT additive.

Example: a battery pack in a combat robot provides 7.4 volts. If wired as shown in FAQ #19, both drive motors, the weapon motor, both ESCs and the power light can each access the full battery voltage of 7.4 volts. The receiver is typically powered by a voltage regulator built into the drive ESC and will have its voltage limited to about 5 volts.

The voltage requirements of the components are not additive, but the amperage requirements of the components ARE additive. If the two drive motors have a maximum current draw of 4 amps each and the weapon motor has a maximum current draw of 8 amps, the battery may have to deliver 16 amps in a 'worst case' situation. The current draw of the ESCs, receiver, and power light are trivial.



Q: Hi Aaron, how do i connect the channels in the R/Cs to the ESCs so that they control the robot and the weapon? and how do you do a "channel mixing"?

A: See FAQ #19 for help in connecting up the ESCs and the receiver.

Channel mixing is a function commonly built into R/C transmitters and speed controllers. Consult your radio or ESC manual for specific instructions on use of the function. For additional info search this archive for "proportional drive mixing".

Q: After I saw the FAQ, i noticed that there's a "link" between the cables from the ESCs and the batteries. What is the use of those links? what's the consequences if I dont use it? where can i get it?

A: That's your power switch. Many competitions require the switch to be in the form of a removeable link. Search this archive for "removeable link" for an explanation and sources.

Q: 1 more question, FAQ #19 showed the wiring for 2 [motor] robots. How do you connect [four] motors to their ESCs?

A: Robots with two motors per side typically connect those motors in parallel to the output of a single ESC. Search the Motors & Controllers archive for "run 4 motors" for a diagram and design considerations.

You can save both of us time and effort if you check the archives before asking a question.

Basic combat robot wiring.

Two sets of two motors connected to a two-channel ESC.



Q: Dear Aaron, I see that you should not go cheep on the radio (I could not agree more, I have seen this in sumo bots), but what do you think is a 'threshold' price is for a radio that we see on line? for example, a $700 radio is going to be great, but $7 dollar radio is too cheap. $100 might be about right. (I don't know if that is true). what do you think? Thank you, New York

P.S. I have ordered Grant Imahara's book and already got another one based on your recommendation. Thank you very much!

A: Ten years ago a 'good' R/C system in common useage by combat builders cost twice what a 'good' radio does now, so I don't feel comfortable setting a fixed price line. Also, the demands of combat robotics are different than for other R/C applications -- one $200 radio model might be great for robot combat, while another $200 radio might be a real headache.

As with many combat robot components, I recommend that you see what radio systems are popular with successful builders. Until you have enough experience to go your own way, following the lead of other teams is a good plan. If winning teams aren't using that $40 'Super Bamboo 3000' radio you found on-line there's most likely a really good reason.

Q: The [Popular R/C transmitter] with [Super Bamboo receiver] combo has been used successfully by both [team #1] and [team #2] over the past year or so. Total cost for the [popular transmitter]/[Bamboo receiver] combo is around $[very little]. (West Chester)

A: I know that [team #1] has looked into [Super Bamboo] radio gear, but I can't verify use of that specific R/C system in combat by either of the teams you mention. If they are using it they're in the minority and taking chances I can't recommend. Don't push your luck -- you'll feel really dumb if your $[very little] radio glitches and boots you out of a tournament.



Q: Does the [Spektrum] DX5e [R/C transmitter] Failsafe?

A: The failsafe behavior of the DX5e depends on the receiver used with it.

  • The Spektrum AR6255 receiver will failsafe to setable positions on all channels and meets all current failsafe requirements.

  • The AR6115 and AR600 receivers kinda-sorta failsafe by sending out a 'null' signal that relies on the behavior of the device plugged into it to respond properly.
I recommend use of the AR6255 receiver. Don't go cheap on your radio!

Q: Does the [Spektrum] DX6i [R/C transmitter] Failsafe?

A: Same deal as the DX5e.

Q: is the ar600 receiver legal for combat robotics? This question is related to the question on the dx5e

A: Mark J. here: the failsafe requirements for combat robotics address the behavior of the entire robot when the transmitter signal is lost.

  • The Spektrum AR600 itself is neither legal nor illegal -- proper failsafe response with this receiver depends upon correct interpretation by your ESC/controllers of the 'null' signal this receiver sends on loss of transmitter signal.

  • The Spectrum AR6255 receiver is entirely legal. It sends specific shut-down instructions to the ESC/controllers on loss of transmitter signal which guarantees correct failsafe bahavior.

Q: The Hobby King "orange" receiver will failsafe on all channels when bound to a DX5e or a DX6i.

A: I cannot confirm that, and I DO NOT recommend Hobby King products for use in combat robots. Like Aaron said above, don't go cheap on your radio.

Update: an article in the January, 2012 issue of Servo Magazine says that the Hobby King R410 and R610 receivers will bind to the Spektrum DSM2 transmitters and implies (but never actually states) that they do failsafe on all channels. You can roll the dice if you like, but I'm not going into combat with a $5.95 receiver.

Q: IS the [Tactic] TTX404 transmitter with the [Tactic] TR-624 receiver legal for combat robotics? Is it good?

A: Pay close attention because I'm getting tired of saying this:

 DO NOT GO CHEAP ON YOUR RADIO! 

I'm not going to go thru all the chinese knock-off radio systems individually to point out their flaws and list the things that would make you wish you'd spent a few more bucks. Legal or not, I recommend that you stay away from cheap, off-brand radio gear!

Most robot builders choose Spektrum radios, and for good reasons. Here are a few:

  • Good resale value.
  • Readable documentation.
  • Established record in combat robotics.
  • An army of robot builders to ask for set-up help.
A single radio glitch can take you out of a tournament, and nothing is more frustrating to work with than a balky radio system. Don't sabotage your efforts by trying to save a little on your R/C gear.



Q: Hi Aaron, how does the R/C in a Combat Robot control the wheel and the weapons motors? does it need a motherboard(mainboard)?

A: An Electronic Speed Controller (ESC) takes the signal from the R/C receiver and translates it into controlled power levels for weapon or drive motors. There is a great deal of discussion about the selection and use of ESCs in the Motors & Controllers archive.



Q: Hi Aaron, do i have to include the blade's motor on the R/C for a spinner bot(Hypno-Disc,Last Rites,Hazard....) or just let the motor run by itself?

A: Holy cripes man -- read the event rules!! No respectable event would allow uncontrolled weapon motors. you will also (I hope) be required to have an R/C system that shuts down all weapon and drive power if the R/C signal fails. Safety first! See FAQ #18.



Q: Hi Aaron, how does the gyro work on 4 wheeled robot? it looks really complicated.

A: There is no difference in the operation of an R/C peizo gyro on robots with 2, 4 or more wheels. The turn signal from the receiver is sent to the gyro where it is compared to the actual turning motion of the robot. The gyro modifies the turn signal and sends it on to the signal mixer. The mixer sends the modified signal to the left and right side ESCs to keep the robot on the desired course despite minor mechanical or environmental influences.

  • Two-wheeled robots are difficult to keep pointed straight under power -- they tend to turn even when the R/C signal calls for a straight course. A gyro will hold the course steady.

  • Four-wheeled robots have no difficulty tracking straight, but do have trouble maintaining a smooth turn. A gyro will juggle the turn signal to keep the turn rate smooth and constant.
For more information on gyros, see the Team Run Amok Gyro Guide.

Q: It's me gyro guy again. Would i need a gyro if i used 2 motors to run 4 wheels like BioHazard?

A: First, 'BioHazard' runs six wheels - not four. Six wheel robots tend to be both stable and very maneuverable.

I'm not sure any robot 'needs' a gyro. Build it and see how it handles. Handling depends on factors like weight distribution, wheel track, wheelbase, and tire width. If you have trouble holding a straight line or carving a smooth turn you can add a gyro to assist.



Q: Would the Airtronics SD-5G be a suitable radio for combat? I know its pretty basic.

A: Mark J. here: yes, the Airtronics SD-5G is suitable for robot combat. It meets the minimum requirements for all weight classes and weapon types, it has the usual features to adjust the radio response to your preferences, and the manual is complete and written in understandable english.



Q: It seems that the few lithium cutoff devices that are available on the Robot Marketplace are designed for non-reversible throttle ESCs. Will these devices be at all usable in a mixed, 2 1-channel esc setup? If not, how will I protect the Lipo from over-discharge?

A: Mark J. here: think about what you're asking. If you're in the middle of a robot fight do you really want circuitry that shuts down your robot to 'protect' your battery?

In combat, robots do not run fuses, circuit breakers, or any device that could decide to protect the robot by shutting it down in the middle of a fight. Size your battery to provide ample power for a 'worst case' match. If you're worried, you can use an audible warning module to let you know when your battery is getting low and give you the option of shutting down, but don't use a cutoff!



Q: While looking on Ebay, I noticed that some of the radios, mostly Futaba PCM radios, have an option to add a radio module to change its frequency. If I add a 2.4 Ghz module to a 72Mhz radio, would it be legal for ground use? I know it won't be cheap, but it will help save some money than buying a new radio.

A: Mark J. here: some Futaba transmitters (3PK, 7U, 8U, 9C, T10C, 12FG, 12Z, 14MZ) do allow for replacement of the frequency module with a 2.4 GHz conversion package that includes a new receiver and antenna. The resulting transmitter/receiver package is entirely ground legal, just like a standard 2.4 GHz system.



Q: Any idea if the Turnigy 9X radio is any good? I'm getting mixed results online.

A: Mark J. here: I don't recommend going cheap on electronics, and the Turnigy 9x (AKA SkyFly 9x) is a really cheap knock-off radio system. It doesn't even come with a manual. If you have never owned a 2.4 gHz radio, I certainly wouldn't recommend starting with this one. Features are limited, programming is awkward, and quality control is poor.

You're getting what you pay for, which in this case is very little. In fact, you get too little for a combat robot -- there are no failsafes. That makes it useless for robots with active weapons. Save your money and buy a quality radio.



Q: Hi Aaron, where can I get information related to RF circuits??

A: I don't know what level of information you're looking for, and this is not a topic where you're going to pick up useable skills from reading a couple of websites. Try a web search for "RF circuits" and browse the results to see what's available. I can recommend a good school for an EE degree if you like.



Q: Hi Aaron. I want to control my robot with a PS2 controller. Firstly I don't want it to be an RC(wireless) controlled robot...just a simple manual wired robot. Is it possible? If yes then how can I do it? Thanks!!

A: Yes, it can and has been done with either wired or wireless PS2 controllers. You need a microcontroller interface, which is not particularly simple. Lynxmotion has the parts and tutorials to help guide you thru the process.



Q: Dear Aaron I'm working on a antweight full body spinner for franklin institute 2011 and was wondering, how long does it take to charge a 9 volt 600mah rechargable liion pack using the smart charger from battery mart.com? I know that batteries or chargers with out critical specks makes it difficult to know what's going on but the price range is reasonable. It says the battery can handle a 550 mah charge rate and weighs only 1.28 0Z do you think i should get these batteries or find another site? Please advise. Anthony...AKA team warpz

A: Stay away from these, Antony. There are no specs given for either the battery or the charger. Buy a quality li-poly battery and charger -- this is not a place to scrimp.



Q: What is the most idiot-proof rechargable battery in terms of charging? I know the SLA is one of them, but what gets 2nd place?

A: Mark J. here: yes, Sealed Lead Acid (SLA) batteries are the most forgiving of the battery technologies available for combat robots. Our charging technique for the 'Hawker' brand SLAs used in 'Run Amok' and 'The Gap' was really simple: plug into constant-voltage power supply and walk away. It simply wasn't possible to over or under-charge them, and there was no possibility of damaging the batteries.

The next step down on the 'idiot proof' scale are the Nickel Cadmium (NiCad) and Nickel Metal Hydride (NiMHd) cells. A good quality constant amperage 'peak detector' smart charger will pretty much automate the charging process, although you do have to look out for 'false peaks' that can prematurely stop the charging. Buy a quality charger and you should have no trouble, even in a busy combat pit.

Another step down brings you to the newer Lithium Iron Phosphate (LiFePO4) cells, also known as 'A123' or 'M1'. Although less 'dangerous' than other lithium battery types, the A123 cells do require considerable care to perform well. Special 'cell balancing' equipment is needed to keep the battery packs competition ready, but relatively simple 'voltage cutoff' charging can be done between matches.



Q: What is an AM radio?

A: There are two widely used ways to encode information on a radio wave: 'amplitude modulation' (AM) is the simpler method that requires less complex and expensive equipment, but is more vulnerable to interference. R/C toys use AM radio systems. Hobby grade radios generally use the more complex 'frequency modulation' system to assure better control. Only very light combat robots without active weaponry are allowed to use AM radios.



Q: For how long do you think a switch that uses 20awg wire could handle ~35 amps? the length is about 6 inches.

A: Mark J. here: you can't directly judge the amperage capacity of a switch by the size of the wire attached to it -- but 20 gauge wire itself is rated for only 11 amps max. It is likely that any switch using such small wire would be rated for no more than 5 amps. Trying to push 35 amps thru such a switch will cause catastrophic failure very quickly.

Q: My mistake: it is more likely 19/18 awg. The switch is roughly the same size as this switch. It would just need to handle 35 amps for a half second in case the weapon motor stalls. It is a big weapon motor. What do other beetleweights use for a switch for similar levels of current?

A: Consider how bad you would feel when this switch failed and took you out of a tournament after all the work you put into building your robot and getting there. The switch in the picture is an R/C receiver switch, designed to handle less than an amp. DO NOT USE THIS SWITCH FOR YOUR WEAPON MOTOR!!

The preferred method for high current control in a small and light package is to make a 'removable link' from a suitable wire connector. See the diagram at right for proper use. We use Deans connectors for this purpose.

Reply: That idea never occurred to me. Thanks!



Q: Can the Dx6i dsmx spektrum transmitter shut off your system's gyro during a death spin?

A: No transmitter can 'shut off' a gyro unless the gyro has a dedicated second channel for sensitivity control. Pick a gyro with a remote sensitivity control and any radio system with a spare channel can turn off the gyro.

A 'death spin' happens when a gyro enhanced robot gets itself inverted. The 'left-right' controls don't change when the robot is inverted, but the gyro corrections are reversed! The robot will spin uncontrollably as the gyro keeps adding more and more 'correction' in the wrong direction. Unless you can shut down the gyro your only option is to turn off the transmitter and let the 'fail-safe' shut the whole robot down -- you're hosed!



Q: While my friend and I were setting up a new Lipo battery, there was a tiny, tiny, tiny short circuit. The spark was barely visable, there has been no problem for weeks, and the pack did not buldge or get warm a single bit. Is it safe to use?

A: Heat is the big enemy of LiPo batteries. A tiny spark with no heating or signs of swelling likely did no damage. Inspect after every use, as you should do with any LiPo battery, and you should be fine.

Q: How paranoid should I be about charging a Lipo? Keep in mind that my definition of paranoid is much more extreme than those of other people. For quick reference, what other people call "extremely careful" is what I would call "moderately concerned." Example: When I first heard that you should never charge a Lipo unattended, I thought they meant you had to sit and stare at it the whole time or else you risked Armageddon.

A: LiPo cells give ample warning of trouble. Use a quality charger intended for LiPo batteries, keep the battery within its discharge amperage rating, and inspect after use for signs of swelling. If you want to be extra careful, there are flameproof 'charging bags' available from hobby suppliers. Maybe you should keep your friend in one?



Q: Do most brushless escs failsafe upon loss of signal? The majority of the ones I'm looking at don't say anything about it.

A: Mark J. here: ideally you shouldn't rely on the ESC to decide on what to do on signal loss -- it should be handled by the receiver. If your receiver takes no decisive action and simply defaults to 'no signal' then the ESC should correctly shut down. To be certain, I would recommend that you select an ESC that specifies its action on signal loss.



Q: It is quite hard to drive my 2wd bot in straight line. Should i use gyro sensor to help me with this problem? FYI, i'm using Futaba 6EX 6-Channel 2.4GHz Radio System with R607FS Receiver and the Sabertooth 25Amp Dual Motor Controller for the ESC. What type of gyro would be compatible with my current system & can you help guide me on how it should be done?

A: A solid-state peizo gyro will greatly improve both straight-line and turning control on a two-wheeled robot. All of your questions about gyros have been previously answered in posts found in this archive - start reading! You will also be interested in the Team Run Amok gyro guide. Write back if you have specific questions not already answered.

Q: Hi Aaron, follow up on the gyro issue. I manage to find GWS GW/PG-02 Dual-Rate Piezo Gyro. and this stuff is just within my budget. The problem is i'm not really sure whether it will work with my system & honestly i'm not really sure about the wiring/connection. What i can tell you is that the ESC (Sabertooth) that i use allows channel mixing, and i use the left analog stick on my Futaba transmitter to control both Forward/Reverse & TurnLeft/TurnRight motions. I have browse through your gyro guide but didn't understand it quite well. Please help me further. Thanks!

A: OK, here are the high points...

  • All hobby radio systems output the same signal from the receiver, so all hobby gyros are compatible with all hobby receivers.

  • No gyros are compatible with transmitter-based radio mixing -- transmitter channel mixing must be turned off. You must use the mixing function of your speed controller or an on-board stand-alone mixer downstream from the gyro.

  • There is a gyro wiring diagram in this archive: search for "I want to build a two-wheeled robot with a gyro".

  • For steering and throttle on left stick, the gyro will plug into channel 4 output on your receiver and the Sabertooth 'channel 1' lead will plug into the gyro. The 'channel 2' Sabertooth lead will plug into the channel 3 output on the receiver.

  • Gyros are NOT INVERTIBLE. If your robot is invertible you must use a gyro that can be turned off via a spare R/C channel or the robot will spin uncontrollably when inverted. the GWS GW/PG-02 gyro does have a remote sensitivity adjustment, but the description of that adjustment in the manual does not describe it well enough for me to tell if it will work for inverted robotic operation.



Q: i would like to know what circuit board is being used and where do i get the boards found on the picture on the 2.007 Arduino Nano Carrier page. thanks

A: The 'Arduino Nano Carrier' board was a prototyping board developed for a class at MIT. It is not, AFAIK, commercially available. The 'Arduino Nano 3.0' board itself is available thru a link on the page you reference.

Additional questions about the Arduino microcontroller should be directed to the Arduino Forum -- it is not a component commonly used in combat robots.

Q: Do you have any diagram or info for the circuit board?

A: No. Please direct Arduino questions to the Arduino Forum.



Q: Why do some teams have a separate radio for their bot's weapon, [rather] than having it on one radio?

A: Mark J. here: in the BattleBots era many teams had a 'weapons officer' who operated the weapon via a small switchbox on a long, hard-wired 'tether' plugged into the transmitter. This freed the driver from worrying about the weapon status and kept his attention focused on maneuvering the robot.

Now that reliable radio systems are much less expensive, it's simpler and more convenient to give the weapons officer their own transmitter with a dedicated weapon receiver in the robot.



Q: Ok I have a sabertooth 2x5 for my antweight and an ar500 reciever with dx5e radio. When I plug the sabertooth into the reciever, no lights come on on the reciever [assume you mean ESC]. I know its not a faulty esc because it gives power to an am one I have. Do I have a faulty reciever or am I doing something wrong?

A: Lots of potential trouble points here. My first guess is that you haven't 'bound' the receiver to the transmitter. Follow the instructions in the AR500 user guide or on page 9 of the DX5e manual. Read the rest of the manual while you're at it.

Q: Hi, I cant bind the reciever to the transmitter because the light on the RECIEVER (there's supposed to be an amber one) won't light up. The Esc's light works. I think the ESC is giving it power. The amber light on the Reciever should be flashing but it's just staying off.

A: Check to make sure the receiver is being powered by the ESC -- it sounds as if there is no power to the receiver. If the receiver is getting power and you're getting no response from it, I think the receiver is fried.

Q: To check the power to the reciever can I use a voltmeter? If sowhere do I check. (which wires?)

A: Power the electrics up and unplug one of the ESC connectors from the receiver. Check the power at the connector plug with a voltmeter. The red(+) and brown(-) wires carry 5 volt power. Be extra careful to avoid shorting the power connections with the voltmeter probes.

Note that it is possible to insert the ESC plugs into the receiver upside down. This will do no damage, but the receiver will have no power. The correct insertion has the brown wire closest to the bottom edge of the AR500.



Q: I just finished competing with my 3lber. After one win and 1 close loss, my robot suddenly had very strange driving problems. As my team was preparing for our third match, the robot began to move very slowly. We did a voltage test, and found that the motors which should be recieving 12v were only recieving 8. The battery was fully charged, and the transmitter was set on "high." However, after turning auto-calibrate on, the problem went away.

Yet shortly into our third match, the robot suddenly lost reception for a bit and then started moving backwards. This cost us the match and the competition.

The robot used to have this problem, yet this has been the first major incident for a long time. All the motors have capacitors, and the 2.4ghz receiver has about 1/3rd of it's longer antenna completely outside of the aluminum armor.

Oh ya: The robot was an electric hammer, and I liked its performance.

More information can be provided if needed.

A: I don't think that more information is going to help. You don't mention what specific radio system you're using, but I'm going to guess that it's at fault. You can't afford to rely on a radio that won't hold settings and goes glitchy at a competition. Replace it, and make sure it's a quality radio.

Q: It is a spectrum 5, with an AR500 receiver.

A: Mark J. here: electronic trouble (especially intermittent radio trouble) is very tough to diagnose remotely. I think Aaron's suspicion is reasonable; your radio receiver and/or transmitter has gone glitchy and should be replaced.



Q: Does the [Spektrum] ar6200 [receiver] have a failsafe that meets the RFL regulations for a one pound robot?

A: RFL regulations do not require radio failsafes for one-pound robots without an active weapon. Antweights with an active weapon require a failsafe only on the weapon channel.

The Spektum AR6200 receiver has a suitable failsafe response only on channel 3. The other channels do something undesirable for a combat robot: they lock in the position they last had before signal loss. This behavior is not allowable for classes heavier than antweights, but does meet minimum RFL specs if the weapon is controlled by channel 3.

The event organizer has the final say on what is and is not allowed at the event, so best to check with them before you commit to this receiver.



Q: Hi Aaron, I'm building a antweight robot. Where does the crystals go?

A: Replaceable 'crystals' are used by some hobby grade transmitters and receivers to control the precise frequency upon which the radio commands are sent and received. You may remove the crystals and plug in crystals of a different frequency to switch the radio system to another 'channel' within the same 'band' to avoid interference from another radio on the same channel. Instructions for swapping out crystals will be in the user manual.

Newer 'spread spectrum' radios operating on 900 MHz or 2.4 GHz do not use replaceable crystals and will adjust their signal to avoid interference without any operator involvement.

Toy R/C systems do not have replaceable crystals. Toy radios in the US operate on either 27 MHz or 49 MHz and generally do not allow for channel change. The use of toy radio systems for antweights is limited to robots without active weapons and should be cleared with the event organizer prior to the event.



Q: Is the Spektrum AR6110 DSM2 Microlite a quality reciver for and antweight spinner?

A: I haven't personally used the Spektrum Microlight receiver, but the comments at Robot Marketplace are all quite positive, and Spektrum in general makes quality products.

Note that the receiver has non-standard failsafe response on all but the throttle channel -- make sure the weapon ESC you select will respond correctly to a 'no signal' input from the receiver and shut down the weapon motor.



Q: Is it leagle in the rfl to add on my own fail safe to a radio that doesn't have one?

A: The Robot Fighting League (RFL) regulations require radio systems that 'failsafe' by bringing the robot drivetrain and weapon to a stop on loss of radio signal for all robots weighing more than 12 pounds and for robots of any weight with active weapons. The regulations do allow for 'homebrew' radio failsafe systems that meet specific criteria:

  • Amplitude modulation (AM) radios are not allowed for control of robots that require a failsafe system, even if equipped with a failsafe device.

  • Any homebuilt or modified radio system must be approved in advance of the tournament by the event organizer.
See FAQ #18 for a table of specific RFL radio requirements by weight and weapon type. The event organizer always has the final say in these matters.



Q: Would it be possible to use the mixing on my Dx6i in a way that steering would be on the rudder channel as opposed to the usual aileron?

A: Mark J. here: you have a mode 2 DX6i transmitter and you want throttle on the right stick and steering on the left? No problem. Instead of using Elevon (Delta-Wing) mixing, use V-Tail. Your motor outputs will be channels 3 and 4 on the receiver. Consult your manual for specific instructions on setting up a wing-tail mix.

Here is the setup matrix for modes and mixes to put the controls on the sticks you want to use:

 

Left Stick
Steering

Right Stick
Steering

Left Stick
Throttle

Mode: 1
Mix: V-Tail
Mode: 1
Mix: Elevon

Right Stick
Throttle

Mode: 2
Mix: V-Tail
Mode: 2
Mix: Elevon

It is possible to change a DX6i transmitter from Mode 2 to Mode 1 thru a hidden menu. You can also swap the spring centering mechanism from the right stick to the left. Here's a tutorial.



Q: I have two electrical systems running in my 'bot. Would it be possible for them to share the same power switch by having their ground wire run through the switch?

A: Mark J. here: diagram this and you'll see the problem. It works only if the voltages of the two systems are identical. If there is a voltage difference there will be a reverse current flow thru the lower voltage circuit when the switch is 'off', likely destroying the electronics and the battery. Power diodes in the ground circuit could prevent this, but it isn't worth the trouble. Use a DPST switch or energize a relay with one circuit to switch the second circuit.



Q: hi first time. can i use a 100amp hobby wing platinum esc with cheap radio system like exceed t6a or fly sky model fs-ct6b i hooked up the esc and blew out both receivers

A: Mark J. here: there should be no compatibility issues between any hobby receiver and any hobby Electronic Speed Controller (ESC). In this case, I'm suspicious of a problem with the Battery Eliminator Circuit (BEC) in the Hobbywing ESC.

  1. You should have only one source feeding power to your receiver. If your main drive ESC also has a BEC you need to cut the power lead (the red wire in the three wire receiver lead) on any other components that may be feeding power to your receiver.

  2. The BEC output from the Hobbywing is programmable - it may be set incorrectly. If you're going to use the BEC on the Hobbywing, check the voltage output to make sure it's in range and stable. It's entirely possible that your 'bargain' ESC has a erratic voltage regulator that fried your two 'bargain' receivers.
I'm not a fan of 'cheap' when it comes to R/C electronics because of issues just like this. Next time, buy components that come with a user manual written in a language close enough to english that you can set up the ESC properly.



Q: Hello i am building an ant weight for fun so i dont want to put a ton of money in to it. I am making a simple wedge to start with and i would like to know if the GWS 2.4GHz 4-Channel GP Transmitter System from the robot market place would be a good place to start? Also i am am not getting in to the speed controllers yet i just want forwards and backwards. Thank for your help.

A: If you're building an ant I recommend going one of two ways:

  • Go really cheap and pull the electronics and drive motors out of an R/C toy - see the Toy Hacks & Kits archive for more info; or

  • Buy quality electronics that won't have to be replaced immeditely when you upgrade your robot and will retain good value if you decide to get out of the sport - search thru this archive for recommendations on this option.
Going the middle road with cheap component R/C gear has many problems. You end up spending a fair amount of money and getting a collection of parts that doesn't really meet your needs and that no one else will want when you're done. The GWS GP transmitter is a very basic radio with extremely limited capacity. Although it does comply with the RFL minimum radio requirements for an antweight wedge, it does not meet the failsafe requirement for heavier robots with active weapons. I don't think you'd be happy with it and I don't consider it to be an acceptable radio for a combat robot.

As to just going forwards and backwards, you'll still need an interface to interpret the output from the receiver and send power to the motors. The R/C relay interfaces needed to do that are both heavier and more expensive than small speed controllers. There are small, light, inexpensive R/C switches, but they only provide on/off control - no reversing. You'll be much better off to go with full function speed controllers.

I suppose you could modify a pair of R/C servos for continuous rotation. Servos have a very small speed controller built into their internal controller board. It's an inexpensive approach that gives proportional forward and reverse control and simply plugs directly into your receiver - but servos are very slow compared to current antweights or hacked toys. I haven't seen a servo powered ant in many years.

Q: hey what would be a good toy to hack for an antweight?

A: There are several hackable toys discussed in the Toy Hacks & Kits archive. The Johnny Lightning 'Battle Wheels' R/C toy is widely available and could be a good bet.



Q: I recently upgraded my robot from an competition-unworthy, dirt cheap GWS transmitter to a nice Spektrum one. Unfortunately, when I tried to plug in my Pg-03 gyro into my new reciever, I found that the plug had a special notch that prevented it from fitting.

When I looked it up, the gyro did not mention it was compatible with Spektrum. Is this true? What would happen if I filed off the groove on the plug and put it into the receiver? If it wouldn't work, where can I find a spektrum compatible gyro?

A: The output signal from all modern R/C receivers is the same, but there are differences in the plugs and wire colors. Some PG-03s came with Hitec 'S' connectors with orang/red/brown wires, and some came with the Futaba 'J' style plug with white/red/black wires.

Your PG-03 has the 'J' plug with the little tab that keeps it from being inserted upside-down into a receiver socket that has a groove for the tab to fit into. See the Servo City Connector Types page for a full explanation. Just shave off that tab and you'll be fine. If it doesn't work, you've plugged it in upside down.



Q: Hi Aaron, I have been using the auxiliary channel on an Ant 150 esc to power a small pneumatic solenoid on my antweight flipper at 7.4 volts, but the dimensions of the speed controller are too bulky. I have been looking for a compact and light on/off/reverse switch that plugs into my spektrum reciever and can control my solenoid. So the question is: where could I find one?

I don't want to use the 1 amp Picoswitch from the RobotMarketPlace because it requires an antiparallel [flyback] diode, while the 10 amp Battleswitch is too heavy and overkill since the valve probably uses around 1 amp at most. The Pololu switches are a possibility but I am not experienced enough in electronics to understand the description well. I have realized that I could use a Fingertech tiny esc to control my valve as it is both compact and lighweight, using the switches on my DX6 instead of the control sticks for the on/reverse effect. What is your suggestion? Thank you.

A: Mark J. here: hold on... the auxiliary channel on your Ant 150 had no reverse, so why do you now need on / off / reverse for your solenoid?

  • Circuit diagram showing placement of a flyback diode.Adding a flyback diode to the output of the Picoswitch is trivial, so if it will otherwise meet your needs you can certainly use it. The Picoswitch is a simple on/off switch - no reversing capability.

  • Team Run Amok has used the RCE200 R/C switch to control pneumatic solenoids with excellent results. It is larger than the Picoswitch, but requires no external diode and has a few extra bells and whistles. It is also an on/off switch with no reversing.

  • The Pololu R/C switch with MOSFET is inexpensive, very compact, and has the capacity to control a solenoid. It does require a bit of knowledge, soldering, and puzzling out to hook up correctly. Very possible to goof up. Again, a simple on/off switch.
If you really do need reversing, the Fingertech tinyESC could be a fine solution.



Q: I have a question about servo motors. How can they be programmed only to turn in specific increments (such as 2 or 3 degrees) instead of its full rotation? What happens when a servo motor is turning but runs into a strong force and stalls? Would it eventually break? Could I solve my "limited rotation" problem by simply stalling it? These are small servo motors, the kinds you would find in a model airplane or helicopter.

A: It sounds like you aren't controlling these servos with standard hobby radio gear - true?

A standard hobby servo responds to 'pulses' sent along its signal wire to it from the radio receiver (or a device emulating a receiver). The position of the servo depends on the length of the pulse. With a hobby grade radio system it is possible to rotate the servo fast or slow as many degrees as you like. The servo will then attempt to hold its assigned position against external forces. You can find an explanation of the pulse code system at the Seattle Robotics Society servo page.

A servo should not be stalled. The motor draws large current when stalled and the heat build-up from this current will damage the motor rather quickly.

More on servo control



Q: Hey Aaron, I have a few questions. After seeing quite a few builders switch to the safer A123 batteries, I am looking to do the same. For that switch I need to get the astroflight 109d as the charger (maybe not need) anyways on the astroflight website it says that the charger requires a 12-15 Volt power supply, and .1 to 16 Amps input. My question is, I have a 12V 8.3A power supply that I have used for a smaller charger, would it supply enough amps to power the astroflight charger? Thanks!
Best Regards
Daniel

A: Mark J. here: general note - the new LiFePO4 and A123 cells require a different charge cycle with a lower cutoff voltage than other lithium cell types. Do not attempt to charge these cells with a charger designed for Li-Poly or Li-Ion chemistry.

The AstroFlight 109d charger requires the 555 Software upgrade chip in order to be useable with the A123 cells. The upgrade chip will convert a 109 or 110 Deluxe charger to charge only A123 cells -- you will lose the capacity to charge other cell types. The Robot Marketplace sells a 109 with the upgrade chip installed, but there are less expensive A123 chargers that can switch between different cell types (example) including some that do not require an external power supply (example). I'd suggest looking into alternative chargers to see if they meet your needs before buying an upgraded 109.

Back to your question: the AstroFlight 109 requires the full specified DC power input in order to provide the full specified voltage and amperage output. If you will not be pushing the charger to these limits, a DC power supply with less capacity may be used. You didn't tell me how large a battery pack you plan to charge so I can't comment directly on the suitability of your 8.3 amp power supply.

Q: Thanks for the A123 help! and about my power supply, I will mostly be charging 2 and 4 cell A123s so I belive it should work. I called Battlepacks and they were quite helpfull. On their website they sell an Astroflight 109, and they probably already installed the chip as well. As far as balancers, they have an A123 balancer as well. And seeing so many builders use the Astroflight I figure it might be a good investment. Thanks You for all your help!
Daniel

A: You're welcome, Daniel.

I suspect that a lot of builders use the AstroFlight 109 for their A123 batteries because they had a 109 to charge their LiPolys and they were able to convert for the small price of the upgrade chip. I like AstroFlight chargers for their reliability and simplicity of use, but if you don't already have a 109 it's expensive for the limited functionality you get. A lot of the R/C aircraft guys seem to be using a Thunder Power charger that is very flexible and includes a cell balancer.



Q: Hi Aaron, I've decided to graduate from running 8 Custom series Battlebot toys to the real deal, but I have some questions for you.

1) When buying a transmitter and receiver, do they have to be from the same company? the same # of channels? Or does simply having matching frequency crystals make them work together?

A: Mark J here: the currently favored 2.4 GHz radio systems do not have crystals. The transmitter and receiver 'talk' back and forth and 'agree' on a frequency that is free and uncluttered. This DSM system is still evolving and has some significant differences between manufacturers and even between current and earlier radios from the same manufacturer. The number of channels does not have to match, but check the documentation carefully for compatability with your chosen receiver. Stick with the same manufacturer.

If you are considering an 'old school' FM radio system, be advised that some combat robot tournaments no longer allow their use. There are two 'flavors' of FM signal used by different manufacturers: positive shift and negative shift. A receiver designed for one shift type will not recognize the other signal regardless of crystal frequency compatability. HiTec and Futaba use negative shift, while Airtronics and JR use positive shift. Some third-party receivers will work with either type of shift -- check their documentation for compatability. Again, the number of channels does not have to be an exact match, although there are some differences in the signal from two or three channel 'pistol grip' transmitters that may cause incompatability in receivers with four or more channels.

Check FAQ #18 for minimum radio requirements for specific weight classes and weapon type. Again, some events may have higher requirements. Check the rule set of specific events you plan to enter. I advise picking out a nice 2.4 GHz system that will be accepted anywhere in any class.

2) Once I get my servos spinning, how do I translate that into larger motors driving the robot? Thanks again, -Russell

A: Inside your servo is a very small electronic speed controller (ESC) that translates the signal from the radio receiver into variable power and polarity to control the servo motor. To control a larger motor you need a larger ESC. See FAQ #21 for help in selecting drive motors and matching ESCs. FAQ #19 shows how everything gets wired up.



Q: Is it possible to program how far a servo can rotate? That is, make it only rotate 45 degrees instead of 180 degrees?

A: Sure. Your computerized transmitter's 'Adjustable Throttle Volume' (ATV) function can set the amount of servo travel on any channel, and even do it differently on each side of the 'neutral' position. See our Futaba transmitter programming guide for more info.

Q: Can you also do the ATV thing with the 5 Channel Spektrum DSM2 2.4ghz?

A: Spektrum calls it 'Travel Adjust'. I believe it's available on every radio they currently make - except the budget 5-channel DX5e: you only get the basics for $60.

Q: Thanks. In addition... damn. That was the one I wanted to buy.

A: I know it's tempting to go for the 'bargain' radio package, but the additional functions on a 'full featured' radio can tame an unruly robot and turn a sitting duck into a tiger. You'd outgrow that DX5e quickly, so spend the extra dollars now and save the expense of a future upgrade.



Q: Hi, Aaron. I own a Vision Peak Ultra battery charger that still has a 5-year warranty on it, but sadly the battery connector is broken beyond repair. Is there anyway I can contact the Robot Marketplace for a new connector or do I have to contact the Dynamite company themselves?

A: Obtaining warranty service is covered in the manual. The charger has a 5 year guarantee "against workmanship and manufacturing defects" thru the manufacturer. You can call their tech number and ask, but I wouldn't think that a crushed connector would be covered.

Either way, a new Tamiya connector is about $2 at your local hobby shop. That's cheaper than shipping the unit in under warranty -- replace it yourself.



Q: I've been told that a PWM motor controller can provide a motor current of approximately twice the current that it is pulling from the battery- something about inductance and circulating currents. Do you know anything about this? I didn't get much beyond Ohm's law, and it has me a little worried about sizing wires and connectors- afterall, power is (I squared times R), so it should make a big difference for heating and melting things!

A: Mark J here: pure technical fluffery. The condition you describe only occurs at partial throttle where the controller output voltage is well below the battery level and the current flow is a fraction of the full-power maximum. The peak amperage for which you must design is never higher than the motor draw at the maximum load it will see in operation. The controller cannot stuff more amps thru the circuit than the motor can draw, nor can it supply more total wattage than it draws from the battery.

Consult the Team Tentacle Torque & Amp-Hour Calculator to calculate the expected maximum operating amperage draw for your drive motors.



Q: I am building a robot for my job at the NASA Glenn Research Center. I am using two 24v electric bike motors (attached to each freewheel via bike chain), the Sabertooth 2x25 ESC, a 24v NiMH battery, and a simple 2-channel remote. Once I hooked everything up and tested it, the motors lagged and were very jumpy (however they did seem to react in the correct manner). Also, the further motor from my ESC/Battery sometimes didnt respond at all. I know my ESC and motors are working properly. Could this be a mix in signals or a bad transmitter/receiver?

A: Hey, how come I didn't get hired for that job?

Faulty radios are uncommon. What is common is to hook everything up and test it without first fully charging the battery pack. A low battery charge will cause symptoms exactly as you describe. With a low battery, the red 'error' LED on the Sabertooth will flash as the motors sputter and draw the voltage below required levels for operation of the electronics. Give the battery a full charge before proceeding.

The next likely culprit is that your Sabertooth ESC is not set-up correctly for R/C control. You say you know it's working properly, but take a minute to step thru the Sabertooth DIP switch wizard and correct any errors in the switch settings. You most likely want:

    Sabertooth DIP switches
  • 'Other' battery
  • Radio control
  • Radio and receiver
  • Independent mode (for tank steering)
  • Exponential response
 
This will require DIP switches 2, 3, and 6 'on' and 1, 4, and 5 'off' (as pictured above). If you have a pistol-grip transmitter or otherwise want 'differential drive' steering, turn on switch 4.

With the DIP switches set correctly, block the wheels up off the ground and power up the robot - leave the transmitter off. The Sabertooth should display a single dimly glowing blue LED labeled 'status1'. Turn on the transmitter. The 'status1' light should glow brighter to indicate that a control signal is being received. Exercise the transmitter controls and watch the motor response and Sabertooth indicator LEDs. The single blue 'status 1' light should remain brightly lit and no other lights should come on while the motors respond to forward/reverse commands.

If still no joy write back. Report your findings and give me your email address - we can step thru isolating individual components to find the trouble.

[No follow-on received. I'll bet it was the battery.]



Q: How do you figure out the correct charge rate for a given battery - as in, how many amps to set the charger up to? I am working with a NiMh atm, but it would be nice to know for the other types as well...

A: There are different grades of each battery chemistry type and the maximum charge rate varies with both the chemistry type and the manufacturer. Wherever possible, find and follow the manufacturer's specs for charge rate. This is particularly vital for batteries using lithium based chemistry -- don't fool around with them, they can catch fire if improperly charged!

That said, as a very general rule NiMHd cells should be charged at a rate no greater than '1C' - the rate needed to recharge the fully discharged cell in one hour. For example, a battery made from 4000 mAh NiMHd cells should be charged at no more than 4 amps (4000 mA). Small cells (like AA or AAA size) typically require lower charge rates (try 1/2 C). Monitor the temperature of the battery during the first few charges. Too high a charge rate will make the cells very hot, especially near the end of the charge. Lower the charge rate if the cells get hot.

Q: I can't find a battery charger for Nimh's that is low enough to charge my beetleweight's 400 milliamp 7.2 volt nimh battery pack. what should I do?

A: You should look harder. There are multiple NiCad/NiMh chargers at the Robot Marketplace that have adjustable outputs suitable for your small pack.

Q: Oh ya. Battery guy here again. I ordered my battery using the robot marketplace's custom antweight pack section. Who exactly would I contact for charging information?

A: My 'manufacturer' I mean the company that builds the cells, not the guys who put them together into a pack. In your case, that would be GP batteries: see their Data Sheet for GP40AAAM cells. They recommend 200 to 400 mA for a fast charge.



Q: I just put a gyro in my robot, but it is acting a bit odd. When the robot gets tilted upwards, the wheels jerk in an odd way. Also, it is hard to really tell if it is having an effect on the driving.

My guess is that I didn't install the gyro correctly. What is the proper way of aligning and installing a gyro?

A: Mark J here: I'd agree that you have installed the gyro incorrectly. The manual that comes with your gyro should refer to a 'corrective axis' or 'gyro axis of rotation' and diagram how the gyro should be installed relative to that axis. In a combat robot the corrective axis is a line running from the center of the chassis vertically straight up to the sky -- same as in a helicopter, which is likely diagramed. This often means mounting the gyro case 'standing on end' rather than flat on the chassis. It sounds like you currently have the gyro axis of rotation oriented along an axis running thru the robot from left to right.

When properly aligned, an outside force pushing the nose of the robot to one side will result in the robot self-activating the drive system and returning the nose to the original orientation. As noted in the Run Amok gyro guide, a solid-state gyro cannot be used with receiver mixing. You must go 'tank style' control or use an on-board mixer.

Write back if this doesn't clear up you problem.



Q: Could a big, beefy capacitor be used to "jump start" a weapon motor? The idea is somewhat like a buffer tank in pneumatic bots. When the weapon is at rest, the robot is charging up the capacitor. When the weapon motor is fired, the capacitor discharges into the motor, giving it a huge but brief overvolt.

In theory, this should be enough to dramatically increase the motor's power for a short period, yet be short enough to not fry the motor. (I know from experience that 3 volt motors can survive 150v discharges) That could decrease a spinner's spinup time, make an electric hammer more destructive, give some electric lifters more speed, etc.

A: Mark J here: if you directly charge a capacitor from a 12 volt battery you'll get a capacitor charged to 12 volts - no more. The voltage marked on the side of a capacitor is its maximum operating voltage, not a voltage to which the capacitor will magically charge. The charged capacitor could deliver additional amperage on weapon motor start-up, but no additional voltage. If your battery is adequate for the task you'd see no appreciable performance difference. If your battery in inadequate you'd do better to use the weight of the capacitor toward a larger battery.

It is possible to use a voltage multiplier circuit to charge the capacitor to a voltage higher than the battery supply. You would need to isolate the capacitor from the main weapon circuit, charge it, then switch it in just as the weapon is fired. This is difficult, dangerous, may require advance approval under RFL rules (section 6.2), and runs the risk of frying not just the motor but the battery and any electronics in the circuit. Even if you did all that I believe the results would be negligible as the additional true power (watts) delivered to the motor would be small.

Don't get fancy. If you want more power use a larger motor and/or boost the battery voltage and cross your fingers.



Q: Hi Aaron. I have a question that's been bugging me for a while. Would it be legal for a robot to have a weapon not controlled by the radio, e.g. a flipper controlled by a switch that is pressed by the other robot? This is particularly relevent to me because I only have a two-channel radio.

A: The short answer is 'no' -- not with a two-channel radio.

You can have an active weapon that fires autonomously, but the weapon system must de-activate and become inert on radio signal loss. Section 4.4.1 of the RFL ruleset says:

Radio systems that stop all motion in the robot (drive and weapons), when the transmitter loses power or signal, are required for all robots with active weapons or any robot over 12lbs.

I can't think of a way to accomplish that without a dedicated, failsafe weapon radio channel.



Q: What radio do you recommend for a lightweight horizontal spinner, or which ones are most commonly used? I need 1 channel per motor (two motors) one for the spinner and one for the killswitch.

A: A lighweight with a spinner requires a coded FM or digital 900 MHz/2.4 GHz radio with failsafes on weapon and drive channels. The most popular radio for your needs is the Spektrum DX6i for its full set of features and low price. You'll probably want to select the Spektrum AR500 receiver to go with it.

Note: there is no R/C 'killswitch' on a modern combat robot. The current rules require the radio system itself to 'failsafe' to zero throttle and weapon power on loss of radio signal. The Spektrum DX6i conforms to this requirement.



Q: This does not concern combat robotics, but it is similar. I am attempting to build a small 3 Channel RC plane, a model of an F-22 Raptor to be exact. I am just confused on the servo set up. This image explanins what I would like to do. How do I set up the servos so that if I press up my right DX5e stick (elevator) bot servos move up and down so they are both either up or down, but yet how do I set them up to diffirentiate so that I can turn? Not sure if this is the right place to ask this, but thanks anyway!

A: Combat robots have the same need to combine R/C control signals so that the input from two sticks control two output devices in different ways. It is called 'mixing'.

Computerized R/C transmitters generally come with mixing options built in. You're looking for 'elevon mixing' (although technically you have a 'taileron'). Check your R/C manual to see if you have this option and how to set it up. If your radio does not have built in mixing, you can add an inexpensive stand-alone mixer between the receiver and the servos.

Flying a plane with this type of two servo set-up is not particularly easy; you only have elevator and aileron control, no rudder. To turn you have to roll the plane to one side with the ailerions, then apply elevator to 'climb' the plane into the turn. Best luck.

Thank you very much!



Q: I'd like to run 3 robots off of one 6 channel Futaba transmitter. Not at the same time of course. What's the best way to install a remote kill switch on each bot so I can turn on/off each robot in order to control each bot with Channels 1 and 2? Thanks for the help!

kill switch diagram A: Mark J here: I'd suggest a Team Delta RCE220 Dual-Ended Switch in each robot. Hook each into channel 3 (throttle) on their respective receivers.

  • Wire ESC power for robot 'A' thru the 'normally open' terminals on relay X
  • Wire ESC power for robot 'B' in series thru the 'normally closed' terminals on both relays X and Y
  • Wire ESC power for robot 'C' thru the 'normally open' terminals on relay Y

  • With channel 3 pulled 'down' - relay X will be active and only robot 'A' will have power.
  • With channel 3 'centered' - both relays will be inactive and only robot 'B' will have power.
  • With channel 3 pushed 'up' - relay Y will be active and only robot 'C' will have power'
This assumes that your ESCs can be powered up and down without any attention to startup sequencing, and that the robots pull no more than 12 amps each (capacity of the RCE220). With proper threshold settings the RCE220s in robots 'A' and 'C' could be replaced by single-relay RCE210s, but the explanation is simpler if I assume all RCE220s. Have a 'power light' visible on each robot so you can tell which one has power at any given time.

NOTE: if you're willing to take a couple of seconds to switch between model memories to switch robot control I think this can all be done with transmitter programming. No 'kill switches' needed and all ESCs would remain powered at all times. It's pretty sticky programming and I'll need to test it before I pass it on. Let me know if you're interested (and tell me which Futaba transmitter you have).



Q: Can I feed the receiver [output] of my [Spektrum] DX5E [radio] into a basic stamp [microcontroller] and with proper programing get more than the five channels the radio works with?

A: How many more channels do you need? You aren't going to get more simultaneous independent proportional control channels by post-processing the receiver signal. It is possible to 'piggyback' additional on/off control signals onto the existing channels (see VANTEC Hitchiker KeyKoder) but this requires extensive modification to the transmitter as well as the receiver.

It is also possible to split a single proportional control channel to provide simple on-off control two (or more) devices (see: Team Delta R/C Dual-Ended Switch) but control of the two devices will not be simultaneous. Something similar to this could be done with microcontroller receiver post-processing.



Two battery packs in series. Q: I need to make a battery series using two 7.4 batteries. I need to be able to have two plugs so I can plug the two batteries in. I need a diagram please. I can solder.

A: I think I understand. You want to be able to plug two 7.4 volt batteries into your 'bot to produce 14.8 volts, right? I'm guessing you want to keep the batteries separate because your charger won't handle a 4-cell battery. Diagram at right.

Q: If you have two 7.4 volt 800 mAh batteries hooked up to make 14.8 volts, would that battery series hold 1600 mAh?

A: No. Two 7.4 volt 800 mAh batteries in series will give 14.8 volts with an 800 mAh capacity. Two 7.4 volt 800 mAh batteries in parallel will give 7.4 volts with a 1600 mAh capacity. You can get either double the voltage or double the capacity, not both.



Q: I am having a hard time deciding which battery to purchase. I'm building a 3lb beetle with 2 Bane Bots 28mm 16:1 brushed motors geared down 1.5:1 using a 15A Esc. The Team Tentacle Torque calculations:

  • Total Peak Amps: 4.56 Amps
  • Amp Hours Required - 5 Min: 0.266 AH

I'm using an Axi 2808/24 running off of a 30A Esc for the active spinner, rotating a steel bar with dimensions 10" x 1" x 1/4". I'm planning on running on 7.2v giving 8568 RPM. With 20 spin ups I'm getting 0.16 [AH from the Run Amok spinner spreadsheet]. So total need 0.266 + 0.16 = 0.426 [AH]

Now I'm entirely clueless on how 'Continuous Discharge' relates to 'Max Continuous Current'. I thought the E-Flite 1500mAh 7.4V Double Cell 2S LiPoly Pack, 13g would cover my beetles needs but it's only a guess. Do you have any ideas on which battery would efficiently handle the robots power needs, and more importantly how you calculated which one was effective?

A: Mark J. here: you've done a great job using the appropriate tools -- the Team Tentacle Torque & Amp-Hour Calculator and the Team Run Amok Spinner Excel spreadsheet -- and you've come to a spot where you're unsure about how to proceed. That's what we're here for!

First, let's clear up the 'Continuous Discharge' and 'Max Continuous Current' confusion using that 1500 mAh E-Flite LiPoly pack you mentioned. Continuous discharge for that pack is '20C'. The 'C' relates back to the capacity of the pack -- 1500 mAh. That pack can discharge at a maximum continuous rate of twenty times 1500 milliamps = 30 amps = maximum continuous current.

There are two main considerations in selecting a LiPoly battery pack: capacity and discharge rate. Capacity measured in milliamp hours (mAh) is needed to provide enough power to get thru a match. The discharge rate measured in amps must exceed the current used by the robot in order to protect the battery from damage caused by discharge heat. LiPoly batteries are very sensitive to heat damage.

  • Capacity: if your calculations are correct you only need 426 mAh to power your 'bot for a 5 minute match. I like to have a comfortable buffer of battery capacity above the calculated requirement, so anything over about 600 mAh would cover this requirement with ample spare capacity.

  • Discharge rate: this is a little tougher to estimate. A few seconds of current averaging 1.5 times maximum continuous current won't do harm to the battery, so you do have a little room for error.

    • For the drive, the Tentacle calculator says the peak amp requirement is no greater than 4.56 amps at maximum 'push'.

    • For the weapon, I use the maximum amperage rating of the weapon motor divided by 1.5 as an estimate of the needed continuous current. The Axi 2808/24 has a max amperage rating of 22 amps. The Axi can draw much more than that - over 60 amps at stall - but a properly designed weapon drive will not allow the weapon motor to stall.

Rounding up, the total continuous current requirement is 5 amps for the drive and (22 / 1.5) = 15 amps for the weapon, totaling 20 amps.

The 1250 mAh E-Flight pack should cover your power needs with a good margin to spare. It has almost three times the power capacity you need, and the 25 amp continuous current rating exceeds the estimated requirement of 20 amps. Just don't bog down the weapon motor!



Q: Do you know if you could overvolt the Hitec HS-985MG servo to 7.2 volts? I am running it directly off the reciever, and I want more speed. If I cannot overvolt this servo, what is a way to bring my battery down to 6 volts for the servo? Space is really tight. Thanks.

A: Mark J here: the main problem with overvolting a servo is the small ESC that controls the servo motor. When you raise the voltage you also raise the amperage the servo motor will draw, and that can fry the ESC when the servo is heavily loaded. I don't know how heavily you load the servo, but I do know that if I tell you to go ahead at 7.2 volts and the servo smokes you're gonna be ticked off with me -- so no, I can't say it's safe to overvolt it.

If you're running the receiver from a battery eliminator circuit (BEC) in your electronic speed controller, it's already regulated to 5 volts -- but a high-torque servo draws a fair amount of current and may overload the BEC. If the receiver is running direct from the battery, dropping the voltage correctly requires knowledge of the amps the servos draw under load, and I don't know what that number is for your application.

I guess my recommendation is to either find another way to get more speed or just roll the dice with the servo.

Q: Hi Mark, about the servo, could I coil up a bunch of the red wire leading to the electrical servo lead to lose some voltage? How long would I need it to get around 6 volts, when my initial battery charge is 8.4 volts? Thanks.

A: This is going to take a little explaining, but I would be lax to deprive you of a complete answer. Pull up a chair...

Adding impedance does not directly control voltage, it controls current. You will get a voltage drop from extra wire length, but the amount of drop will fluctuate with the relative values of your added 'constant' impedance and the highly variable impedance of the servo as its load changes -- see Wikipedia article on 'voltage divider' for the math. If you have enough impedance to reduce the voltage to 6 volts at low servo loading, you're going to get so large a voltage drop when the servo motor is heavily loaded that the electronics will stop functioning! No, adding impedance in an attempt to hold voltage constant is a poor approach.

There are ways to directly reduce voltage but, as pointed out previously, dropping the voltage correctly requires knowledge of the amps the servo draws under load. The venerable 7805 voltage regulator chip can provide a stable 5 volts at up to 1 amp from a DC power source as high as 35 volts. The problem: your high torque servo will most certainly pull more than 1 amp if asked to supply full torque. You could verify this with an amp meter.

Another possible solution: a forward-biased silicon diode will drop the voltage flowing thru it by about 0.65 volts. Place several of these in series and you can get the voltage drop you need -- but the diodes must be capable of flowing the current you require for the servo. Again, we don't know what that amperage load is, only that it is fairly large.

All this brings us to one simple solution that does not require knowledge of the amperage draw: place a 'tap wire' into your battery pack, a couple cells short of the last cell in the pack. This tap wire will draw 6 volts from the first 5 cells in the pack to power the receiver and servo, while the full pack powers your drive motors. No calculations, no semiconductors to overload, just a clean 6 volts.

Don't worry about the actual 'peak' battery voltage. When a servo says it has a max voltage of 6 volts they are allowing for a little higher voltage from a freshly charged 5-cell NiCad/NiMHd pack.

Battery pack with tap wire



Q: How do you control a pnuematic ram's movement remotely? What do you use to remotely open and close the valves that control direction?

A: Combat robots typically use solenoid-operated air control valves that open and close electricaly, with an R/C switch interface to allow remote operation. The Team Da Vinci Understanding Pneumatics page gives a good overview of combat robot pneumatics.



Q: I'm planning on building a Beetle class robot with an open source microcontroller at its core. It's going to be using an arduino micro controller as the brains and an xbee added on to communicate with the xbox controller Im going to be using for control. Hurray for HCI experimentation! The arduino has pwm ports and i2c capabilities.

I'd like to know if you recommend a dual motor h-bridge that can use i2c or should I just hook escs to the pwm ports? Im not entirely sure what the difference is between an esc and h-bridge, other than an esc has safety mechanisms. Being a 3 pounder Im trying to watch my weight.

Im planning on 2 drive motors that put out about 5-6mph on maybe 6" wheels, maybe the [BaneBots] 28mm Planetary Gearmotor, RS-385 Motor (not sure the right ratio but Im sure the calculator you provided will. Which is fantastically helpful btw!) Maybe around 20 Amp stall amperage?

A: Mark J. here: I'm a fan of the educational and recreational aspects of building your own electronics, but I have to point out that there is precious little advantage to doing so when building a combat robot. I've seen many homebrew radio systems and speed contollers, and there wasn't one of them that I liked as much (or that performed as well) as standard off-the-shelf products. A combat arena is a hostile and expensive place to find the weaknesses of your electronics design.

First, check with the event organizers of tournaments in which you expect to compete to see if they are even willing to consider allowing a homebrew system. There will be concerns about interference and failsafe requirements so be prepaired for a lot of questions, skeptical frowns, and extra scrutiny from the tech inspector. Note that failsafe requirements are different if you plan on an 'active' weapon - see FAQ #18. Be aware that event organizers may overrule the RFL radio recommendations.

Let's sort out the difference between an H-bridge and an ESC:

  • An H-bridge allows three control states: full forward, full reverse, and off. Attempting to control a quick robot in a small arena with a H-bridge is not recommended!

  • An Electronic Speed Controller (ESC) allows full range control over speed from full forward thru full reverse. Note that model aircraft ESCs don't have reverse 'cause airplanes don't back up.

  • Some ESCs have failsafe mechanisms and some don't. Failsafe electronics are usually built into the radio system - as are useful features like variable exponential response, adjustable travel volume, custom rate channel mixing, dual rates, and position trims. Best luck in emulating those features on a Xbox controller.

The Team Tentacle Torque & Amp-Hour Calculator will give you a maximum expected amperage draw based on your motors, voltage, gearing, weight, and wheel diameter. You only have to worry about stall amperage if the motors actually stall, which a properly designed drivetrain should not allow. There are multiple posts on ESC selection in the Motors & Controllers archive.

A pair of BaneBots 36mm 20:1 RS-385 gearmotors at 7.2 volts in a beetle spinning 3" wheels will give a top speed near 7 MPH in a small arena and will break the wheels free in a dead push at less than 3 amps each. If you're tight on weight, the BaneBots 24mm 20:1 RS-370 gearmotors weigh 35% less (3.3 oz. vs. 5.1 oz. each) and will provide comparable performance. A five amp dual-channel ESC should do fine for such an application and can weigh well less than one ounce.



Q: I check in with your site frequently - really value your low key, common sense approach to the sport (activity? hobby? giant money hole?, whatever). This time I am interested in your advice about LiPo fires.

My team travels with a steel pot with lid, welding gloves (to the elbow style) & a small (3 lb) ABC fire extinguisher. Assuming safe handling/ charging practices (OK some would say LiPo & combat is inherently unsafe, but that to one side for the moment), are we correctly equipped in case of a fire?

A: Mark J. here: thanks for the kind words! As chief battery officer for the team, I'll take this one.

You're much better equipped to handle a LiPoly battery fire than most teams. Manufacturers recommend charging LiPoly batteries in a fireproof container with an ABC dry chemical fire extinguisher handy. You have that covered, and the welding gloves could certainly be useful. I'd be just a little worried about the lid blowing off your steel pot -- LiPo ignition can be fairly violent.

You are right to treat LiPoly batteries with respect. An improperly charged, damaged, or too-rapidly discharged (shorted) LiPoly can burst into flame. YouTube has plenty of videos of this happening. Typically there is a ball or jet of smoke and flame from the ruptured pack which may propel the battery some distance, so just placing the battery on a fireproof surface won't do -- it needs to be contained.

If you do get a fire, use a dry chemical extinguisher or just back away if it can burn safely. There may be multiple flame-ups as individual cells ignite, so it's best to stay back and deal with it from a distance. Do not use water on a LiPoly fire! Sand will smother the fire effectively if a dry extinguisher isn't available.

If you're tired of hauling around that steel pot, there are specially designed bags for charging and storage of LiPoly batteries. They're easy to pack, are less likely to short out your charger if something goes 'poof', and the lid won't blow off.

Always charge your Lithium battery with a correct lithium battery charger, never use a 'puffy' battery (indicates damage and gas release), and do observe safety protocols. Read thru the Great Planes LiPoly manual for safe handling practices.



Q: hi aaron. where can i get a good quality remote control system? upto how much current rating it can work satisfactorily?

A: Read thru this archive for suggestions on suitable combat robot radio systems.

The Electronic Speed Controller (ESC) determines the current your electrical system can handle, and the ESC is a separate item from the R/C radio. Any hobby radio system can be used to control any ESC that has a standard R/C interface. The capacity of available ESCs ranges from 2 amps to well over 300 amps. Information on ESC selection is also in this archive.



Q: Am I able to hook up a rc motor to a radio receiver without a speed controller?

A: No. The reciever receives the radio signal and translates it into a coded series of low power pulses. The ESC interprets these pulses into speed and direction information and supplies high current to the motor. You need both pieces!



Q: I am building a ant weight using the sabertooth 5 dual esc for my motor controller and a brushless esc for my weapon, what transmitter and receiver do you suggest I use?

A: We've said before that no matter what type of robot you're building it makes very little sense to purchase a 'cheap' radio system. If you continue to build robots you will soon want to upgrade your radio and will find that the resale value of that cheap radio is small. Buy a good radio system and your robot will benefit from the improved performance and your wallet will benefit from better resale if you do need to sell.

I'd recommend a full-featured DSM 2.4 Ghz system with a display screen and full failsafes. Such a radio meets all performance requirements for all weight classes. The popular Spektrum DX6i transmitter with BR6000 receiver is an example.



Reader comment: I bought the exceed rc 2.4 ghz transmitter before I found your site and it was a huge disappointment. I highly agree with your suggestion not to get it!!

[See our comments on the Exceed radio system]



Q: Hi Aaron. I have an antweight lifter controlled by a [Spektrum] BR6000 receiver. I have the [lifter] servo on the last channel of the BR6000. When I turn off my transmitter the servo snaps to a specific point then stops moving. Is that supposed to happen with the BR000? In the radio and electrical section of your archive you said that channel 3 on the AR500 [receiver] would snap the servo to a pre-set position, and the same thing is happening with my servo but I have a BR6000 with failsafes on ALL channels. Whats happening?

A: That's exactly how the failsafes on the BR6000 receiver function - all channels are driven to a pre-programmed position on loss of transmitter signal. You can set the failsafe position wherever you like: see Spektrum's BR6000 failsafe position instructions.

The AR500 receiver would only failsafe channel 3 to its programmed position and would send no position information to the other channels. Other manufacturers have different options on failsafe positioning, but your Spektrum is working as designed.



Q: I need to limit my 11.1 v battery to 6 v for my receiver. I was thinking of using a zener diode in series with a regulator. Is this a good choice or is there any better way to do it?

A: Mark J. here: first, check to make sure your receiver needs to be fed no more that 6 volts. Some receivers - Spektrum, for example - can handle 12 volts or more.

Second, many ESCs have a built-in voltage regulator that will supply a nice safe 5 volts to the receiver thru the servo lead with no direct battery connection needed. If your ESC has such a Battery Eliminator Circuit (BEC) you will neither need or want to supply power from an additional source.

You can use a zener diode as a voltage regulator by placing the zener diode in parallel with the power terminals of your receiver with a current limiting resistor upstream. You may have to fiddle with the resistor value to get proper regulation. A better and more stable solution would be to use an inexpensive 7805 voltage regulator chip.



Q: Hello, I am making some custom NiMH packs for a beetle. Do the individual cells need to be charged before the pack is assembled, or will they charge correctly the first time?

A: It's best to have all the cells at a similar state of charge when the pack is assembled, but a long and slow (~50 mA) 'trickle charge' of the pack will bring all cells up to full if there is an imbalance.



Battery Eliminator Circuit bypass for a servo. Q: Hi Aaron. I have a very compact antweight lifter and I want to increase the voltage on my analog lifter servo. I am running the servo off of a [Spektrum] BR6000 [receiver] which is connected to an Ant 100 [ESC] which is connected to a 7.2 volt li-po. I don't have a whole lot of room, so an extra battery is not an option. It is running at 5 volts right now [from the BEC on the Ant 100] and I want to increase it to 7.2. Could you put a y-connector or something to split the voltage? Thanks.

A: Sure. The Speed 100 Electronic Speed Controller has a built-in Battery Eliminator Circuit (BEC) that regulates the voltage fed to the receiver down to a safe 5 volts regardless of the battery voltage. Your lifter servo is plugged into the receiver and is operating off this same 5 volt feed. To get full battery voltage to the servo you'll need to disconnect the positive power wire from the servo plug (the red wire on most servos) and patch it into the positive lead from the battery. The signal and ground wires remain plugged into the receiver. This will bypass the BEC for the servo and leave the receiver at 5 volts.

Q: So it would be fine to take off the servo pin, cut the individual pin off the red wire, strip the end, and put it in the screw terminal [on the Ant 100] with the other red battery connector? It wouldn't cause any shorts or anything? Thanks.

A: Mark J. here: yes, you've read the diagram correctly. The receiver uses the 'signal' and 'ground' wires to communicate with the servo controller - which is actually a very small ESC. The red 'power' wire supplies positive 'juice' to the servo controller and motor, which also use the ground wire as a return path to the battery. The ground is continuous and unregulated thru the receiver and back thru the ESC to the battery -- no shorts, no conflicts. Assuming that your servo can handle 7.2 volts, you're home free.

P.S. - Aaron is sulking because you didn't believe him, but we both understand that it's better to ask than to watch your electronics go up in smoke.



Q: I read some where that the [Spektrum] DX6i [radio] was recalled for suddenly killing air craft mid flight. It also could turn on propellers at unexpected times. Is it safe to use on a bot?

A: Mark J. here: in early 2009 Spektrum discovered that a few specific production runs of their DX6i transmitter had bad stick potentiometers. The affected date codes and instructions for returning those transmitters for service are listed at the Spektrum website. Anything you buy now should be fine, but check the date code on older transmitters.



Q: Do you have any pictures of a robot's electronics?

A: See Zpatula's page - at the bottom. See also a diagram in FAQ #19.



Q: Which controller should I use for controlling a lifter for a BB series toy like zpatula?

A: We used the Team Delta RCE220 Dual Ended Switch to control the lifter on Zpatula. It's overkill for a beetleweight, but there was room in the chassis and we had one in our parts bin.

The lifter is built to operate with the motor running in a single direction, but it is much better to use a bi-directional (forward/reverse) motor controller for quicker and more predictable response. The RCE220 can be connected as a bi-directional 'H-bridge' and has special input connections that allow the use of 'limit switches' to stop the motor when the lifter reaches maximum and minimum height. This makes control of the lifter much simpler.

The lifter is also built with a 'slip clutch' that limits torque and prevents the motor from stalling. You will want to lock-out this clutch to allow full lifting power, but this may cause the motor to stall under heavy lifting. The stock motor consumes about 2 amps when stalled @ 6 volts. If you use an ESC to control the lifter you'll want one with at least that much peak capacity. Something like the FingerTech tiny ESC could be marginally adequate to control this lifter motor up/down. The RCE220 we use is rated at 12 amps.



Q: My question is... can you explain how a transmitter and receiver work and why exactly the spektrum receiver is so much better than a 4 channel pico receiver on antweight robots? Thank you.

A: Mark J. here: that's not a simple question and I cannot answer it in a short paragraph. I'd suggest you start by reading the article describing the Spektrum DSM technology at the Spektrum website.



Q: How is the HK-T6A HobbyKing 2 [radio system] being sold at FingerTech? Is it worth it?

A: Mark J. here: This is the Exceed 2.4GHz radio offered under another name. I have reviewed this radio previously -- search for 'Exceed' in this archive. Fingertech claims that their 'tinyESC' will correctly failsafe with this radio, but I'm not sure which other ESCs will failsafe properly.



Q: We are building a combat robot and are going to buy the Dx6i transmitter and the BR6000 receiver. I was just wondering how the receiver interfaces with a microcontroller.

A: Building your own speed controllers? The output of an R/C receiver is a pulse width modulation providing a 4 to 6 volt 'high' signal for a length of 1.0 to 2.0 milliseconds on a 20 millisecond cycle. A detailed description of the signal and an example of interfacing can be found at Chuck McManis' website.



Q: Does the Spektrum DX6 transmitter have the feature of Adjustable Throttle Volume? Thanks

A: Sure - the Spektrum DX6 is a full-function transmitter with all the bells and whistles. Spektrum calls it 'Travel Adjustment' but it's the same as ATV.



Q: Hi, I have a 11.1 volt lipo for my antweight. I accidently left it out in the sun and it got kinda puffy. I checked the voltage and its at 7.4 volts. Do you think the battery's okay? Should I charge it and see if it works or is it bad from leaving it out in the sun? Thanks.

A: A puffy LiPo is a bad LiPo. Heat is a LiPo's worst enemy. It's toast. Do not try to charge it - charging a damaged LiPo can cause a fire or small explosion. Dispose of it properly (see instructions in the Materials & Components archive) and go buy a new one.



Q: Hi aaron.Can u tell me how to code for using pulse width modulation in atmega8 microcontroller?And to which pin of motor driver l293d should we connect the pwm output?

A: This info is widely available on the 'net, so I'm not going to repeat it here.



Q: I recently lost my wallet so my budget just got a lot tighter! So, instead of Spektrum, I'm thinking of going with an Exceed system. Other than being cheaper -in both senses of the word- what are the downsides? Thanx.

A: Mark J. here: we've warned people off of the Exceed 2.4Ghz radio in earlier posts. The last place you want to 'go cheap' is on your electronics. How functional a radio system do you think you're going to get for $45? The radio is manufactured in China by a company called 'Fly Sky' and is marketed under several names. It is (IMHO) entirely unsuitable for use with combat robots.

  • The manual is an unintelligable translation from chinese. Example: "When you buy the CT6 series remote systems, with a programming line you can set up the program by your own, this will bring you happiness."

  • No on-board function adjustments except mechanical trims. All other functions must be adjusted via a computer connection, so plan to bring a laptop with you to the tournament.

  • If you do sort out the software and computer connection, the functions available are limited: endpoint, reverse, subtrim, dual-rates (Ch 1, 2, and 4), stick mode assignment, and some sort of exponential rate adjustment for Ch 3 and another channel I can't identify.

  • No standard channel mixing (elevon, v-tail). Either figure out how to adapt a helicopter swashplate mix to your robot or program your own user mix.

  • No batteries - buy your own.

  • No failsafes - relies on a correct response from the ESC to be legal for active weapons or classes above hobbyweight.
I will repeat my opinion - this system is unsuitable for robot combat. Check question #16 in the FAQ and then go look around for your wallet some more.



Q: My thwackbot is moving forward while spinning. I was wondering if I can control this so I can turn it into a FBS and still use a AM radio?

A: I suspect that your robot is moving 'forward' because your test area isn't level and it's actually moving 'downhill'. There are several posts in the Design & Construction archive about 'meltybrain' systems to get controlled movement from thwackbots. You'll spend a WHOLE lot more money trying to get meltybrain to work than you would on a top-flight FM radio.



Q: Hi would the Robot Marketplace's Sabertooth 5 dual motor ESC work with the Exceed 2.4 MHz transmiter and receiver?

A: Yes - I'm not a fan of the Exceed radio system, but the receiver has standard outputs that any R/C component can plug right into.



Q: Are lithium-ion nanophosphate batteries really good batteries? How come no one seems to be using them?

A: The Lithium Nano batteries have a very high discharge and recharge rate, as well has high energy density. Drawbacks are the need for a specialized charging system and a bulky cylindrical shape -- too bulky for use in most insect-class designs where the high energy density might best be used.



Q: Hi. I need a 11.1 or 12 v battery that can run at 5a for ten minutes. It should be under 3oz, .5" x 4" x 2.5" and under 30 dollars. I found a couple at BatterySpace, but too expensive. Thanks.

A: I'd suggest that you pay the extra $5 for that LiPoly pack you found at BatterySpace and stop wasting your time (and mine).



Q: If I have an Spektrum AR500 receiver and a Sabertooth ESC which has a failsafe, will I meet the failsafe requirments for an Antweight with a servo lifter?

A: Mark J. here: if the lifter servo is digital, yes. If the lifter servo is analog, maybe/maybe not. There is a certain amount of 'interpretation' here.

The Sabertooth ESC with the failsafe DIP switch turned on will take care of the drive motors, but the AR500 receiver sends no signal at all (except on channel 3 - see below) when it looses contact with the receiver. A digital servo will freeze in place when this happens, but an analog servo may creep. Technically it wouldn't be in compliance and a picky tech inspector may call you on this.

The AR500 will failsafe channel 3 to a pre-set position on signal loss. A servo lifter controlled by channel 3 would snap to the pre-set position. The rules say that all motion is supposed to stop with signal loss but interpretation of this rule can vary from event to event. I would suggest contacting the organizer of the event you plan to enter for their opinion.

Q: Could you recommend a digital servo under $50 with 100 oz/in at 5 volts?

A: I can come close: the Hitec HS-5645MG Digital High Torque servo produces 143 oz/in of torque at 4.8 volts. Metal gears, 2.1 ounces, $54.99. There are some no-name chinese digital servos well under $50, but I woudn't use them. You'll forget about spending the extra money a lot quicker than you'd forget about a cheap servo failing and taking you out of a tournament.



Q: I am thinking about buying an Integy charger, mainly the INDI 16X705 Multifunction. Is there any reason not to get these?Also, I can't find what style connectors it has. Do you happen to know?

A: The INDI 16X705 charger is an all-in-one charger with a built-in power supply -- no external power supply needed. I've seen mixed reviews for Integy chargers, but the list of features and price are tough to beat. The charger comes with a standard Kyosho connector, but you can take it off and put on anything you like.



Q: I have a Futaba 3003 servo, a Sabertooth 2x5 ESC, and a Spektrum DX5e transmitter with an AR500 receiver. When I wire my motors, receiver and servo my ESC doesnt give power to the receiver or servo.

A: I'll assume that you have a charged battery in there someplace that delivers between 6 and 18 volts. Take a look at the wiring diagram in the FAQ (#19) to make sure you've got everything hooked up correctly, and double check battery polarity. If you're certain that the wiring is correct it's time to suspect a faulty ESC.


Q: I'm building a spinning weapon on my ant and i have 2 speed controllers, a dual for the drive motors and a single for my weapon motor. I'm using a li-po 800ma 7.4v battery. How do i wire it to both ESCs?

A: See the wiring diagram in the FAQ (#19).



Q: What do you think of the E-flite 5 channel transmitter (LP5DSM)?

A: Largely useless for robots. It has a very low power output and does not meet robot failsafe requirements.



Q: Does a servo have more torque moving forward than backwards? Are both directions equally timed? Thanks.

A: Servos are 'neutral timed' for equal speed and torque in either direction.



Q: Robot Magazine warns you not to twist the antennas together on the Spektrum BR6000 receiver. Why not?

A: Twisting pairs of wires together is a well known technique to reduce transmission or reception of radio frequency signals. See the Wikipedia article on twisted pairs. Robot motor leads are often loosely twisted to prevent them from radiating radio interference that can cause trouble for receivers.

The Spektrum BR6000 receiver is really two receivers in a single package, each operating on a different frequency. The two antenna wires on the BR6000 receiver lead to a separate receivers inside the housing. Twisting the antenna leads around each other forms a 'twisted pair' that may interfere with reception. Keep the antennas away from each other and away from other electronics.



Q: I want to use the Exceed RC 6-Ch 2.4Ghz radio for my hobbyweight. Can I use a Spektrum receiver for it?

A: Mark J. here: I strongly recommend against going cheap on your electronics -- particularly on your R/C gear. It would be false economy to save a few bucks on the radio only to get washed out of a tournament when it glitches. I don't see any mention of failsafe capability for this system and it's anyone's guess if it is compatible with Spektrum receivers. Don't do it.



Q: I'm having problems with my NIMH 7.2 4200 miliamp racing battery pack. I fully charge it, and after driving my robot around in the backyard for 20 minutes, its dead. Whats wrong? The robot isn't drawing that much juice.

A: Mark J. here: three possibilities:

  1. the pack isn't being fully charged;
  2. the pack is old, damaged, or has a dead cell;
  3. the robot really is drawing that much juice.
You haven't told me anything about your robot weight or drivetrain, and you haven't mentioned the type of charger you use. A high-end charger will tell you how many mAH the battery takes to charge and could rule out options 1 and 2.

Q: The racing battery pack is a Duratrax DTXC2146 7.2V 42000mAh HiMH Battery. It is about 6 months old,and I have a Duratrax IntelliPeak Pulse Charger. My robot isn't really a robot, its an old Tamiya King Tiger tank with clutch mechanism. I'm not sure what motor it has, but the motor is about 2 inches long and 1 1/4 inches wide. I am peak charging the battery at about 2 amps. THANKS!

A: It's unlikely that your motor could be sucking down enough amps to drain a 4200 mAh pack in 20 minutes. Make sure there is no binding in the drive mechanism, but I suspect a battery problem.

NiMH packs can be damaged by recharging while still 'hot' from a recent rapid discharge. 'Old' refers to the number of charge/discharge cycles the pack has been thru, not just it's physical age. The Duratrax charger does not display the amp-hours put into the pack during the charge, so it is of no direct diagnostic help. Still, a 2-amp charge rate should take better than two hours to fully charge your drained pack -- is that about how long it's taking?

Try charging the pack at 1 amp and letting it drop into automatic trickle charge mode overnight. If that doesn't give better results I think it's time to buy a new pack.



Q: Hey guys, I have a soldering question. I'm having great difficulty tinning and soldering the ends of my wires. The wires are 20ga silicone insulated premium wires, similar to Dean's Wet Noodle wire.

My soldering iron is a Weller soldering station with a 42 watt heater and all sorts of tips. I've tried narrow screwdriver-shaped tips that Weller says will heat to 600 and 700F (depending on the tip), and have had only moderate success so far. The solder is rosin-core, I think the alloy is 60SN, if that makes a difference.

No trouble soldering PCBs, but the wire is giving me fits. At best, I get a shoddy coating of solder on the wire, and it doesn't seem to soak up the solder like I think it should. I have 18, 16, and 12 gauge wire yet to solder, and I'm afraid to even try it. Any advice? Is my equipment too wimpy for the job? [Dave B.]

A: Mark J. here: your equipment is more than up to the job, Dave. What you need is some paste flux. Dip the end of the wire into the flux to coat it. Apply a tinned iron and the solder will soak into the wire like water into a damp sponge.



Q: I did a dumb thing. I cut the connector off of my fully-charged 11.1v LiPo with a knife -- both wires at the same time. As you would expect, there were some minor fireworks and I now have a pretty rainbow weld spot on my knife.

The battery was only shorted for a moment (maybe one second). It did get pretty warm, but not hot. What are the chances that I damaged my battery? Do you think it would still be combat worthy? The robot is yet to be completed, so I won't be able to test the battery for awhile. [Dave B.]

A: Mark J. here: everybody gets a case of the dumbs now and again, Dave.

LiPolys do not appreciate being shorted. Examine the battery casing: it should be taught and flat. If there is any bulging or 'inflation' the battery has been damaged and is unsafe to use. If it looks OK I'd put it on a heat-resistant surface away from flammables and run it thru a couple of charge/discharge cycles. If it behaves normally, run it.



Q: Can I change my Spektrum DX6i transmitter from Mode 2 (throttle on left stick, elevator on right) to Mode 1 (throttle on right stick, elevator on left) without a physical re-wire?

A: Most computerized transmitters have a menu page to swap modes. The DX6i has one, but it's undocumented and really well hidden:


  1. Press the selection roller and hold down while turning on the transmitter.

  2. Release the roller when 'SETUP LIST' appears.

  3. Rotate the roller to highlight 'COPY/RESET' and press the roller.

  4. Rotate the roller to highlight 'RESET' and press the roller.

  5. When prompted with 'SURE NO/YES', rotate the roller to highlight 'List'.

  6. Toggle the 'AIL D/R' switch (top right of the transmitter) from position 1 to position 0 a few times.

  7. Secret hidden menu! Select your desired mode with the roller and press to accept.
Spektrum DX6i hidden mode switch menu

I don't have a DX6i here to try this on -- let me know how it works.



Q: I intended to use the Spektrum D6xi, and the Sabertooth 5RC to control my robot. Is there a way I can get throttle on the left stick and steering on the right (or vice versa if required) and integrate a gyro? Will it be invertible?

A: Mark J. here: I suggest you read the Run Amok Guide to Combat Robot Gyros for details on mixing and gyro inversion. Briefly:

  • Gyros are not compatible with transmitter mixing. You will need to use the on-board mixer function on your Sabertooth to run a gyro.

  • If you use the Sabertooth mixer you can assign the throttle and steering to any stick and axis you like: for left stick throttle with right stick steering plug throttle into channel 3 and steering into channel 1.

  • An inverted robot with a gyro will go into an uncontrolable 'death spin' unless the gyro can be shut off with a spare radio channel. The 'invert' function on the Sabertooth will not solve this problem - you must have a gyro with a remote shut down feature.
If you decide to forego the gyro, the Run Amok Transmitter Programming Guide will give you info on transmitter mixing options that are more refined than the ESC mixer. Again, briefly:
  • Left stick throttle with right stick steering requires a Mode 1 transmitter and Elevon transmitter mixing.

  • Right stick throttle with left stick steering requires a Mode 2 transmitter and V-Tail transmitter mixing.

Q: Thanks Mark. Could I have two gyros and use the spare chanel to switch between the one for right side up and the one for inverted with two transistors spliced into the logic cable? As an alternative, could I use the "Remote Gain Dual Mode Heading Lock Gyro MS-044"?

A: Are you all that sure that your 'bot will need a gyro at all? I'd suggest trying it before going to a gyro.

I don't like the 'two gyros' idea. If you want to try something unusual, how about mounting the gyro on a tiny servo and rotating it back upright when you invert?

The MS-44 manual does talk briefly about 'gyro sense reversal', but this will not correct the 'death spin' problem. Helicopter gyros simply never face the same problem as a skid-steer robot - when inverted, helicopter direction controls reverse and the gyros are designed for that.



Q: Dear Aaron, I need help on setting up the Spektrum DX6i for combat robot use with a BR6000 receiver. Please help, from Anthony.

A: Hi again, Anthony. I'll be glad to help, but I need to know more about your robot and where you are in setting up the electronics. I also neen to know how you'd like to have the controls set up: do you want to have forward/back and left/right on one stick or have throttle and steering on different sticks? Would you rather have independent throttle for the two sides of the robot on two sticks (tank steer)?

You can get information on some of the set-up options in the Run Amok Transmitter Programming Guide. It was written for a Futaba radio, but you have most of the same options on your Spektrum. For a little help in wiring up the receiver and your ESC, see the wiring diagram in the FAQ (#19).

Write back and let me know what specific help you need. Oh, and tell me if your Spektrum is Mode 1 or Mode 2.

Q: My robot uses three channels -- 2 for drive and one for weapon. It is an antweight with a vertical saw and two hacked Hitec servos for drive. I want tank steer and my transmitter is Mode 2. My electronics are already set up. What ports do I plug these parts into? Thanks for your help!!!!!

A: OK, Anthony - tank steer is the simplest control system to set-up, but your left control stick does not spring-center like the right stick. You will need to modify the spring centering system by opening up the case and transferring the spring assembly from the left-right axis to the up/down axis. Tricky, but some help with this can be found at www.max3design.com.

  • Your left drive motor plugs into channel 3 and will be controlled by the left stick on your transmitter.
  • Your right drive motor plugs into channel 2 and will be controlled by the right stick on your transmitter.
  • The weapon ESC plugs into channel 5 and will be controlled by the landing gear switch on the upper left corner of your transmitter.
Set the robot up securely on a block so that the wheels are off the ground. Disconnect the weapon drive belt or remove the blade. Set the transmitter controls to the 'everything off' positions (sticks centered, gear switch in whatever you want the 'off' position to be). Follow the 'binding' instructions in Step 5 of the Quick Start Guide that came with your system, or in the receiver guide sheet.

Once the receiver is 'bound', you can check the control response. Each drive motor should spin 'forward' when it's stick is moved forward and 'backward' when the stick is pulled back. They should not move at all when the sticks are centered. You can adjust the 'no movement' point with the channel 2 (elevator) and 3 (throttle) 'trim' adjustments (see your manual). If direction response is reversed, it can be corrected by the 'servo reversing' function (see your manual). Likewise, if the weapon motor is 'on' with the switch in the 'off' position, use servo reversing on channel 5 (gear). If the weapon motor spins backward, reverse the power leads from the ESC.

That should get you rolling, Anthony. Take it off the block and drive it around. Leave the weapon disconnected outside safe containment - PLEASE!



Q: Can I connect a Battle Switch and an ESC to the main drive ESC power connections?

A: I'm not sure I understand the question. If you are trying to use the Battle Switch like an ESC to control a weapon motor, the switch and motor system should be connected to the battery in parallel to the drive ESC. Connecting the weapon switch and motor to the drive ESC battery connections would be OK. Wiring would be similar to the wiring diagram in the FAQ #19.

Team Delta also makes R/C switch interfaces. Theirs have built-in radio fault failsafes.



Q: I want to put a master switch in between the battery and ESC. How do I wire an on/off switch? Do I use 3 or 4 post switch?

Charging jack wiring diagram. A: The device labled 'Removeable Power Link' in the diagram is the master switch. In large robots this switch is usually a 'removeable link' for safety purposes. A removeable link is just a connector with one side shorted by a loop of wire -- pull the connector apart and the circuit is broken. See Team Delta switches/links and Robot Maketplace power switches.

Smaller class robots can use a simple single pole single throw (SPST) switch of suitable capacity, or you can make a removeable link out of a small connector. Some form of master power disconnect switch is required by all current rulesets.

See also the wiring diagram in the FAQ #19.



Q: I have a 7.2V battery with a Futaba connector that I am trying to wire to a Sabertooth 10 RC Dual Motor ESC and a BB-3-9 ESC. I looked everywhere on the internet and I can't find the right adapter. What would be a good solution to this problem?

A: A Futaba connector is not suitable for high amperage connections -- it's only good for about 3 amps. Replace the Futaba connector with a Deans Micro Plug and run wires from the plug to the Sabertooth screw terminals. Connect the weapon ESC power leads to those same screw terminals and you're in business.

Can't solder? Now would be a great time to learn.



Q: I have a Sabertooth 2x25 ESC connected to two drill motors. When I hooked them up they worked for a second and then pulled up a red code and shut down. Hasn't worked since. Is the controller fried?

A: Probably not, but you haven't given me enough info to say for sure. Several different problems can trigger a shutdown and give you the red error light.

The first thing I'd check would be the battery. If it's lithium the #3 dip switch should be down, if not the switch should be up. If the battery does not have enough output capacity to deliver full amperage to your motors the supply voltage will drop and the Sabertooth may shut down. Fully charge the battery and try again. If no joy, test the setup with a larger battery (or smaller motors).

Make sure the other dip switch setting are right, and double-check all the connections to make sure they are correct and tight.

Q: I have the dip switches set right. The only thing I connected wrong was the pigtail servo lead off the 5v side of the ESC - I hooked the ground wire to the S2. Plenty of bats.

A: Mark J. here: you didn't think it was worth mentioning that the receiver was incorrectly wired when the ESC failed? More info needed:

  • What are your dip switch settings?
  • What battery are you running -- type, voltage, capacity?
  • What drill motors?
  • What receiver?
  • When the pigtail ground was incorrectly connected to S2, what was the pigtail signal lead connected to?
  • If you power up the ESC now, are any of the indicator lights lit? Which ones?
  • If the receiver has a power light, does it light up when the ESC is powered?
Verify that the receiver is still operational. Hook it up to a suitable battery and test each of the outputs with a servo.

Check the 5 volt power supply from the ESC with a voltmeter. You may have cooked it when you wired the receiver incorrectly.

- Dip switches 1 and 4.

Assuming that you mean that 1 and 4 are 'off' (down toward the numbers) and the others are 'on', that puts you in R/C mode with no mixing. That's fine...

- Battery is 12v lead acid, 7.2 AH

A single 12 volt battery -- OK.

- Craftsman 19.2 drill motors.

Whoa! Even at 12 volts those motors will pull more amps than the 'peak' output rating of the ESC if loaded down near stall. At 24 volts they'd pull a whole lot more. I wouldn't try to run those motors on a Sabertooth 2X25 -- it's gonna overload and shut down anytime the motors are pushed.

- Futaba R606FS 6 channel 2.4.

Should be fine...

- Pig tail was connected to receiver.

OK, but you told me that the ground wire was connected to 'S2' on the ESC. To what were the other two wires (signal and power) connected?

- [Update] Signal to 'S1' and power was connected to the 5V.

From the dip switch settings I'm assuming that you're doing mixing in the transmitter. Grounding the 'S2' input to the ESC should have done no harm, but you would have had no steering.

- All the lights on receiver work.

Good...

- No lights on ESC work.

Not good...

- It has exactly 5v on ESC.

OK, no lights on the ESC has convinced me that there has been a failure in the Sabertooth. I think it's a poor choice to control those motors -- it's going to shut down everytime the motors are put under any real load. Still, the claimed overload protection should have kept the ESC from frying during your very short test. Get in touch with Dimension Engineering and give them all of the information you've given me. They may replace it under warranty or offer repair service, but I wouldn't use that ESC for those motors.

Q: The drill motors have the factory planetary gearing then geared down 4:1 gearing on top of that. Will that make any difference in the amount of amps it would be? Does that effect it at all? Trying to not spend $500 on a Vantec. BTW Thanks for all the help.

A:This project sounds like something other than a combat robot.

The amperage draw of the motors will depend on the voltage, gearing, wheel diameter, vehicle weight, and the resistance the vehicle is encountering. You can use the Team Tentacle Torque & Amp-Hour Calculator to approximate the amperage draw when the vehicle is pushing full force against an immoveable object. This is a common condition with a combat robot, but maybe not for whatever you're building.

The Craftsman 19.2 volt motor/gearbox is not one of the motor choices available on the Tentacle Calculator, but you can use the 'DeWalt 18v Low' motor selection as a reasonable stand-in. Enter your 'Operating Voltage' (12), Robot Weight (fully loaded - ??), Wheel Diameter (??), and Gear Ratio (4:1). Leave the other values at default. The 'Amps (per motor) to spin wheels' output will give you the maximum expected amperage draw.

Robot builders generally make sure their ESC can deliver their maximum expected amp demand for a good long time. Buying a $500 ESC that meets your requirement is a lot cheaper than buying a succession of marginal ESCs that keep failing and knocking you out of tournaments. Happy robot builders do not go cheap on their ESCs.

D9 Caterpillar Bulldozer Q: You are correct sir, I'm actually building a 1/10 scale D9 dozer. I know you don't deal with these models, however it's very similar to a robot and you are the only one that I've found that knows what I need to do. So here is what I have:

  • Weight - 78 lbs
  • Track on the ground is 17" long and 2.75" wide
  • Blade is 17" wide and 8" high
How would I enter that into the calculator? What ESC would you recommend? THANKS!

A: OK, now that I know what we're working with I can be of more help.

Track and blade size are not factors. I'll guess that the diameter of the tread drive sprocket is about 4": that gets entered into the calculator in place of wheel diameter. Plugging all that in, I get a peak amp draw of only around 6 amps when pushing hard with the treads spinning for traction. You might pull more amps working in thick mud or another surface where the treads could really dig in -- I don't know exactly what dozer modelers do with their toys. Top speed is about 1 MPH.

If this were a robot, I'd convert that excess torque into speed, but the slow speed will be realistic for a model dozer. The added advantage is that the projected amperage draw is well within the capacity of the Sabertooth 2X25 ESC.

We still don't know what blew out your ESC during the test, but let's chalk that up to some hidden defect. Get the Sabertooth repaired and you should be fine.

Q: Mark J, YOU ARE THE MAN!!!!! I appreciate all the help. And I know that in the future, I can count on you. Is there anyway that I could send you a pic of what I'm building?

A: Happy to help. I've advised all sorts of non-robot projects here -- small trains, camera booms, electric bicycles, automotive interiors -- but this is my first mini bulldozer. I'd like to see a picture: send me your email address (I won't publish it) and I'll contact you.



Q: I was looking through the Hexy Jr. build pics at the Team WhoopAss website. They seem to have a LOT of onboard "stuff" that other robots don't have, like modified X-Box controllers and custom built control boards. Is this really necessary or what?

A: First, it's hard to argue with the record Team WhoopAss has put together. If something is on their robot, it's there for a reason.

Hexy Jr. was built during the BattleBots era when IFI Robotics controller systems were the rage. Most of the odd electronic bits, including the custom controller board, were for the IFI radio system. You wouldn't need all that with the current radio systems.

The other things I see on their parts list are standard items for a well designed pneumatic weaponed 'bot. They did it right.



Q: I am using the Spektrum BR6000 receiver and will be powering it from a circuit board. Assuming the input voltage to BR6000 is 5V, how do I work out the current draw?

A: The Spektrum FAQ says that their AR6000 receiver draws about 40 mA -- the BR6000 should be the same. If you need to peg the consumption more precisely just supply the BR6000 with 5 volts and monitor power consumption with an amp meter.

Note: a receiver passes power to servos and gyros (but not ESCs). If you have such devices plugged in to the receiver you'll need to measure power consumption with everything installed and under simulated load.



Q: What type of wire connectors (PowerPole, Deans Ultra) should I use on a beetleweight?

A: We've always used Deans connectors on our sub-light robots and we've never had a problem with them. The new Deans Micro Plugs should be fine for a beetle.



Q: Is it safe to wire the Spektrum BR6000 receiver to a 7.2V battery? I know about over-volting when it comes to motors but I don't know if it applies to receivers. Also is it possible to use it with the Spektrum DX5e 5-channel radio system instead of the Spektrum DX6i 6-channel system?

A: It's good to be cautious with voltages applied to radio systems. Some receivers are more sensitive than others, so always consult the manual when going outside the normal range. Spektrum says that all of their Digital Spectrum Modulation (DSM) receivers have an operational range of 3.5 to 9 volts, so you should be fine at 7.2 volts.

The BR6000 receiver can and should be used with either the DX5e or DX6i transmitters. It has full fail-safe features that are required in most weight classes and weapon options.



Q: Can I connect batteries having different current ratings? They are same type ones.

A: Not recommended. If you connect the batteries in series, the weaker battery will drain down before the other and current flow will try to reverse-charge it. At best this could damage the discharged battery and at worst it could burst into flame. Connecting them in parallel would be safer, but I think I'd avoid the whole thing.



Q: What is GPS?

A: What does this have to do with combat robots?



NiMH discharge voltage chart Q: What is the voltage where NiMH batteries don't work properly?

A: I'm not sure that I understand your question. NiMH cells in series can deliver as much voltage as you like, so I'm guessing you mean the voltage where the cell is effectively drained? The chart shows a typical voltage vs. discharge state for a NiMH cell. When fully charged the cell can provide a little better than 1.3 volts under load. By the time it gets down to 1.1 volts it's pretty much drained. The discharge rate and cell temperature will impact the discharge curve.



Q: What is the best way to keep NiMH batteries?

A: Lithium batteries are picky about storage conditions, but Nickel Metal Hydride cells aren't. Store them at room temperature and give them a charge once a year. They'll loose charge during storage, but a couple of charge/discharge cycles and they'll pop right back to full capacity.



Q: I am not very experienced with R/C Robots and I am planning to use Spektrum DX6i 6-channel 2.4Ghz Radio System Mode 2 with the Sabertooth 5 RC Dual Motor Speed Controller. Will this create proportional drive mixing with the left joystick controlling forward-back, and the right joystick controlling left-right?

A: Mark J. here: that's the joystick set-up I like to use. It's possible to set-up the Sabertooth to give you left/right and forward/back control on any sticks you like. For left stick throttle, right stick steering with a mode 2 Spektrum:

  • Leave transmitter mixing off (default).
  • Flip the #1 option switch on the Sabertooth 'ON' to enable on-board mixing.
  • Plug the Fwd/CH1 connector from the Sabertooth into the throttle output on the receiver (channel 3).
  • Plug the Turn/CH2 connector from the Sabertooth into the aileron output on the receiver (channel 1).
There are two disadvantages to doing this:
  • The 'flip control' option for invertable robots on th Sabertooth does not work when on-board mixing is turned on; and
  • On-board mixing disables many of the control adjustment functions available with transmitter-based mixing.
You'd be better of to use the more adjustable mixing options available in the transmitter and leave the Sabertooth in no-mixing mode. The catch is that you'd need the mode 1 version of the Spektrum to get left stick throttle and right stick steering with transmitter mixing. The set-up for a Mode 1 Spektrum would be:
  • Turn transmitter elevon mixing on (consult the manual).
  • Flip the #1 option switch on the Sabertooth 'OFF' to disable on-board mixing.
  • Plug the Fwd/CH1 connector from the Sabertooth into the elevator output on the receiver (channel 2).
  • Plug the Turn/CH2 connector from the Sabertooth into the aileron output on the receiver (channel 1).
More detail on transmitter mixing functions, an example of transmitter settings for a robot using left stick throttle and right stick steering, and troubleshooting help can be found in our transmitter programming guide.



Q: I have gotten out of combat robots and I have some lithium-ion 3 cell batteries. Is there any way to keep them for longer without having the batteries lose capacity? Thanks.

A: Mark J. here: Li-ion batteries do irreversably lose capacity as they age, whether they are used or not. The loss is greatest when the cells are fully charged and at high temperatures. For best results drain them to about half capacity, seal in plastic bags, and store in your refrigerator. This will cut your loss by about 90% compared to fully charged batteries at room temperature..

Alternate solution: sell them to somebody who can use them now.



Q: Will using a gyro negate the timing on my DeWalt drill motors?

A: Mark J. here: DeWalt drill motors spin faster counterclockwise than clockwise because the brush timing has been advanced to improve efficiency when spinning counter-clockwise. Used in a robot, the left-side motor will run a little faster than the right-side motor and the 'bot will curve to the right rather than go straight under full power.

A peizo gyro is used in an R/C system to detect turning movement that has not been initiated by the R/C transmitter. When such turning is detected the gyro modifies the output of the receiver to correct for that movement. Peizo gyros were developed for R/C helicopters to monitor the action of the tail rotor and keep the chopper pointed in the right direction, but they can be very handy in specific robot applications as well.

Back to your question: yes -- since the turning of the robot happens without R/C turning input a heading-hold type peizo gyro would sense this as 'drift' and would correct the turning motion by reducing power to the left-side motor.



Q: Hi Aaron. I watched a thing a few years back on Tech TV (before G4). This guy made a remote control thing that had a wire connected to the remote out of stuff like old VCR parts. Do you know what I am talking about? If you do, how do it work?

A: No clue.



Q: How many volts can the BR6000 receiver supply -- 4.8 or 6.0 volts?

A: Receivers do not 'supply' voltage -- they pass on the voltage from the source that powers them. The Battery Eliminator Circuit (BEC) of a typical Electronic Speed Controller (ESC) provides 5 volts. The Spektrum receivers can operate directly from a power source as high as 9 volts and would pass that voltage directly thru to the receiver power output line.



Q: I found a RC radio for $35, but I'm not sure its 75 Mhz. If its not, can I buy a receiver that I know is 75MHz? The radio in question is a 4 channel Futaba.

A: Take a look at the removable crystal for the transmitter and/or receiver. They should both be labeled with a frequency channel number. If that channel number is between 61 and 90 (75.410 to 75.990 MHz) then the radio is 75 MHz ground frequency. If the Transmitter is not 75 MHz, a 75 MHz receiver will not work with it -- they have to match.

A certified technician can retune a Futaba radio from 72 MHz to 75 MHz, but it would be less expensive to buy a radio on the correct frequency band to start with.



Q: Would the DX5e be a good buy for a inexpensive 2.4 GHz radio?

A: The Spektrum DX5e is brand new and I have received no feedback from robot builders, so I cannot make a recommendation. I can tell you that it is a very basic 5-channel transmitter that lacks many useful features that make a robot much easier to set up and more comfortable to drive.

Q: What features does it lack compared to the DX6?

A: The DX5e has few features compared to the fully computerized DX6. Some examples:

  • no LCD display;
  • no batteries or charger;
  • no multi-model memory;
  • no adjustable travel volume;
  • no user programmable mixes;
  • no exponential response settings;
  • fixed dual-rates - not selectable by channel;
  • only one non-adjustable mixing mode (elevon).
Read the DX5e manual for a complete (if short) list of features.

Q: Could you use the DX5e with the BR6000?

A: Yes, and you should use the BR6000 receiver for the full failsafe features. The AR500 receiver does not correctly failsafe for robot applications.

Q: You can buy the DX5e on 'Mode 1' or 'Mode 2'. What does that mean?

A: There are two 'standard' ways that input channels are assigned to the two control sticks. Mode 2 is the U.S. standard with the elevator channel assigned to the vertical stick axis on the right side of the transmitter. Mode 1 is the European standard and has the elevator channel on the left stick. Computerized transmitters generally allow you to switch between modes, but the DX5e doesn't so you have to pick.

If you're going to use the included 'elevon' mixing and want 'single stick' control of throttle and steering on the right stick, pick Mode 2. Elevon mixing in Mode 1 will give you throttle on the left stick and steering on the right. More information on modes and mixing can be found in our transmitter programming guide

Q: Does the DX5e have a low battery light or something?

A: Yes, that's covered in the DX5e manual. See page six, and read the rest of it while you're there.



Q: Could you solder right angle pins onto the BR6000 receiver to make it a horizontal pin receiver if you needed to?

A: Yes, but I'm not sure why you'd need to. If you're short on space, it would be easier to take the connector plugs apart and plug in the individual wires.



Q: If your receiver antenna is cut, can you solder and heatshrink it back together?

A: Sure -- just keep the total length as close to the original as possible. Search this archive for 'original length' for a full discussion.



Q: I was driving my beetleweight around when suddenly it caught on fire. Everything was destroyed. I hooked two li-poly batteries in series and connected them to 2 Victor speed controllers. I also charged the batteries with a NIMH charger. Could that have been the problem?

A: Lithium batteries are very sensitive to abuse, and an abused lithium battery can burst into flame.

  • never charge lithium batteries with a charger not specifically designed for them;
  • never discharge lithium batteries at a higher amp draw than they are rated.
I suspect that you damaged the batteries by charging them with a NiMH charger and the battery then failed under load. Electrifly.com has a good article on Li-Poly care.



Q: Dear Aaron, what is the difference between digital and analog servos? Thanks.

A: The whole difference between digital and analog servos is in the electronic controller board inside the servo. Digital servos have a microprocessor on that board that can process the signal from the receiver and send more precise, quicker control information to the servo motor. If you're flying a helicopter this is important. If you're powering an ant lifter or hacking a servo for a drivetrain, you'll never notice a difference.



Q: What do you think is the best 3 channel wheel controller for an antweight?

A: Spektrum DX3R.

Q: What inexpensive 3 channel 75 MHz transmitter would you suggest?

A: The 75 MHz band is on the way out. If you buy an inexpensive 75 MHz radio you're going to need to upgrade soon and nobody is going to want to buy your outdated R/C system. Buying the Spektrum radio will save you money and trouble in the long run. I've said before, I don't recommend scrimping on electronics.

Q: What do you do if you are using the Inertia Labs chassis, but the Spectrum BR6000 receiver is to big to fit inside?

A: I'd make a new cover for the chassis to fit the BR6000, but I have a feeling you aren't going to be happy 'til I give you the name of a 3 channel 75 MHz pistol grip system. We don't use pistol grip transmitters, but if I wanted an inexpensive 75 MHz pistol system exclusively for antweight use I'd go with the Futaba 3PM.

Q: Are there any 2.4 GHz receivers that are the size of the GWS micro receivers? It is supposed to fit in the Inertia Labs chassis.

A: Although not as small as the GWS pico receiver, the Futaba R603FF receiver that comes with the 2.4 GHz version of the Futaba 3PM pistol grip system is 1.5" by 1" x 0.55" and will fit in the Inertia Labs chassis next to the Barello ESC. I recommended the Spektrum system over this Futaba because I don't know any robot builders who have used the Futaba 2.4 GHz systems, but the specifications look great and it is less expensive than the Spektrum.

Q: Would you suggest the 3 channel 75 MHz Hitech Aggressor SRX-3 for a inexpensive antweight transmitter?

A: There are two different 75 MHz Hitech Aggressors: AM and FM. The AM Aggressor is very cheap, but combat robots create a lot of electrical 'noise' that interferes with AM radio. AM radios are not allowed for robots with active weapons, and not at all in some competitions. I cannot recommend that you purchase an AM radio. The FM Aggressor is more expensive than the Futaba 3PM and has no useful additional features. I like Futaba.

Q: Can the Futaba 3PM 3 Channel transmitter use a GWS micro receiver as the receiver?

A: No. The GWS receivers work well with 4 channel and higher transmitters, but there have been many problems reported by builders trying to use them with 2 and 3 channel transmitters. Something's different about the signal coding in transmitters with less than 4 channels.

Q: I'm thinking about using a Futaba 14MZ 2.4GHz Radio System for my antweight. I only have $80, and I can't find the price of the 14MZ. Can you tell me how much it costs and if its a good radio for me?

A: You can't find the price of the 14MZ??? I put "Futaba 14MZ" into Google and it spit prices all over me. Very nice radio, but a little out of your price range. Street price is about $2300.

We could have saved a lot of time if you'd mentioned your budget at the start.



Q: So ant robots competing at Robogames 2008 cannot use a 75mhz frequency?

A: Mark J. here: once again, Robogames has screwed up their ruleset with conflicting statements. We had the same problem last year. Section 4 of the Robogames ruleset says ants can use any approved ground frequency, even AM radio. It says ants require no drive failsafe. It says ants do not require coded radio as long as their weapons will failsafe. It also says:

"All robot radio systems must be coded, mated pairs between transmitter and receiver. This means that no other transmitter, operating on the same frequency, can communicate with your receiver, and your transmitter cannot send signals to any other receiver than your own."

This effectively restricts all weight classes to 2.4 GHz spread spectrum systems.

Last year, under the same confused ruleset, Robogames did let antweights compete with 75 MHz systems. It is unclear if that was a one-year exemption. I can only recommend that you write to Dave Calkins (dcalkins@robolympics.net) and ask for clarification. Let me know what you find out.



Q: Dear Aaron, is the inexpensive GWS transmitter that Inertia Labs sells 4 channel or 6 channel? Also, what are the advantages and disadvantages of that GWS transmitter?

A: GWS offers both 4 and 6 channel systems. The Inertia Labs desciption of the GWS transmitter is messed up -- the title and picture are of the 4-channel GWT-4A but the text describes the 6-channel GWT-6A. The price matches up to the 6-channel unit, but the receiver offered is 4-channel.

I'd suggest refering to Robot Marketplace and their GWS systems. Their descriptions are correct and their price for the equivalent transmitter and receiver package is the same.

The GWS radios are low-end, no frills systems. They do not have any computerized functions, mixing, or failsafes. Their only advantage is low price. I'm not a fan of skimping on electronics.



Q: I accidentally put the connector on a 7.4 volt battery into two of the pins of a GWS pico receiver. There was a big puff of smoke. Can I do anything or do I have to buy a new receiver?

A: A big puff of smoke is always a bad sign. I'll assume you've tried hooking the receiver up correctly and seeing if it works? I wouldn't get my hopes up, but depending on which two pins you happened to overvolt it's possible that you just blew out one channel output. Try it. Even if it's fried, it probably won't be the worst mistake you'll make in robot combat. We've made some really big ones!



Q: My Spektrum DX6 radio crapped out, so I replaced it with a DX6i. I have 3 models with programmed AR6000 receivers in them. How do I bind the old AR6000 receivers to the new DX6i? Will I have to reprogram them?

A: Mark J. here: the DX6i is compatible with the AR6000 receiver, although some people have reported problems getting the receiver to bind. Keep the transmitter at least 6 feet away from the receiver during the binding. It may take a few tries.

  1. With the power off, plug the bind plug into the BAT port of the AR6000.
  2. Power on the receiver by connecting the battery to any unused port. The blue LED should be blinking.
  3. Position the throttle stick in the desired fail-safe position.
  4. Pull and hold the trainer switch on the top of the transmitter and turn on the power switch. It may take several seconds for the system to connect. The LED on the receiver should go solid green, indicating the system has connected.
  5. Remove the bind plug from the receiver.

Repeat for each receiver. Your prior receiver programming should be intact.



Q: My old JR receiver had most of it's antenna eaten up by a drum at it's last event. I was considering using the MicroBotParts receiver, but those are no longer being produced. Space is tight, so my only option now is the GWS pico receiver as sold by Inertialabs. My ant is a spinner -- will this receiver failsafe properly?

A: An antweight with an active weapon requires an FM radio, and the weapon must shut down on loss of radio signal. The MicroBotParts and GWS pico receivers are both FM-PPM single conversion receivers, but FM-PPM radio systems do not themselves provide a failsafe. Check this: explanation of FM-PPM and FM-PCM radio systems.

Some weapon motor controllers, like the Team Delta Solid State D-Switch, have failsafe circuitry built in that is independent of the radio system. Alternately, several manufacturers make tiny failsafe modules that plug in betwwen the receiver and the controller. Either of these options would meet the weapon failsafe requirement.

If your old JR receiver provided correct failsafe, I'd suggest soldering a new antenna wire onto it.



Q: I'm building my first robot, but I don't know much about radio control. What is the difference between a 3 channel radio and a 4 channel radio?

A: A 'channel' has the ability to independently control one item on your robot, like speed and direction instructions for motor(s) on one side of the robot. Two channels could control motors on each side of the robot, and a third channel could control activation of a weapon. A single channel can control mutiple motors, as long as they are all doing the same thing at the same time.

Note that there are also two two basic designs for R/C transmitters: 'pistol grip' and 'twin stick'.

  • Pistol Grip and Twin Stick transmitter styles. Most hobby grade 2 or 3 channel transmitters are the pistol grip design with a throttle trigger, a large self-centering steering knob, and a smaller knob or switch for the third channel. These are designed for controlling R/C cars. It is possible to find 4 channel pistol grip transmitters, but they are uncommon.

  • Most hobby grade transmitters with 4 or more channels are twin stick design, intended for use with model aircraft. These transmitters have two joysticks side-by-side, with each joystick controlling two channels: one channel in the up/down direction and another in the left/right direction. Additional channels may be controlled by knobs or switches. Some 2 or 3 channel stick transmitters exist, but again they are uncommon.
Most robot drivers use the twin-stick design, but it is entirely up to your preference.



Q: I want a circuit design having astable multivibrator output 12 volt a voltage amplifier convert 12 volt to 24 volt to a relay.

A: Mark J. here: good morning, and how are things in Mumbai? If I understand your request, you're looking for a circuit design to convert 12 volts to 24 volts to power a relay. I'd rewind the relay for 12 volts, but if you want to do it the hard way take a look at Harry Lythall's Practical Voltage Converter (archived).



Q: How do battery eliminator circuits work? What is a good one? How do I set them up?

A: Mark J. here: I hope you don't think that you can run your 'bot without a battery!

A battery eliminator circuit (BEC) is a voltage regulator or converter that takes the voltage from your main battery pack and reduces it to 5 volts to power the radio receiver. This allows you to 'eliminate' the separate battery pack for radio gear.

Most small electronic speed controllers (ESC) have a BEC built in. High quality stand-alone BECs are available for larger applications from Team Delta.

There is no set-up required for the built-in BECs; just plug your receiver into the ESC and connect the ESC to your battery pack. The ESC will feed power back to the receiver thru the 3-wire connector. The Team Delta units come with full instructions and require four solder connections.

Q: If a voltage regulator can take a larger voltage and bring it down to 5 volts, could it also take 24 volts and bring it down to 12 volts? That way I could run my 12 volt drive from my 24 volt weapon battery pack.

A: The bad news is that voltage regulators and converters are limited in the amperage they can provide. A radio receiver takes far less current than your drive system. A regulator with enough capacity for your drive would be very expensive, bulky, and heavy.

The good news is that you don't need a voltage regulator to run your 12 volt drive from your 24 volt battery. If your transmitter has 'ATV' (Adjustable Throttle Volume) you can set the transmitter to send a maximum 1/2 throttle signal to your drive ESC -- your drive motors will only 'see' 12 volts from the speed controller. As long as your ESC can handle 24 volts, you're home free.



Q: What is a 'gyro'?

A: See the gyro post in the FAQ (#20).

Q: Do you have to put a gryo in a `bot?

A: Absolutely not. Very few combat robots use gyros, but many that are difficult to control would be better off if they did.



Q: Can a Vex radio controller be used to control a bot?

A: The only electronics from a Vex Robotics Kit that can be used with combat robot components is the R/C transmitter. Nothing else will interface with standard hobby electronics. Search for 'Vex' on this archive page for more info.



Q: How does the new Spektrum DX6i radio system compare to the old DX6? Is it better?

A: Mark J. here: I haven't had my hands on a new DX6i yet, but the DX6i manual (13.3 megabyte PDF) goes over the new features:

  • The transmitter case has been redesigned. It looks more modern and has a larger LCD display.

  • The clunky programming interface has been improved with a 'roller' design to move thru menu selections.

  • The new DX6i may have more range than the old DX6, which was recommended only for moderate-range applications. The DX6 had plenty of range for robot combat applications.

  • The DX6i has additional programming functions for airplanes and helicopters, but no added features that will be useful for most robot applications.

The biggest difference seems to be the price.
  • The Spectrum DX6 came with four servos and the optional full failsafe BR6000 robot receiver for $150. You could sell the servos on EBay and knock your total cost down close to $100.

  • The Spektrum DX6i comes with the new AR6200 receiver which is not combat robot suitable and no servos for $180. Add on $50 for the required BR6000 receiver, sell the AR6200 receiver on EBay, and your cost is close to $200.
The DX6 was a great bargain and had all of the features a robot combat guy needed. The DX6i has a few more bells and whistles, but you'll end up paying nearly twice the price. My advice is to snap up an old-stock DX6 while you can still find one. Check with Spektrum to see if they are still exchanging the AR6000 receiver that came with the DX6 for the BR6000 receiver at no cost.

Q: I just wanted to let you know, the Robot Marketplace removes the AR6000 if you purchase the BR6000 receiver with the DX6. I'm not sure if this is the case with the DX6i, but it's likely.

A: It's a different deal. Robot Marketplace was able to substitute the BR6000 for the AR6000 receiver at no cost with the Spektrum DX6 system. The new DX6i system comes with the AR6200 receiver for $180, and for an additional $50 you buy the BR6000 robot receiver. For $230 you get both receivers.



Q: Can I "unbind" a Spektrum BR6000 receiver from a specific transmitter if I need to?

A: The simple way is to bind the receiver to a new transmitter. That will unbind it from the old transmitter.

If you want to unbind without a new bind:

  1. Turn on the receiver without the bind plug.
  2. Turn on the transmitter while holding down the bind button.
  3. Wait 30 seconds.
  4. Turn off the receiver.
  5. Turn off the transmitter.



Q: I've got problems with my new robot turning around and not going forward or backward like it should. I'm using a Futaba 9CAP transmitter and a Vantec RDFR23 speed controller. I'm set-up to use the right joystick for all the driving. Vantec already has mixing built in, the speed controller's 2 cables are S = steering and T = throttle. I followed the instructions for the transmitter programming on madoverlord.com for the 9CAP.

A: Mark J. here: the mixing instructions for the 9CAP at Mad Overlord assume that your speed controller has no mixing capability. You can set up either your 9CAP or your RDFR23 to mix, but not both!

  • Simplest solution: follow the instructions in your Vantec manual to turn off the onboard mixing on your RDFR23.

  • Alternate solution: reset your 9CAP to a clean configuration -- no mixing! Plug the Vantec steering cable into receiver channel 1 and the throttle cable into channel 2.
Either solution will get you back on track.



Q: Is it possible to use barcoding to program a real combat robot?

A: Yes, but I don't know why you'd want to.



Q: What is the equation to determine the size of the capacitors I need to place on my motors to best absorb energy spikes and reduce radio interferance without slowing motor response?

A: Mark J. here: electrical noise reduction is more of an art than a science. The optimum capacitance value varies with the RPM and load on the motor, so there is no single 'best' value for a combat robot application. Try 0.1uF as a starting point. See also the Team Delta Application Note #1 on reducing motor electrical noise.



Q: Can I use LiPoly and NiMH batteries in one robot as long as they don't intersect at all? (Different ESCs, no BEC, ect.)

A: Sure. The problem comes if you try to use different battery types wired in series or parallel for a common current draw. Don't do that!



Q: I'm considering the Thunder Power 2200mAH LiPoly Quad Cell 4S 14.8V Pack - Extreme Series for my featherweight. How much should I expect the actual amp-hours to vary from the stated?

A: Mark J. here: all battery types have reduced amp-hour output under heavy current draw. How much reduction depends on the internal resistence of the battery and the discharge rate. Thunder Power claims very low internal resistence and very little reduction in amp/hour capacity at high current draw for their latest series of extreme performance LiPoly batteries (chart). The R/C helicopter guys say the chart is about right. Under combat robot loads, you can expect very close to the full rated power from a fresh Thunder Power battery pack.



Q: Could I run 2 of my Astroflight 110 chargers off one power supply?

A: There's no such thing as a free lunch. An AstroFlight 110D charger can pull up to 16 amps at full output. If your power supply can't pump out 32 amps then you can't run two 110Ds at full output, but you might get away with two at less than maximum charge rate.



Q: Can I replace the 12 volt, 10 amp lead acid batery in my electric bike with a 12 volt, 10 amp LiPoly battery? Is a 500 watt rc compabile with a 500 watt pancake motor?

A: Mark J. here: LiPoly batteries don't come in exactly 12 volts -- but then your lead acid battery wasn't exactly 12 volts either. The closest you can get in LiPoly would be 11.1 volts for a 3-cell battery. Two problems:

  1. LiPoly batteries can be destroyed and even burst into flame if the amperage draw is too great. You need to determine the maximum amperage draw of your bike and obtain a LiPoly capable of handling that large a draw.
  2. LiPoly batteries require a different charger than lead acid batteries. Do NOT attempt to charge LiPolys with a lead acid charger!

As for your second question, I don't know what you mean by a '500 watt rc'. If you are talking about R/C speed controllers, they are rated by maximum amperage draw and voltage. Again, you would need to determine the maximum amperage draw under operating conditions of the 500 watt motor you reference and match that to a specific speed controller.



Q: Can I wire SLA and NiMH batteries in parallel?

A: Mark J. here: do not mix dissimilar batteries! Way too many things could go wrong and result in a battery explosion. I can't figure out why you'd want to mix SLAs and NiMHs.



Q: What kind of trickle charger should I use with SLA batteries?

A: You don't want a trickle charger for combat robot applications. Trickle charging simply supplies a very limited current at an appropriate voltage. Is is designed to charge a battery over a long time period or maintain the charge on a battery between infrequent uses. If your SLA is an automotive/motorcycle/marine type, an appropriate charger can be purchased at an auto supply store.

Note that some specialty SLA batteries do not respond well to trickle charging. Hawker batteries for example must be charged at a high initial amperage rate or they loose capacity. Check with the manufacturer of your battery for their charging recommendations if in doubt.



Q: If my 'bot lost its radio signal, would my Spektrum DX6 radio failsafe cause my solenoid actuated spinner to stop?

A: The Spektrum BR6000 receiver can be programmed to failsafe to any input position on any channel. Instructions for programming the BR6000 receiver are at the Spektrum website. If you program your weapon channel to failsafe to "off" position it will shut off the R/C switch controlling your weapon solenoid when signal is lost.

The Spektrum AR6000 receiver does not have full failsafe capacity and is not legal for robots that require failsafes on weapon and drive systems.



Q: To reverse a servo, would I just switch the positive and negative wires? Also, to wire two servos on the same channel, would I just wire the two servos in parallel? Thanks.

A: Do not switch the polarity of the servo - instant fried electronics. Almost all hobby R/C transmitters have a 'servo reverse' switch or function that will invert the servo direction. Use that! You can also buy a tiny electronic servo reverser that plugs in between the receiver and servo.

You can wire two servos in parallel and plug them into a single receiver output. Your local hobby shop can provide a 'Y-connector' for that purpose, or you can splice the wires yourself



Q: How do I wire a servo so that it's powered directly from the battery instead of the receiver? I know I'll probably burn out a few, but I have many extras, so it's fine.

Overvolting servos. A: Mark J. here: Some readers may wonder why you want to do this. Servos can be 'hacked' to provide continuous rotation and used as drive motors. 'Overvolting' the servo will provide more speed and power at the cost of reduced lifespan. Many receivers and other R/C electronics, however, are very sensitive to increased voltage and can fail or malfunction if pushed to higher voltages. The trick is to run the receiver at a correct voltage while providing higher voltage to the servo.

A servo lead has three wires:

  • the red wire is power (+)
  • the black wire (brown in JR servos) is power (-)
  • the remaining wire (white, orange, yellow, blue, whatever...) carries the coded receiver signal (S).
Disconnect the red wire from the receiver end of the lead and route it to your main battery positive terminal. Splice a new ground wire from your main battery negative terminal into one black lead. The black lead from the servo to the receiver must remain intact as part of the signal loop.

Note: although extensively used in the past, servos are inferior to other insect class drivetrains. I don't recommend them.



Q: Which competition required spinners to use gyros to prevent the death spin? I remember something about that in the RFL rules previously, but I can't find it in the same draft.

A: Gyros don't prevent the death spin, they CAUSE the death spin if they invert. See the gyro guide for info. I don't know of any event which has either required or disallowed the use of gyros.



Q: I bought a Gardner crimping tool. How do I use it to crimp my 45 amp powerpoles?

A: Mark J. here: the inexpensive Gardner crimper is best used with the 30 amp Anderson Powerpole connectors. The contacts for the 15, 30, and 45 amp connectors are the same, so actual power capacity of the connectors are also the same -- about 100 amps. The difference is in the size of wire the connectors will accept. The 30 amp powerpoles will accept up to 10 gauge wire (really). You may want to solder the 45 amp connectors rather than trying to crimp them. See: this archived article for instructions on crimping with the Gardner tool.



Q: Is it legal for several bots to share one transmitter?

A: Sure. Most teams have only a single transmitter that they use for all of their 'bots. One word of caution: at some large tournaments the insect classes run in a small arena at the same time that the larger 'bots are fighting in the big arena. In that case you may need a backup driver and a spare transmitter. Same deal if you have more than one 'bot entered in a single weight class -- you may end up fighting yourself!

Q: If you end up fighting yourself, can't you forfeit one of ur bots and still win?

A: Mark J. here: --- WEAK! ---

If you enter a robot in a tournament you should be prepared to fight it under any and all circumstances; if not for your pride then for the benefit of the spectators. Allowing such a forfeit would be at the discretion of the event organizer. I'd throw both your 'bots out.



Q: How do I tell if a radio can use failsafe? I'm looking to buy an R/C system for under $200 from Robot Marketplace.

A: If a radio system has failsafe capability, it will say so somewhere in the descriptive text -- it's a feature they want you to know about! AM and regular FM (PMM) systems generally do not failsafe. Coded FM (PCM or IPD), 900 MHz, and 2.4 GHz systems may failsafe.

Part of the problem you're having is that Robot Marketplace currently sells only one radio system under $200 that is full failsafe: the Spektrum DX6 2.4 GHz with BR6000 receiver.

Note: All Spektrum 2.4 GHz systems failsafe, but systems with the AR6000 receiver fail to the last speed setting received from the transmitter. This is not legal for robot combat which requires drive and weapon systems to stop on signal loss. The Spektrum BR6000 receiver corrected this problem.



Q: How can I calculate the battery capacity needed for my A28-150 AmpFlow powered drum weapon?

A: Mark J. here: lots of variables! Rule of thumb: a typical spinner adds about 40% to the capacity needed for the drive motors.

If you're determined to go thru some rough calculations, you can use the Team Run Amok Spinning Weapon Excel Spreadsheet to calculate the energy capacity of your weapon in joules. Each time the weapon is depleted and must spin up it will consume approximately:

Amp Hours = weapon joules / (1800 * voltage)

Additionally, the weapon motor will consume some power just maintaining the weapon at speed. This can be very roughly approximated by:

Amp Hours = motor no-load amps * match length / 60

Example: an AmpFlow A28-150 motor spinning up a 10,000 joule weapon 5 times during a 3 minute match:

  • Each spin-up will use: 10,000 / (1800 * 24) = 0.23 Amp Hours.

  • Five of those will use 5 * 0.23 = 1.15 Amp Hours.

  • Maintaining spin speed requires: (3.4 * 3) / 60 = 0.170 Amp Hours.

...so the estimated battery capacity requirement for this scenario is around 1.32 Amp Hours.



Q: Can I use the GWS GWT-4A 75Mhz FM R/C System in a battle if I have a spinning weapon?

A: The current RFL rules require all robots with active weapons to have fail-safe electronics which will stop all robot motion if the radio signal is lost. Robots weighing 12 pounds or above must use FM radio with PCM or IPD coding, or digital 900 MHz or 2.4 GHz systems.

The GWS GWT-4A does not meet any of these requirements. It may be used for passive weapon robots up to 12 pounds, or for active weapon robots up to 6 pounds if add-on fail-safes are included.

Note that a few tournaments now require digital 900 MHz or 2.4 GHz systems for all robots. Check with the specific event organizer to be sure.



Q: Are there any mixing functions in the Spektrum DX6 I can use in my two wheeled 'bot?

A: The Spektrum DX6 is a full-featured R/C system. It has multiple pre-set and custom programmable mixes and couplings built in. A full description is in the owners manual.



Q:The modified RFL rules adopted by Robogames state:

4.4.2. All robot radio systems must be coded, mated pairs between transmitter and receiver. This means that no other transmitter, operating on the same frequency, can communicate with your receiver, and your transmitter cannot send signals to any other receiver than your own. Examples of such systems are Spektrum, IFI, and XPS XtremeLink - these are just examples and should not be taken as a comprehensive list or an endorsement.

But then:

4.4.4. Toy radio systems are allowed at this event for robots up to 12 lbs with no active weapons.

4.4.5. RC systems on the AM band are allowed at this event for robots up to 12 lbs with no active weapons.

4.4.6. All robots that are either: a.) 30 lbs or above or b.) 12 lbs or above with an active weapon MUST use a radio systems on the FM band with PCM, IPD coding, a digitally coded 900 MHz or 2.4GHz system (for example IFI), or an approved custom control system.

So, would I be able to use a normal FM system with my Antweight?

A: Mark J. here: From the discussion on the on-line forums, I think the intent is to require ALL combat robots at Robogames to run 'spread spectrum' radio systems -- sections 4.4.4 thru 4.4.6 should have been removed. However, I've always had trouble with Dave Calkins' logic. Write to him for clarification: dcalkins@robotics-society.org. Dave sometimes ignores his email, so 'cc' a copy of your question to Simone Davalos: simone@robotics-society.org.

Q: I talked to Dave Calkins. He told me:

"Ant [you can] 75, Lw cannot."

A: Six words? I guess that helps a little, but it still leaves beetles, sublights, and AM radio uncertain. Robogames was once a popular and well-run event. I'm sorry to see it fall into confusion.



Q: Would it be possible to charge two 12 volt BattlePacks as one on an AstroFlight 110 Deluxe charger? What about two 24V packs? Would it just depend how they're wired?

A: The AstroFlight 110D can charge up to 24 NiCad or NiMHd cells in series. That's a nominal 28.8 volts. A single 12 volt pack can be charged at up to 8 amps, but the maximum charge rate starts to drop above 14.4 volts. A single 24 volt pack (or two 12 volt packs in series) can be charged at a maximum 5 amps.

It's possible to charge multiple packs in parallel, but the charge amperage gets split between the packs. Two 24 volt packs charging in parallel would each charge at only 2.5 amps maximum. That won't save any time compared to charging each pack by itself.

If you need to charge higher voltage packs or multiple packs in series, pay a few extra dollars for the AstroFlight 112D. It can charge up to 40 cells in series (48 volts nominal), can pump 8 amps into a single 12 or 24 volt pack, and can charge two 24 volt packs in series at 4 amps.

You might also consider the ElectriFly Triton2 charger.



Q: How do 2.4 Ghz R/C systems avoid interference without crystals?

A: Each time the transmitter is turned on it 'listens' to radio activity on the 2.4 GHz band, selects an unused channel, and transmits a code to the receiver to tell it what channel to use. Your 900 MHz or 2.4 GHz wireless phone works pretty much the same way.

More information at Spektrum R/C.

Q: So from what you're saying, the 2.4Ghz radio only works with the receiver it comes with? If not, do you have to sync it with a receiver?

A: When a 2.4 GHz receiver is used for the first time with a transmitter it has to be 'taught' the receiver's code in a process called 'binding'. You only need to do that once, and you can bind multiple receivers to the same transmitter.



Q: Where can I buy sensors for a self-controlled antweight?

A: For a start, browse:

Q: After seeing Team Tentacle's 'Thinkling' I've decided to go with the Baby Orangutan MCU and two Devantech SRF08 ultrasonic rangefinders. How do I wire all of this? Is there anything else I need for the electronics? Thanks.

Thinkling robot A: Mark J. here: hold on there, Cowboy... I'd say you were more than a little ahead of yourself. Nobody who's ready to build an autonomous combat robot is going to write to me and ask how it all gets wired up. Even if you can get your sensors, MCU, motors, weapon (you're missing a weapon controller), and remote activation R/C (you're missing that, too) wired correctly, the robot is just going to sit there without software. How are your programming skills?

Sit down with a good book on autonomous robots, like Robot Programming: A Practical Guide to Behavior-Based Robotics by Joe Jones. Technical information on how to communicate with the SRF08 is at the Devantech website, and the Pololu website has links to documentation for the Baby Orangutang.

Once you're confident that you can 'wire up' and program the robot controller and peripherals, write back and we can discuss design issues and combat theory.



Q: Will one radio be compatible with all bots as long as the bot and radio have the same crystal?

A: Mark J. here: it isn't quite that simple. AM systems are incompatible with FM systems. 'Negative shift' coding (Futaba, Hitec) is not compatible with 'positive shift' (Airtronics and JR). Some 'third party' receivers can use either shift pattern. If the transmitter and receiver have the same coding, operate on the same frequency band (27/49/50/53/75/900/2400 MHz), and use the same modulation (AM or FM) then yes, matched frequency crystals (75 MHz and lower) will probably make them compatible. The 900 and 2400 MHz radios do not use crystals.

Stick with a single manufacturer for transmitters and receivers and you should be OK.



Q: Is the GWS GWT-4A 75Mhz FM R/C System a good system for a first time bot builder? I'm going to make an ant to start out with so I make $3 mistakes instead of $300 ones while building my 'bot.

A: For your first R/C system, you can:

  • Start with something simple and upgrade when you need to,
  • Start with a system that will handle your future needs and save the trouble (and expense) of a later upgrade.
I like the second option.

The GWS GWT-4A has no fail safe capability, no channel mixing, no dual rates, no exponential response, no receiver battery pack, and it will require at least one extra set of frequency crystals for competition use. Just plain vanilla R/C. It's cheap to purchase (a little over $100, with listed options), but it's also gonna return little of the purchase price when you go to sell it -- and you're gonna need a better system as soon as you move up past the hobbyweight class or go to an active weapon.

For less than $200 you can purchase a system like the Spektrum DX6 with the BR6000 receiver. You'll get all the features you're gonna need for any practical combat robot in any weight class. It requires no extra crystals, has full fail-safes, three channel mixes, dual rates, exponential response, a receiver battery pack, and four micro servos that you don't need and can sell on EBay to further reduce the price. If you should decide that combat robots aren't your thing, you can sell the Spektrum for a good price to someone who made the $100 mistake of buying a cheap system for their first robot.



Q: Are 2.4ghz systems legal in the featherweight and above weight classes?

A: Sure -- the 2007 RFL Standard Extensible Rule Set allows use of any ground-legal frequency (27/49/50/53/75/900/2400 MHz in the United States) in any weight class. See the FAQ (#18) for specific fail-safe requirements.



Flexi Flier Q: How do I control the two articulated halves of a robot with a design like 'Flexy Flier' with only one transmitter?

A. The usual solution is to use one transmitter and two receivers all on the same frequency. Put one receiver in each articulated section, plug the local ESC or weapon controller into the appropriate output slot and you're set. It's more reliable than running long extension leads from one side to the other and risking failure from flexed and twisted wires.

Some non-articulated 'bots also use two receivers for redundant back-up. JuggerBot / Tricerabot used twin receivers for separate dual-channel front and rear speed controllers. If either receiver or ESC failed, they could continue the match with at least partial power.



Q: Is there any advantage of using a Vex Transmitter-Receiver as compared to a HiTec Laser 6? We do not need to program, it will be only RC driven.

A: The Vex Robotics System radio control gear has a different data format than hobby R/C systems. It is not compatible with standard combat robot components. Use the HiTec!

Comment: I have some information to add to your answer to the question about the Vex radio system. The Vex transmitter uses pretty much standard Futaba data encoding, it's the receiver that's the problem. If you replace the receiver the system works just fine with standard R/C equipment.

I've tested the Vex transmitter with Microbotparts and Futaba AS receivers. They work well and can even use the Vex crystals. Microbotparts have a 6 channel receiver available for $29.95, though it is not on the webite -- just ask. I've also used the Vex transmitter with the Sombra Shadow 3 and Polk Hobbies' Seeker 6 synthesized receivers.

Great site, keep up the good work. [Wreno - North Texas Battle Group - BattleBots on the water with a WW2 theme]

Reply: Thanks, Wreno. I knew the data outputs of the Vex system weren't compatible with standard R/C, but didn't know the source of the problem. A 6-channel transmitter with crystals for that price is a bargain, but robot guys should be aware that the Vex transmitter has only primitive channel mixing and no ATV, fail safes, exponential response, or dual rates.



Q: How do you control a bot that uses automotive steering? The only way I can can think of is to use a pistol grip radio.

A: A pistol grip radio will work, but we use standard twin-stick transmitters: left stick forward and back for the speed controller, right stick 'side to side' controls the steering servo. No mixing required. You could put both throttle and steering on one stick, but we like them separate.

We set up our differential steering 'bots the same way, throttle on left stick and steering on the right, using elevon mixing. See our transmitter programming guide for more info on channel mixing.



Q: How do I get an 'inverted' switch and how do I wire it?

A: Mark J. here: when an invertible robot is flipped upside down, left / right steering response remains correct but the throttle response is reversed; a forward transmitter command will back the robot up and vice versa. An 'inverted' switch reverses the response of your throttle to compensate for this, but you don't usually add-on an inverted switch -- it's already there:

  1. Many twin-stick transmitters will have a toggle switch that controls channel 5, or channel 3 on pistol style radios. Some speed controllers (like the Scorpion HX) and some on-board mixers (like the IMX-1) have an extra input lead that will reverse throttle response. Plug that lead into appropriate channel on your receiver and the toggle on your transmitter becomes an inverted switch. The Scorpion HX ESC can also reverse throttle automatically in response to a 'gravity switch' on the robot itself, but there is too much vibration on a combat robot for me to recommend that.

  2. If you have a computerized transmitter with user-defined functions, you may be able to assign a custom function to one of the available switches on the transmitter to reverse the throttle response. See my tutorial on Programming the Futaba 6XAPs for Combat Robotics for more information on computerized transmitters.

  3. Non-computerized transmitters usually have a mechanical 'servo reversing' slide switch for each channel. You can simply flick the reversing switch for your throttle channel when inverted. You may want to fasten an extension onto the small reversing switch to make it easier to find in combat. I suppose you could replace the small slide switch with a larger toggle switch if you really want to do a mod.
All of the above only work if you're using some form of channel mixing. If you're using two-stick 'tank steering' you're out of luck -- reversing the throttle direction would goof up your steering response.



Q: Is a 50 MHz radio legal to use on a bot?

A: Mark J. here: the Robot Fighting League accepts use of 27/49/50/53/75/900/2400 MHz radio systems for combat robot control in the USA, as long as the radio system has the required failsafe protocol for your weight class and weapon type. However, you must have an amateur radio operator license to legally operate a radio transmitter on either 50 or 53 MHz.

Be sure to check with the event organizer to make certain they don't require some specific radio type. Some events are talking about going to only 2400 MHz 'spread spectrum' radios.

Q: How come a 72 MHz radio is not legal to use on ground-based vehicles?

A: Model aircraft are particularly sensitive to interference because their altitude gives them increased 'line of sight' reception. The Federal Communications Commission (FCC) made 72 MHz 'airborne only' to assure R/C pilots of clear channels without interference from somebody playing with an R/C dune buggy in a backyard miles away. To even things out, 75 MHz is restricted to 'surface only' uses. All other approved R/C frequencies may be used for either surface or air.



Q: My 'bot has Lexan plastic side armor and an aluminum top, bottom, and front. Will the radio signals go through OK if the antenna is internal?

A: You have to pretty much fully enclose an antenna in a box of metal or other conductive material to block the radio signal by forming a Faraday cage. You'll want to keep your antenna spaced away from the metal surfaces and any source of electrical 'noise', but the signal will be able to penetrate thru the plastic sidewalls. Check the Ask Aaron Radio Reception Problems page for more info.



Q: Is a 2.4 GHZ radio considered coded FM?

A: The 2007 RFL Ruleset says:

All robots that are either: a.) 30 lbs or above or b.) 12 lbs or above with an active weapon MUST use a radio systems on the FM band with PCM or IPD coding, a digitally coded 900 MHz or 2.4GHz system (for example IFI), or an approved custom control system.

So, digital 2.4 GHz and 900 MHz radio systems are cleared to control any weight class 'bot with either active or inactive weaponry -- equivalent to coded FM.



Q: Can R/C signals go through UHMW polyethylene?

A: Non-conductive materials like wood, glass fiber composites, and plastics are transparent to commonly used R/C radio frequencies.



Q: Is there any advantage of using a Vex Transmitter-Receiver as compared to a HiTec Laser 6? We do not need to program, it will be only RC driven.

A: The Vex Robotics System radio control gear has a non-standard data output format from the receiver. It is not compatible with standard combat robot components. Use the HiTec!



Q: Is it true that radio signals can't go through carbon fiber? If it is, how do bots covered in carbon fiber work? And can R/C permeate metal, or is that why most bots have at least a tiny polycarbonate window?

A: Both metal and carbon fiber block radio signals. If the metal or carbon armor completely encloses the 'bot, the radio antenna must stick out thru the armor, or be mounted under a radio-transparent 'window'. See the Ask Aaron Radio Reception Problems page for more info.



Q: In LiPo battery performance stats, the maximum discharge rate is often given as 'XC', where X is a number. Is this some other form of measurement, or does that stand for constant?

A: The 'C' relates to the capacity of the battery pack: a pack with a capacity of 3000 mAh has a 'C' of 3000 mA, or 3 amps. If the maximum discharge rate for that pack is given as '20C', it would be 20 times 3 amps = 60 amps. If you exceed the maximum discharge rate you will risk overheating and damaging the pack, and even setting the pack on fire!



Q: How do I configure the radio system for a four or three wheeled omnibot? Where would I buy the mixer?

A: Robotlogic.com makes a three-wheel omni mixer called the OMX-3. They also make a Mecanum mixer for four-wheeled Mecanum omnibots.

With the mixer installed, you may assign any stick axis you like for forward / reverse, left / right, and rotate. Search the Ask Aaron archive for more information on omnibots.



Q: Is it safe to connect my HSR-5995TG digital robot servo to a 7.4V Li-poly battery? When the Li-poly is fully charged, it's around 8.4V, and I'm afraid of burning out the servo electronics.

A: All freshly charged rechargeable battery packs will read a higher resting voltage than their rating. When placed under load, the voltage will drop down toward the rated voltage. Hitec says the servo will run on a 7.4 volt LiPoly, so don't worry about the extra volt in the resting state -- run it!



Q: A couple of times I've drained my 3-cell Lipoly battery down until the weapon ESC starts cutting off at 9 volts. Am I harming my battery? Will I get less cycles out of the pack? Will this effect the battery's capacity and discharge rate? What is the average number of cycles for a lithium polymer battery?

A: Mark J. here: a three-cell lithium polymer battery can be safely drained down to 8.4 volts, so running into the 9 volt cutoff is fine. The usual cause of damage to Lipoly batteries is overheating caused by too high a discharge rate. Temperatures over 140 degrees can damage or destroy the pack. Leave some space around the battery for cooling air circulation and keep the discharge rate within the manufacturer's spec. A well cared for Lipoly battery can exceed 1000 charge/discharge cycles. Check the Electrifly Lipoly Manual for more tips on care and disposal.



Q: I need to be able to spin my 'bot in place to keep the heavy front scoop pointed toward my opponent. Will a controller that mixes channels like Vantec be capable of spinning one wheel one way and the other wheel the other way, or do I need to drive 'tank style' with one stick assigned to each controller to get that?

A: You can mix channels with a computerized transmitter, a plug-in electronic mixer, or a twin-channel controller with built-in mixing. All of them produce the same control style: one control channel (stick axis or throttle trigger) moves the 'bot forward and back, while a second channel (stick axis or wheel) controls turning. If you activate only the turning channel, the 'bot will spin in place without moving forward or back -- just like throwing the control sticks in opposite directions when working tank-style.



Q: I'm building a sumo robot for competition and I was wondering what the differences are with the different types of batteries? Is it possible to just use alkaline 9v batteries to power my motors? The motors that I plan to use are modified Black and Decker screwdrivers. Thanks!

A: Mark J. here: even small combat robots don't use alkaline batteries! Alkalines are designed to operate in devices with a low current requirement. They cannot provide anywhere near the peak current that NiCad or NiMHd rechargeable batteries can, and in a permanent magnet DC motor current equals torque.

A typical alkaline 9-volt battery can provide just over 1 amp of current, and will last a VERY short time at that current draw. Your screwdriver motors will need at least 5 amps each to develop their full pushing torque potential. A sumo robot that can't push is pretty useless.

A modern AA size NiCad or NiMHd cell can pump out 8 to 10 amps of torque-grinding current and keep it up long enough for a full sumo match. An 8-cell pack will give you a nominal 8.4 volts and would be a far better choice for your 'bot than 9-volt alkalines. Simple chargers for NiCad / NiMHd packs are inexpensive and the charging process is uncomplicated.

Your other choice is a lithium polymer battery. LiPolys are very light and compact for their power capacity, but are more expensive and have lower peak current capacity than NiCads or NiMHd packs of equivalent amp/hour rating. LiPolys also require a different type of charger than other rechargeable packs -- never try to charge a LiPoly battery with a charger not specifically designed for the purpose. A good article on LiPoly care can be found at electrifly.com



Making a Y-connection. Q: How do I make a 'Y' shape in my wire?

A: Twist three wires together, solder, and insulate with heat-shrink tubing. Do not use 'bullet crimp' or 'twist' connectors on a combat robot -- they can fail under impact and vibration. Use stranded wire instead of solid-core for the same reason.

Q: Could I make a 'Y' shape in my wire with 3 ring terminal conectors held together with a nut and bolt?

Don't do that! You'd have three 'crimp' connections each subject to failure, plus a nut and bolt that could vibrate loose. It would also be heavy and bulky. Even though I don't like them, you'd be better off using a 'wire nut' twist connector to hold your three wires together. Best solution: learn to solder.



Gyro wireing diagram Q: Hi Aaron. I want to build a two-wheeled robot with a gyro, but I'm having a bit of trouble understanding how that all fits together. Is there a specific type of ESC that I'll have to use to get this to work properly?

Will I be able to use just one control stick (forward, backward, left, right) to control the movement of my robot, or will I have to use two sticks, one for each motor? Thanks!

A: Take a look at my dad's Beginners guide to combat robot gyros page.

R/C gyros are NOT COMPATIBLE with either two-stick (tank style) steering or transmitter mixed single-stick steering. If you're using a gyro, you'll need to use either a stand-alone plug-in electronic channel mixer between your gyro and the ESC, or use an ESC with built-in mixing. Either of these will give you single-stick control.

Many dual-channel ESCs offer built-in mixing: Barello ANT, SOZBots M, Scorpion XL, Vantec RDFR, etc. Check before you buy.



Q: What's a 'mixer'?

A: Most combat robots turn by 'skid steering' where the wheels on one side of the 'bot turn at a different speed and/or in a different direction than the wheels on the other side. This requires independent control of drive motors on each side of the 'bot. You can operate this type of 'bot by controlling the two sides of the 'bot with vertical motion of the two control sticks on a 'stick' style R/C transmitter (called 'tank steering') but most drivers prefer forward/reverse speed control with vertical motion of one stick and steering with horizontal motion of either the same or the second stick.

By electronically 'mixing' the output of two R/C channels, you can have the vertical motion of one stick (or the throttle trigger on a 'pistol' style transmitter) instruct both drive motors to move the 'bot forward or backward, and the horizontal motion of a stick (or the 'pistol' steering knob) tell the motors to spin at different speeds. This is called 'differential steering'.

More expensive R/C equipment and some dual-channel motor controllers have channel mixing capability built-in. See our transmitter programming guide for more info on computerized radio transmitters, and our electronic gyro guide for help integrating mixers with gyros. If your equipment does not have mixing built in, you can buy a small electronic channel mixer that plugs into your system between the receiver and the speed controllers that will provide the same functions.



Q: Hi Aaron: your site has been very helpful -- thanks!

I have two ANT 100 ESCs. I plan to connect two drive motors to one and two weapon motors to the other. Can I connect the two ESCs to one 7.2V NiCad battery? Will the battery be able to provide the power for the two ESC and the four motors? If not, what voltage do I need? Do I need one battery for each ESC?

Thank You.

A: Thanks for your compliment about the site!

The short answer to your question is yes -- you can connect multiple Electronic Speed Controllers to a single battery if you connect them as parallel circuits. See the diagram and description of basic robot wiring in the Frequently Asked Questions section.

The longer answer involves the capacity of your battery to provide the current needed by your motors. Your battery must be able to supply enough current (not voltage) to meet the demands of all the motors at once. NiCad batteries can provide a lot of current to meet heavy load conditions, but if the load becomes too great the voltage output will drop. The Ant 100 ESC also supplies power to your radio receiver and if the voltage drops your receiver can start to 'glitch'. Other types of batteries are less able to meet high amperage drains. Lithium batteries can dangerously overheat if the current demand gets too high, resulting in damage to the battery and even fire!

Adding extra cells to your battery pack to raise the voltage is not a good solution. Higher voltage will create a demand by your motors for even more current and your voltage fluctuation will increase. Add up the maximum current consumption for all four of your motors and use a battery with enough capacity to meet that demand.

One last thing: I mentioned above that the Ant 100 supplies power to your radio receiver. Since you'll be using two Ant 100s, you might run into trouble with both of them trying to power the receiver. You'll want to remove both of the red wire connections from the receiver cables coming from one of the ESCs -- either one. You can clip the red wires, desolder them from the circuit board, or remove the connectors from the receiver plugs.



Q: How do I add a charging jack to my 'bot?

Charging jack wiring diagram. A: Mark J. here: a battery charging jack is a very useful addition to your 'bot. It will minimize charging mistakes in the pits and save critical time between matches.

Select a power plug and wires rated for at least the maximum output of your charger. Find a mounting spot protected from damage but with easy access. The jack will be 'live' to the main battery power, so protect it from accidental shorting!

The jack is wired into the main power cables, between the battery disconnect plug (optional, but handy) and the master power switch. See the diagram at right for details. Check the event rules to determine what type of master power switch is required for your weight class -- a 'removable link' disconnect may be required.

With this set-up, the battery may be charged with the master power switch 'off' for safety, and the battery may be quickly removed or replaced while the charging jack remains with the 'bot.

Q: What type of power plug do you recommend for a heavyweight 'bot charging jack?

A: I use Anderson PowerPole connectors for charger plugs on our larger 'bots. They are available in 15/30/45 amp ratings, have no exposed metal, may be crimped or soldered in place, and require no 'heat shrink' insulation. The PowerPole connectors are available at many hobby shops. Pre-assembled wire sets with PowerPole connectors are available thru Team Delta.



Q: Hi Aaron: My name is Alan. I have purchased all the parts for a tank robot, but I need help hooking everything up. I have: an R/C system, a twin-channel drive motor ESC, a weapon ESC, a master power switch, a 7.2v NiCad battery, and head and tail that should light up when the robot is turned on. Could you please advise me how to hook all this together? Thank you very much.

A: Take a look at the diagram and description of basic robot wiring, Alan. Wire in your power switch where the removable power link is in the diagram -- combat robot tournaments often require a removable link instead of a switch. Your multiple lights will replace the single power indicator light in the diagram.

Q: Thanks for your advice. Just to clarify, do I attach three wires to each battery lead and run one + and one - to each ESC and the lights? Thank you very much for your reply.

A: That's right, Alan -- the ESCs and lights are connected in parallel to the battery. Don't forget to insert your switch between one battery lead and the device connections.

Q: I forgot to ask you this question last time. The different parts for my robot have wires of different gauges. Do I have to replace all the wires in one gauge? Will the robot function with wires in various gauges connecting together? Thanks for your reply.

A: Don't worry about the different wire gauges, Alan. The important thing is that the smallest wire in a given circuit is capable of carrying the largest expected current for that circuit. Bigger wire is OK, but smaller wire can overheat under the load, melt thru the insulation, and short out. As long as your robot components are operating within their rated voltage, you should be able to trust that the manufacturer has provided wire of adequate size. Hook 'em up and run 'em!



Q: Does the diameter measurement of 18 gauge wire include the plastic wrapping around the wire?

A: No -- American Wire Gauge (AWG) sizes measure the conductor only, not counting any insulation. The conductor diameter of 18 gauge wire is about 0.04 inch (1.02 mm).



Q: An antenna on my 2.4 GHz Spektrum receiver got sliced during a fight. It now only has 1/3 of it original length. Will that ruin the reception of my receiver? What is the best way to fix this?

A: Shortening the antenna that much will certainly reduce reception, but you may still have plenty of range. Give it a test run and see how it does.

If you decide to repair the antenna, you can just solder on a length of similar gauge insulated wire to restore the length. Cover the solder joint with heat shrink tubing or a flap of vinyl tape to prevent accidental grounding of the antenna. If you are comfortable soldering PC boards you can open the receiver case, remove the damaged antenna entirely, and solder on a new wire of the correct length.

Q: What is impedance? I hear that even if I solder on a length of similar gauge insulated wire to restore the length, I need the correct impedance as well.

A: Mark J. here: Impedance is a measure of the opposition of an electrical component (like an antenna) to an alternating current (like a radio signal). A proper antenna must have its impedance match the other elements of the system to maximize signal strength.

The main factor determining the impedance of your simple wire antenna is length. Restoring the antenna length by soldering on a length of similar wire will restore the impedance to that of the original antenna.



Q: I'm having problems controlling my newly finished ant. When I push my drive stick straight forward quickly, the 'bot spins in place. When I do it gradually, the 'bot does go forward but it's not straight. The back direction is fine though. I tried to mess around with the trim but it didn't help that much. My bot is using the Spektrum DX6 radio system with delta wing (elevon) mixing. What do I do?

A: Mark J. here: How quickly you throw the stick forward shouldn't matter to the radio system, so I don't think your problem is entirely with the transmitter set-up.

First, check the set-up procedure in the manual for your Electronic Speed Control (ESC) to make certain that the unit is correctly adjusted to work with your radio. An ESC requires careful adjustment to assure that the motors will respond correctly throughout the entire range of transmitter stick motion. Make certain that your transmitter trim settings are centered during ESC set-up.

Once the ESC is correctly set, the transmitter trim should be adjusted so that both wheels start turning at the same time when the stick is gently pushed forward or back. If you're still veering to one side, reduce the Adjustable Travel Volume (ATV) transmitter setting for the side of the 'bot that is running too fast.

If rapid acceleration is still a problem, check for equal weight on the drive wheels on both sides of the 'bot. If one side of the 'bot has more weight it will get better traction and can cause a spin under hard acceleration. You may need to move some components on the chassis to equalize weight. Backing up may mask the problem by reducing traction to both drive wheels.

For a 4-wheel 'bot, make sure all four wheels are in good contact when the 'bot is on a level surface. Adjust the chassis or motor mounts to correct if required.

For additional help on transmitter set-up, I suggest reading thru my guide to programming radio systems for combat robots. I wrote it with specific reference to Futaba systems, but the general tips are useful for any computer radio.



Q: I've seen some people use joysticks to control their 'bot. Where do you get joysticks?

A: You probably saw the IFI Isaac Control System that is commonly used at the BattleBots IQ competitions. Some builders use this system at other competitions as well.

The IFI system accepts a variety of analog PC joysticks as external controllers. These are the same joysticks you can buy at your computer store. PC joysticks are not compatible with standard R/C transmitters.



Battery pack wiring. Q: I'm having trouble building a battery pack from four 'AA' cells. Can you please show me how it's supposed to be done?

A: Check the diagram at the right to see what gets connected to what. Solder the connections with rosin core solder and a non-acid flux. Watch out for sharp points on your connections that could poke thru insulation -- file smooth any rough edges. Large diameter 'heat shrink' plastic tubes are available at R/C hobby shops that will insulate the pack and hold it all together. Smaller diameter heat shrink does a great job of insulating the connector terminals.



Q: How do I connect wires to NiCad batteries?

A: NiCad or NiMHd cells come with or without metal tabs welded to either end. You can solder your wires to the tabs or directly to the cells. If you're soldering directly to the cell, you'll need a soldering iron with enough power to do the job quickly without heating up the whole cell -- maybe 25 watts. Use rosin core solder and a little non-acid flux.



Q: Can I equip a 27 MHz bot with a lifting servo?

A: Mark J. here: the operating frequency of your radio does not effect the ability of the system to accommodate a lifting servo. However, the 27 MHz band is used by both 'toy' radios and 'hobby-grade' radio systems. Toy radios are not compatible with hobby grade components like servos.

Toy radios in the USA are on either the 27 MHz or 49 MHz bands. Hobby grade radio systems are commonly available in the USA for the 27 MHz, 72 MHz (aircraft only), and 75 MHz (non-aircraft) frequency bands.



Q: What is PCM? Is it less susceptible to 'electrical noise' than an FM radio control system?

A: Mark J. here: We need to be careful not to mix apples and oranges:

AM (amplitude modulation)and FM (frequency modulation) are two methods of adding information onto a radio wave. All hobby radios are either AM or FM. Of those two methods, FM is less susceptible to electrical 'noise' interference.

PPM (pulse position modulation) and PCM (pulse code modulation) are two methods of encoding the information for radio control before it is added to the radio wave.

  • PPM is an analog coding system with servo position represented by a time interval between two pulses. It is the standard system used on hobby radios.

  • PCM is a digital coding system with servo position represented by a binary number. A computer in the receiver decodes the number and instructs the servo or speed controller. If no signal reaches the receiver, the computer can send a predetermined 'fail safe' position signal to the servo that can stop the robot and turn off weapons.
PPM and PCM are equally susceptible to interference, but the PCM system responds in a predictable way if the signal is corrupted or absent. Some form of fail-safe encoding is required for larger combat robots, particularly those with active weapons. Check your event rules!



Q: Have you ever seen the bot 'Al'? It has a home made radio system. Where did it get the R/C electronics?

A: Mark J. here: Is that the middleweight flamethrower from Minnesota? I don't know about their radio, but the homebrew radio systems I've seen from the few other teams to try it were unreliable and had no advantages over off-the-shelf systems. If you just happen to have a degree in electrical engineering and a lot of time to spend designing, building, and de-bugging a radio system -- go for it. If you want something that works, stick with a professionally built system.



Q: Dear Aaron, for the Critter Crunch robot combat tournament you are allowed to have tethered robots. Can you explain how to do this?

A: Critter Crunch has been fighting robots for a long time. Their first tournament was years before the first Robot Wars, but their rules are, ummmm... different. They do allow both 2 pound and 20 pound robots to operate with a wire cable instead of radio control. The circuit for each motor and weapon runs from the battery thru a long cable to a control box then back thru the cable to the motor/weapon. The control box has switches and maybe rheostats to control the motors. The cable wires have to be heavy enough to carry the motor current, and the robot has to drag the long, heavy cable behind it.

Critter Crunch also allows R/C robots, so why not build a 'bot that can enter other contests?



Q: Is there any significant difference between the Victor 885 and Thor 883 speed controllers?

A: No functional difference. The only reason Robot Marketplace carries the discontinued Thor 883 is so builders who have a pair and smoke one can get an exact replacement.



Q: Can you use an ant weight speed controller in a hobby weight bot?

A: Sure - as an expensive smoke bomb.



Q: Where can I buy a mixed R/C controller?

A: It would save time if people would check Robot Marketplace before asking me any question that starts with "Where can I buy a...".

Pretty much any place that sells R/C equipment will offer systems with computerized mixing. For a 'bot, you'll want a system on the 75MHz band. Guess what? Robot Marketplace sells a very nice Hitec Laser 6 FM System for less than $135. Check with your local hobby shop as well.

You can also find Electronic Speed Controllers with built-in mixing at (wait for it...) Robot Marketplace.



Q: Does BBIQ use the IFI controller? What is the point having these things?

A: Mark J. here: the IFI Isaac Control System has long been the mandatory R/C system for the BattleBots IQ competition. However, the system has been discontinued by IFI, and used systems are in short supply. BBIQ now strongly encourages the use of the Isaac system, but approves alternate systems on a case-by-case basis.

The Isaac controller is a 'smart' system that prevents radio interference and the need for frequency control and transmitter impound at tournaments. This makes things run much more smoothly for the tournament director. The Isaac also interfaces to the tournament computer and allows the director to remotely shut down power to all 'bots instantly - a big safety plus!



Q: Can you show me how to connect the receiver to an ESC?

A: An ESC designed to interface with a hobby R/C receiver should have a plug already installed that will fit directly into the channel output socket of your choice on the receiver. See the 'bot wiring diagram. Ask the manufacturer of your ESC for additional info, if needed.



Q: What is the difference between 27MHz and 49MHz? [zackrocks1994]

A: Mark J. here: radio systems are assigned to operate on specific frequency 'bands'. Both of the frequency bands you mention are available for use by R/C toys. MHz refers to how many million times per second the radio 'cycles' -- 27 million cycles per second for the 27 MHz radio.

Within each band are several specific frequencies to which a radio system may be tuned. A radio tuned to a specific frequency will not interfere with a radio operating on another frequency.

See also this earlier post.



Q: What is the cheapest 2 channel speed controller?

A: You can scavenge the speed controller out of a toy or servos if you're really cheap, but remember: 'cheap' and 'combat robot' don't mix. If you're going to invest your time and effort, don't allow cheap components to let you down!



Q: What is a 'piezo gyro'? How are they used on combat robots?

A: Two-wheeled robots can be difficult to drive in a straight line without veering to one side and they don't turn in a smooth arc. A piezo gyro senses turning motion, compares it the signal coming from the R/C receiver, and adjusts the signal to the electronic speed controller to keep the robot on the course the driver wants.

My dad has a whole webpage about gyros and combat robots that should answer any additional questions.



Q: How much is the cheapest FM R/C radio system?

A: Mark J. here. The message doesn't seem to be getting thru: cheap and robot combat don't mix. You're going to spend a lot of time and effort building your 'bot and going to the tournament. When some component fails in combat and puts you out of the match, you're gonna wish you hadn't gone cheap.

That said, Tower Hobbies has a Hitec 3-channel pistol grip FM system for $69.99. You'll need either an ESC with built-in mixing or a separate elevon mixer to use it with a 'bot. Inertia Labs will sell you a GWS 4-channel twin-stick FM system for $89.90 (transmitter with crystal $59, micro receiver $21.95, receiver crystal $8.95) that does not require a mixer. Best luck.



Q: What are 'R/C crystals'?

A: Mark J. here: a piezoelectric crystal can be made to vary its electrical properties at a very precise frequency. When incorporated into a radio oscillator circuit, the crystal controls the frequency 'channel' on which the set operates. Hobby R/C systems have removable crystals in both the transmitter and receiver that can be replaced with crystals of a slightly different frequency to tune the system to a different channel and avoid interference from other radios.

R/C systems normally come with one set of crystals. Robot tournaments usually require that you have crystals for at least two different channels, so plan to buy a second set. There are different types of crystals for AM, FM single conversion, and FM double conversion -- make sure you get a set that matches your radio.



Q: How do you control a tank style drivetrain robot with a spinning weapon when you have two hands and three controls?

A: It doesn't take a whole hand to control a R/C channel -- you can easily control two with one finger. R/C airplane guys have to balance throttle, rudder, elevator, aileron, flaps, and landing gear controls. We've got it easy!

For a 'bot, throttle and steering take two sticks if you use simple tank-steer, but you can electronically 'mix' two channels and put both throttle and steering on a single stick operated by one thumb. The on/off control for the weapon can be assigned to a toggle switch that you can flip with an index finger. That leaves seven fingers and a thumb that aren't doing anything except holding the transmitter.

Different manufacturers have slightly different control layouts, but you can see a diagram of a typical Futaba transmitter and get more radio tips at my dad's page: Programming the Futaba 6XAPs for Combat Robotics. I usually run the weapon from the channel 5 toggle switch.



Q: Do LiPoly batteries need to be charged periodically to maintain capacity? I haven't touched my pack for about a month -- do you think the capacity dropped a significant amount?

A: Mark J. here: LiPoly batteries have excellent charge retention and capacity recovery after storage. A LiPoly battery stored for six months at room temperature will recover about 95% of its capacity on the first charge cycle. It's still a good idea to discharge/charge cycle your rechargeable battery (LiPoly, NiCad, NiMHd) a couple of times before a competition to assure full capacity. Always follow the manufacturer's procedure for cycling.



Q: The R/C toy car I just bought only responds within about 5 feet of me. Can I fix this, or should it just be replaced?

A: Make sure the batteries in the car and the transmitter are fresh. If this doesn't help, as a last resort you might want to open up the car to expose the electronics board. There will be a couple of screw adjustments on the board, sealed with a thick, waxy material. Pick one adjuster, scrape away the wax, mark the starting position of the adjuster, and try turning the adjustment screw a little one way and the other to see if the range improves. If not, return it to the starting position and tweak another adjuster. Do NOT try this with the transmitter!



Q: Is 49 MHz the same as the AM band?

A: Mark J. here: In the USA the "AM Band" refers to commercial radio broadcast frequencies between 520 kHz and 1720 kHz -- a kilohertz is 1000 cycles per second. 49 MHz (49 million cycles per second) band is a 'public service' frequency range a little below VHF television broadcast. It's shared by older wireless phones, baby monitors, and five toy R/C channels. Other toy R/C systems operate on 27 MHz on six frequencies squeezed in between CB radio channels.

Toy R/C systems on 27 or 49 MHz use interference-prone Amplitude Modulation (AM) signals, but 'hobby grade' R/C gear on 27, 72, and 75 MHz use either AM or FM (Frequency Modulation) signals.



Q: Where can I find 75 MHz R/C systems?

A: The Robot Marketplace and Tower Hobbies are both good on-line sources of 75 MHz 'ground frequency' radio systems, but don't forget to check with your local hobby shop!

It's also possible to have a 72 MHz radio converted to 75 MHz. Tower hobbies offers this service on new systems, and a web search will turn up other specialty shops that will do this.



Q: I've found micro receivers to be really glitchy and almost uncontrollable beyond 5 feet. Will using a standard receiver increase the range?

A: Mark J. here: Micro receivers are 'single conversion' designs that are more sensitive to interference than the 'double conversion' design usually found in standard receivers, but if you're only getting 5 feet of reception distance you have problems other than your receiver. Electric motors create a lot of electrical 'noise' that can be a problem -- try adding anti R.F. capacitors across the motor leads. Position the receiver as far away from the motors and ESC as possible. Stretch out the receiver antenna and get it out in the open air, away from metal or carbon armor.

Check the Ask Radio Reception Problems page for more info on the topic.



Q: I've boosted the voltage to my toy R/C controller from 6 volts to 12 volts and it's working fine with the toy R/C motor. What will happen if I try the same 12 volts with a larger motor?

A: Tech question, Mark J. here: You'll get a nice puff of smoke followed by a quick trip to the trash bin. Solid state switching controls are rated for a maximum amperage flow. Doubling the voltage to a motor also doubles the maximum amperage, so you've already taken the unit to twice it's design limit. I suspect that if you got into a pushing match you'd fry the controller as is. Bigger motor = more amperage = thermal meltdown. Note that not many R/C receivers can handle 12 volts -- try that at your own peril.



Q: How does an R/C system actually control a robot?

A: A remote control transmitter sends variable control signals for several `channels' based on stick, switch, and knob positions on the transmitter. The receiver deciphers the signal and sends separate signals to the channel ports. You can plug various devices into channel ports that read the signals and turn them into mechanical action (servos), variable current flow (electronic speed controllers), or on/off switches (R/C switches). Connect drive motors to the speed controllers, a lifter to the servo, and a weapon to the R/C switch and you've got control of your robot. For more on robot control systems, see: 4QD Robot Control website.



Q: Is any programming required for ESCs and the radio system?

A: Some combat robot ESCs have a short set-up process, but it isn't programming. Some brushless motor controllers have more elaborate programming capability, but most work fine with the default settings. More sophisticated radio systems can be programmed by selecting options from menus for things like channel mixing, but it isn't required to provide basic radio functions. See my dad's page on programming transmitters for details.



Q: Can I use 22 awg wire to connect electronic components in my antweight?

A: Tech question, Mark J. here: AWG wire sizes run opposite from the way you'd think, with larger numbers indicating smaller diameter wire. For small wire, amperage capacity approximately doubles with each decrease of three number sizes (thicker wire). A 22 gauge copper wire is conservatively rated to carry 7 amps in conditions found in a combat robot. You might get away with it, but I'd suggest 18 gauge for at least the battery to ESC hookup -- or you could use double strands of 22 gauge if that's what you have.

Always use 'multi-strand' wire in your robot, not solid-core. Solid core can break from repeated flexing and shock.



Q: Are there differences between micro receivers and standard receivers other than size?

A: Technical question - Mark J. here: Yes, there are a few:

Some micro receivers are not compatible with 'pistol-grip' style transmitters.

Most quality standard receivers use 'dual conversion' frequency crystals, while most micro receivers require 'single conversion' crystals. The two crystal types are not interchangeable.

Different brands of transmitter have different signal formats -- Futaba and Hitec are 'negative shift', while Airtronics and JR are 'positive shift'. Make sure the micro receiver you buy is compatible with your brand of transmitter.



Q: What kind of cheap radio system do you recommend to control two motors and a spinning weapon?

A: Check with the event organizer before attempting to use any 'cheap' radio system in a 'bot with a spinning weapon. Many combat rules sets require specific safety measures for weapon safety that are not available in inexpensive radios. For robot combat it isn't smart to try to save money on your radio system.




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