Questions and Answers about Combat Robotics
from Team Run Amok

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Ask Aaron Li-Poly Battery FAQ

Li-Poly FAQ
The 'Ask Aaron' Lithium Polymer Battery FAQ
'Ask Aaron' gets a lot of questions about Lithium Polymer (Li-Poly) batteries -- enough to merit a special 'Frequently Asked Questions' page on the topic. I consider this to be a 'beta release' for the moment. What did I leave out? Comments welcomed.

Q: In your LiPoly Battery FAQ, I'd recommend adding a "how to charge" paragraph, highlighting the importance of charging via balance-charge, and discharging only to storage voltage (the latter's particularly important, because lithium ion batteries by contrast SHOULD be fully discharged from time to time) I would never have figured those things out if someone didn't tell me. Also, this video really helped explain how to use a standard charger. [Killeen, Texas]

A: [Mark J.] I appreciate your feedback on the LiPoly FAQ. Adding that charging video is a fine addition. I'll take this opportunity to clear up some misconceptions about LiPolys and Li-Ions:

  • LiPoly batteries sold on the hobby market are lithium ion batteries. They're packaged in a lighter, flexible polymer casing rather than the rigid case used in conventional lithium ion batteries.
  • Discharging any lithium ion cell below its minimum safe voltage will cause permanent and irreparable damage.
  • Lithium ion batteries in commercial products have protection circuitry built-in. The Li-Ion battery in your computer or phone that says it's 'completely discharged' has been shut down by the protection circuitry while the cells are still at a safe voltage.
  • LiPoly batteries sold in the hobby market are raw Li-Ion cells directly wired together without protection circuits. They are succeptable to being over-discharged if the circuit they are powering does not incorporate undervoltage protection.
  • Most speed controllers used in combat robots have a 'LiPoly protection' mode that shuts down the controller when the battery voltage drops to some specified level. Most combat robot builders will (correctly) turn this protection off to prevent the controller from shutting the 'bot down in combat.
Next time 'somebody tells you' about lithium batteries, check their sources.

Q:Thanks for clearing that up for me. I think you've still highlighted an important point worth mentioning: that "raw" batteries for robots need to be treated differently than that of your phone or laptop - it strikes me as an easy (and dangerous) mistake for new builders to make.

A: I think you're right. I'll add a note to the LiPoly FAQ. Thanks!

Q: Hey, I would like to consolidate some of my understanding on lithium-polymer batteries. If I want to run two separate batteries in together in a single robot, then the milliamp hours are not cumulative, but the voltage is -- am I correct? So, if for example, I run two 6,000 mah 11.1 volt batteries together, the result would be a 6,000 mah 22.2 volt power source? Do I also need to be wary of running the batteries in series or parallel? Thanks a lot! [Bellevue, Washington]

A: [Mark J.] With two batteries you have a choice:

  • Two 11.1 volt 6000 mAh batteries in series will give 22.2 volts with a 6000 mAh capacity.
  • Two 11.1 volt 6000 mAh batteries in parallel will give 11.4 volts with a 12,000 mAh capacity.
You can get either double the voltage or double the capacity, not both.

As long as the batteries are the same voltage and capacity you'll be fine running them either in series or parallel.

Two battery packs in series.

Two battery packs in parallel.

Futaba T6J Transmitter Q: Hi, I’m a bit of a noob, so I have a [Spektrum] Dx6i [transmitter] and I’m not sure how to reverse the elevon for mixing?
Thanks! [direct email]

A: [Mark J.] So you've got your transmitter mixing all set-up, but when you move the aileron stick to the left the robot spins to the right? Or maybe when you push the elevator stick forward the robot backs up or spins? Yes, I can help you sort that out.

The Team Run Amok Guide to Combat Robot Radio Systems has a special section on How to setup Elevon Mixing for a Combat Robot that will walk you thru correcting problems like these.

You might also be interested in reading thru the Troubleshooting Robot Control Problems section of the Team Run Amok Transmitter Programming Guide. The guide is written for Futaba transmitters, but the control troubleshooting section applies to all brands.

If you have a specific problem you can't sort out, write back and give me the details.

Q: if i power an electric motor to use it as a weapon for a lifter a little bit like sewer snake with the ''throttle'' stick on my transmitter, will it send a constant signal and so i wont be able to stop the lifter at the end of his course, like if it was a spinner? [Quebec, Canada]

A: [Mark J.] Your weapon motor will not be powered directly from the R/C receiver. The receiver output will instruct either a speed controller or a relay board to do the hard work of controlling the weapon motor. How the weapon system handles power when the lifter at the end of weapon travel depends on the specific device controlling it:

  • 'Sewer Snake' uses an electronic speed controller to control its lifter motor. When the weapon reaches the desired position or the end of travel the driver releases the spring-centered transmitter stick and power to the weapon motor shuts off.
  • Some electronic speed controllers have provisions for limit switches that will cut power when the weapon reaches the end of travel. See the VEXpro Jaguar FAQ for an example.
  • The electric lifter on Team Run Amok's beetleweight 'Zpatula' uses a Team Delta dual relay board that has inputs for 'limit switches' that stop the weapon motor automatically when it reaches the ends of the lifter travel. This allows the weapon to be controlled by a simple toggle switch on channel 5 of the transmitter; flip the switch and the lifter goes up, flip it back and the lifter returns to the start position.
Search the Ask Aaron Robot Weapons archive for "directional limit switch" to find an earlier post on this topic with a link to additional help.
Q: I hot glued some wires to insulate them, and after a particularly intense match, I found that the glue had melted. Other than that, the insides were fine. Is this concerning? It's a Beetleweight robot with a 1000 mAh battery, if that makes any difference. [Andover, Massachusetts]

A: [Mark J.] It's concerning that you'd use hot melt glue on something that has the potential to get hot!

It would help if you'd mentioned what the wires are connecting and where you applied the 'insulation'. Common low-temperature hot melt glue starts to soften around 140 degrees farenheit. Some parts of your wiring -- motor connections, for example -- can routinely get MUCH warmer than that.

If you need to apply a coating to insulate an electrical connection that gets hot, pick up a small tube of Silicone sealant at your local hardware or auto store. It's an excellent insulator, easy to apply, and when cured it withstands at least 400 degrees farenheit without complaint.

Q: Hey Mark, question regarding 'coil' and 'contact' voltage for a solenoid. I've got a solenoid here that has a 24V 'coil' but the contacts can handle up to 48 V. I'm planning on using an 8S lipo for the weapon battery so continuous voltage would be somewhere in the realm of 30V. So I guess I can't power the coil directly with the weapon battery? What am I supposed to do... use a separate battery pack just for my solenoid coil? Seems a little counter-intuitive. [Charlottesville, Virginia]

A: [Mark J.] You bought the wrong solenoid. Manufacturers typically offer a variety of coil voltages for a given solenoid to allow flexibility in the control circuits. There is some tollerance for over/under voltage on the coil, but it's typically +10%, -25%. Check the specification sheet for your specific solenoid.

What you're 'supposed to do' is return the solenoid and get one with the coil rating you want.

If you don't like that option, it is possible to add a power resistor in series with your coil to allow it to operate at voltages higher than its rating. The formula to calculate the resistance needed is:

Resistor Ohms = ((Operating Voltage / Rated Voltage) - 1) * Resistance of Coil

Example: to safely operate a 24 volt coil with a resistance of 60 ohms at 30 volts requires an additional resistance of:

((30 volts / 24 volts) -1) * 60 ohms = 0.25 * 60 ohms = 15 ohms

The power resistor will require a power rating equal to the power consumption of the coil at its rated voltage -- see your contactor documentation.

Q: Mark, I see what you mean, looking at the specs for a White-Rodgers solenoid. My solenoid in question is the Whyachi C1 contactor. The data sheet mentions only a 24 VDC coil and current ratings for 48 VDC for the contacts... not as clear. Any advice on this? Thanks.

A: You have a few options:

  • I make the coil resistance about 50 ohms -- add a 10 ohm 15 watt power resistor in series with the coil.
  • Ask Team Whyachi ( what the actual voltage tollerance is.
  • Run it and hope it holds up.

Q: Do LiPo's come charged fresh out of the box? The other day I was fitting some connectors to an unused, uncharged battery, when I produced a spark and a tiny wisp of smoke. [Andover, Massachusetts]

A: [Mark J.] Lithium polymer batteries are severely damaged if discharged below about 3 volts per cell, so they are stored and shipped with a partial charge -- typically about half of their rated capacity.

Watch that battery carefully for early signs of damage - like 'puffy' swelling of the wrapping. Discard it if you see problems.

Q: Hey Mark -- A user from China just recently asked a question very similar to mine but I want to expand on it just a bit. This is the first year I am planning on using LiPo batteries in my robot for their energy density. My wheelmotor ESCs have Lipo protection so I'm not too worried about pushing their limits there. I'm more worried about my weapon.

The motor I'm planning to use on my spinning weapon has a stall torque of 285 A, and some calculations on the spinner spreadsheet says my required battery capacity should be in the realm of 2 A-h. The Lipo I have (temporarily) selected has a continuous discharge rate of 275 A and burst discharge of 550A. My main concern is that I am activating this weapon with a relay, not an ESC.

As far as I know, main Lipo failure/fire modes are accidental over-discharge leading to low voltage, being damaged or punctured, and being inappropriately charged. Assuming I'm choosing a battery with plenty of extra capacity for safety factor, and it is thoroughly padded from impacts, would you expect a problem? I'm mostly worried about the millisecond motor stalls experienced during relay activation. The weapon is powered with a belt drive so impacts should not completely stall the motor either.

Thanks for the help! [Sayville, New York]

A: [Mark J.] I think you have a good understanding of the issues and have made a good battery selection for your purpose. Watch the LiPo for any signs of outgassing (a 'puffy' expansion of the battery case caused by gas release) and dispose of the battery if you see such an indication of impending battery failure.

About LiPo protection mode on ESCs: unless strictly required by the event rules, turn the LiPo protection 'OFF' in actual combat. You do NOT want your ESC shutting your robot down when you're winning with 30 seconds left in the match! You can get LiPo sensors that emit a loud warning buzzer when the battery voltage hits the danger zone, giving the driver the option of shutting down to save the battery or continuing to stay in the match. Burn the battery if you need to, just win the fight!

Q: This video on combat robot construction says that an LED should be plugged into the receiver so when the machine is on it will light. Is that right? If the Receiver lost connect the LED will not light but the power is still on. [Tokyo, Japan]

A: [Mark J.] The video is correct. The receiver is powered with five volts from the Battery Eliminator Circuit (BEC) in the motor controller. The receiver output has three leads: power + (red), ground (black or brown), and signal (white or orange). The signal lead carries a pulse coded digital signal that is typically shut off when the receiver loses contact with the transmitter, but the power and ground leads are a simple pass-thru from the five volt power source and are 'on' whenever the motor controllers have power from the battery.

A power indicator light can be fitted anywhere on the far side of the the main power switch from the battery, but it is convenient to connect it to the power and ground outputs on an unused receiver port as that provides a constant low voltage suitable for an LED with a small resistor.

Q: Hello Aaron
I want to ask if I can use the Polymer Lithium-Ion Battery when I use the Motenergy ME0708 Motor as weapon motor ? If not Why? Thanks a lot. [Hebei, China]

A: [Mark J.] In theory, you can power any motor with any type of battery you like. The problem here is that the ME0708 motor can draw an enormous amount of current -- more than 4000 amps at 48 volt stall. LiPoly batteries are subject to damage if they are asked to supply more current than they can safely deliver, so you must either find a LiPoly battery with very great current capacity, or you must use some sort of 'soft start' system to reduce the current draw for the first part of your weapon start-up. Search the Ask Aaron Robot Weapons Archive for 'servo slower' to find a previous post on this topic.

Many builders will choose to equip other battery types (sealed lead acid, nickle metal hydride...) that are not prone to damage from attempting to meet the large current demands of very powerful motors rather than risk damage and possible explosive failure of a large LiPoly battery.

Q: Hey, Marc,

When did the combat robot community make the switch from 75MHz FM controllers to 2.4GHz? [Austin, Texas]

A: [Mark J.] See this post farther down this page.

Q: What type of material makes up the functional aspect of the transducer that creates the
   high-frequency sound? [San Clemente, California]

A: [Mark J.] Why do I think I'm doing your homework for you? You might at least put it in your own words and frame it as a combat robot question.

Please allow me to Google that for you: Ultrasonic Transducer.

Q: I am building a hovercraft [HOVERING COMBAT ROBOT], i want to know that can i connect two different ESC's on two Different channels like one on throtlel channel for hull EDF (upward thrust) and one on elevation channel for forward thrust. [San Jose, California]

A: [Mark J.] Yep, that's how it's done. Your rudder servo will connect to (wait for it...) the rudder channel and you'll be ready to hover away.

Q: When you have a robot with 2 active weapons, such as a pincer at the front and a rear axe to self right, how do you change the Remote Control? [Brussels, Belgum]

A: [Mark J.] With two weapons (not a good idea), each is controlled by a separate R/C channel.

Q: And what is needed to programme weapons, and make the 6 wheel drive work?

A: Many, many posts on weapon and drivetrain control in the archives on these topics. Start with Frequently Asked Questions #19, read the Motors and Controllers archive, scan the Solenoid Control of Robot Weapon and Drive Motors FAQ, and then look thru this archive for any questions you still may have.

Q: And how do you control Mechanum wheels on an RC?

A: It's faster to check the archives before you ask a question -- it's been previously discussed in this archive.

You'll need a Mecanum R/C mixer - there's no 'h' in Mecanum. Note: a Mecanum drive robot has four wheels -- not six.

Q: I've got hold of a piezo gyro and a mixer for use in one of my latest projects, and in order to avoid the 'death spin' I have a small r/c bypass switch that allows me to shut down the gyro when the robot gets flipped. However, as a result of this, the steering signal now goes through at least half a metre more wire than the forwards/backwards signal, as well as two separate electronic devices (the gyro and bypass switch), before it reaches the mixer. Will this introduce any delays or interfere with the mixer, and should I think about shortening the wires? [Scotforth, England]

A: [Mark J.] Let's see, half a meter at the speed of light is about 0.000000003 second. I think you can live with that delay. The signals from the receiver to the ESCs aren't relative time-based anyhow.

I would shorten the r/c leads just to get rid of clutter and weight, but a bit of extra signal wire isn't a performance issue. You'll be fine.

Gyro wiring diagram

Q: What is your view on using a 'conversion' kit to convert an older 35/40/72/75 mhz set to 2.4 Ghz? I use one and have found it quite good ( [opinion about really cheap plastic cases on a certain well-known transmitter brand deleted] ) but then I only run antweights and I don't really have the need for anything more complex than dual rates and servo reverse... [Lancaster, England]

A: [Mark J.] If you have an old transmitter that you particularly like, perhaps because you are comfortable with its features and set-up menus, it is certainly tempting to install a kit to convert it to full 2.4 Ghz spread spectrum functionality. This was once an expensive proposition, and available only for high-end radios that offered replaceable transmitter modules. Now there are quite inexpensive kits that can be installed in nearly any transmitter.

The kits do require a bit of work, some minor case modifications, and soldering skill. The transmitter's antenna is replaced, multiple wires from the conversion board must be soldered to specific spots in the transmitter, and the new 'bind' button must be affixed to an accessible place in the case. Here's a real-time video of a builder converting a Futaba transmitter, and this is the kit used in the video.

For most builders, it will be a better upgrade to simply purchase an inexpensive 2.4 Ghz R/C system. If you decide to convert be sure to verify that the failsafe capability of the new receiver is suitable for use with combat robots.

Q: Is it possible to power four servocity 730 rpm motors and a HS-5585 servo off of a 3 cell lipo battery or should the motors be powered by a 3c battery and the servo by a 2c battery with it all connecting to the speed controler? [Bellevue, Washington]

A: [Mark J.] Several points:

  • It's a very poor idea to run a servo above its rated voltage. The problem isn't the servo motor, it's the servo's internal electronics that control the motor direction and position. Exceeding the voltage rating for those components can lead to an abrupt failure.

  • Small hobby servos do not connect directly to the battery or to the speed controller. They plug into the receiver via a three wire lead that supplies both power and a control signal to the servo. In this arrangement, the power to the servo is at the same voltage level running the receiver.

  • The receiver is usually powered at 5 volts by a Battery Eliminator Circuit (BEC) in the Electronic Speed Controller (ESC). The BEC will typically supply enough current to power the receiver and perhaps a couple small peripheral devices, but not enough to power a heavy-duty servo like the HS-5585.

  • It is a very poor idea to run a receiver above its rated voltage. Check the specs on your specific receiver for the acceptable voltage range.

My recommended solution is:

  • Connect your ESC to a 3-cell LiPo battery of sufficient capacity to operate the drive motors for the length of your match.

  • Clip and/or remove the power wire (red) from the control leads that run from the ESC(s) to the receiver to prevent the BEC from attempting to power the receiver.

  • Connect a small 2-cell LiPo to the receiver via a spare servo lead -- red is positive, black (sometimes brown) is negative. You may plug that lead into any unused receiver port. It will power the receiver and servo.

This does add another battery that needs to be charged, but it is the simplest and most reliable set-up. The stall power requirements of the HS-5585 servo are not given in the specs, but I'm guessing at about 5 amps. The receiver requires very little power, perhaps 15 milliamps.

Q: I am spinning a 13 pound disc on a LW with a 4:1 ratio from a 3in AmpFlow. I run 2 Dewalt powerdrive units to 8in carefree tires. I want to now run two 4s Lipo packs in series to run it all. What C rating and Mah rating would you recommend? [Tucson, Arizona]

A: [Mark J.] Greetings, Tucson! Good to get a question from my childhood home. How are things in the 'Valley of the Sun'?

We have tools here at 'Ask Aaron' that can estimate battery capacity requirements for drivetrains and weapons:

  • The Tentacle Drivetrain Calculator" estimates that the drivetrain for a 60 pound robot with two DeWalt PowerDrive gearmotors running from a 29.6 volt battery and driving 8" wheels will use 2.2 amp-hours of current in a typical 3-minute match with a maximum sustained current draw of 63 amps. It also calculates that the 8" wheels are a bit too large for best performance in a reasonably sized arena -- the robot will take more than 40 feet to accelerate to its predicted top speed of 17 MPH. Consider smaller wheels?

  • Our Run Amok Excel Spinner Spreadsheet requires more information that you have supplied to calculate weapon performance and power consumption. As noted in our Spinner Weapon FAQ a '13 pound disc' can have very different performance and power requirements depending on its diameter. You also didn't mention which of the 3" 'mini' AmpFlow motors you'll be using.

    Assuming a 12" diameter aluminum disc 7mm thick with small impactors (13 pounds) driven by an F30-150 AmpFlow motor at 29.6 volts thru a 4:1 reduction, the spreadsheet estimates the weapon will spin up to a peak 6100 joules of energy in about 4 seconds. Guessing at 6 full spin-ups in a 3-minute match, the current useage for the weapon is predicted to be 1.03 amp-hours. Real world peak current draw at start-up may brush 340 amps. Run the spreadsheet again for your weapon setup if my assumptions aren't close.

That all adds up to about 3200 milliamp-hours and a peak draw around 400 amps. I like to add a 'cushion' of about 20% to the battery capacity for unexpected events, so a pair of 4000 milliamp-hour packs at about 100C burst 50C continuous would be adequate.

One observation: the AmpFlow motor should be OK at 29.6 volts, but I'm concerned about pushing the DeWalt motors that high. Dropping the wheel size would make me feel better about the higher voltage, but most builders run the 18 volt DeWalts closer to 24 volts.

Q: Hi - So I'm building a bot, and for the drive I am using the Sabertooth 2x60 motor controller, I have 2 motors @24v each... now I bought 3 Lithium Iron batteries @ 12v 21ah, I was going to run these in series for 36v, but then realized the Sabertooth only takes in 30v max... should I only run 2 batteries in series for 24v for the drive? That would leave me with one 12v battery left, and I have a 24v motor for the weapon, what would be the best thing to do in this situation? Should I run 2 batteries in series for the drive, and connect the remaining battery in series to the weapon to provide 36 volts to the weapon and reduce voltage drop when I use drive and weapon at the same time? Thank you very much for your help! [El Dorado Hills, California]

A: [Mark J.] What on earth are you building that requires 21 amp-hours of battery capacity? That's gotta be way too much for a 3 minute match with any robot that might use a Sabertooth 2x60 ESC. Recalculate?

The Sabertooth 2x60 ESC has an absolute max 33.6 voltage limit. You really don't have a choice but to run just two of your LiFePO4 (12.8 volt) batteries in series for your drive motors. You haven't mentioned any robot details, so I can't comment on performance at 24 volts.

Although it's possible to add the third battery 'upstream' from the drive motors to give 36 volts to the weapon motor, it wouldn't help the voltage drop. Bumping the voltage to 36 volts would increase the current draw of the weapon motor by 50% without adding any additional current capacity to the circuit. The voltage drop would actually be larger. Consider just running two batteries.

Wiring diagram - two ESCs and four motors.' Q: Hi Mark. I have a question about running two speed controllers together. First a bit about my setup. I have 4 drill motors and want a "tank drive" setup using 2 2x12 sabertooth motor drivers. My transmitter/receiver is a FlySky FS-T6. I have searched high and low and can't seem to find anyone with a similar case, let alone a wiring diagram that is similar to my case. Most use 1 2x12 and drive only 2 motors.

My question is, is what I am trying to do possible? I have a photo of my wiring diagram [at right]. Can you suggest any changes? The problem I am having is the motors do not seem to spin the right direction. Moving the stick forward makes the left side motors go backwards and the right side go forward. Moving the stick backward does the opposite (the left side motors go forward and the right side goes backward).

I am also confused about the #4 dip switch (the one that controls mixing). Is it necessary to use the #4 dip switch if I plan on using mixing on my transmitter?

Thank you for any help you can provide. I'm a newbie and am now wondering if I got into the wrong hobby! I'm going insane! Bleh!! [The Land of Opportunity]

A: [Mark J.] Getting 'mixing' correct is one of the more difficult things for a newbie to figure out. Yes, it is entirely possible to use two Sabertooth 2x12 ESCs to drive four motors. Your wiring diagram is fine, your DIP switch settings are correct, and you're very close to getting it to work. All you need are a couple of minor programming tweeks to the transmitter.

I'm assuming that in your diagram the two 'M1' motors are on one side of your 'bot and the two 'M2' motors are on the other side. I'm also assuming that you have channel 1 and 2 mixing enabled on your FlySky transmitter. On most transmitters this is called 'Elevon' mixing but the FlySky seems to incorrectly refer to this as 'V-tail' mixing (channels 2 and 4 on other transmitters). This isn't a problem, but it is needlessly confusing. Just one of the annoying things you have to put up with on a cheap Chinese radio...

Here we go. When you move the elevator stick forward the robot spins to the left:

Step 1: Enter the 'Setup' menu on your FlySky transmitter and select the 'Reverse' function. Select Channel 1 and change it from 'Nor' to 'Rev'. Press OK to leave the menu and try the forward command again:

  • If both motors are now spinning forward, great! Go to Step 2.
  • If both motors are now spinning backward go back to the 'Reverse' function menu, change channel 1 back to 'Nor' and change channel 2 to 'Rev'.

Step 2: Check the turning response. Move the aileron stick a little to the right (spin right command):

  • If the robot spins right, your basic setup is complete.
  • If the robot spins left, swap the positions of the channel 1 and channel 2 plugs in your receiver. Check the forward command again -- you may need to re-do Step 1 to get correct forward/reverse function after swapping the receiver plugs.

That's it! You may need to adjust the V-tail response settings from the default 50% to 100% to get full throttle from the Sabertooth ESCs. See this video on FlySky mixing if you need help with how to do that.

Once you get your 'bot running, you may want to explore some of the other functions on your transmitter that may make the 'bot more comfortable to drive. The Team Run Amok Transmitter Programming Guide will give you some help there. It was written for Futaba transmitters, but the functions covered are similar on most transmitters.

Q: Hi Mark, I am the dual Sabertooth guy running 4 drill motors guy. aka Mr. Land of Opportunity aka Joe H. Thank you so much for answering my question, when I checked the site and saw you answered, I refreshed the page a couple times to make sure it was really happening!

I made a couple errors (of omittance) by not giving more details of the robot. My sincere apologies for that. Quoting you...

"I'm assuming that in your diagram the two 'M1' motors are on one side of your 'bot and the two 'M2' motors are on the other side."

On the diagram I submitted to you, each M1 & M2 represent the sides of the bot (M1 & M2 on left run together, M1 & M2 on right run together). But to clear up any confusion (again, my apologies!) I spent last night making up a diagram that is a little easier on the eyes and more clear. I also clearly labeled the motors this time!

Two ESCs and four motors - don't do it like this!

"I'm also assuming that you have channel 1 and 2 mixing enabled on your FlySky transmitter. On most transmitters this is called 'Elevon' mixing but the FlySky seems to incorrectly refer to this as 'V-tail' mixing (channels 2 and 4 on other transmitters)."

I was using the regular mixing, which was a mistake that led me to contacting you, how confusing! I did a factory reset before trying anything you suggested. When I go into Elevon, it looks like that controls CH1 and CH2. V tail has CH2 and CH4 listed. When I hooked everything back up and ran through the tests you suggested, I was getting some really sporadic results, some of which resulted in the red error light flickering. I decided to call it quits at that point so I didn't damage anything. I tossed my batteries on the charger (thinking that could be a problem if they are low.)

But that makes me wonder if I do actually have this wired incorrectly. I say that because when you said my wiring was correct, you did that with the assumption that both M1s were on one side, and M2s were on the other side.

On page 5 of the Sabertooth 2x12 manual it says:

"If you are using multiple Sabertooths running from the same radio receiver, only one should have the 5v line connected. You can either take the red lead out of the connection housing or just clip the wire with a pair of cutters."

I am assuming when they say "only one" they mean "only one Sabertooth" should have the 5v line connected. The image they use as the example in that document is small and bad quality, so it's difficult to see what's going on there. That might explain the sporadic results and the red error light. I will make another attempt to hook everything up after work tonight, and I will report back. Thank you so much for your time and expertise.

A: [Mark J.] The updated diagram helps a lot, Joe. I had to make a few assumptions that turned out to be incorrect. We'll have you running in no time.

Yes, you do have the 'bot wired incorrectly for the way the motors are positioned:

  • Usually a 'bot with twin ESCs like you have is wired so that each ESC controls one motor on each side of the 'bot. That way if one ESC completely fails the other ESC can still control both forward/reverse motion and left/right turning -- albeit somewhat limited by dragging two inoperative wheels around. This was the set-up I assumed you had.

  • The way your 'bot is wired has each ESC controlling both motors on the same side of the 'bot. This can certainly work, but if one of the ESCs fails you'll be limited to driving back and forth in an arc. Your bot isn't working because each ESC is receiving signals for both the left and right side motors instead of just the signal for the side it controls. We can fix that...

...but let's clear up a couple of other things first:

Mixing: early versions of the FlySky transmitters had problems with mixing and had some of the menu options miss-labeled. It sounds like they've fixed that issue. Yes, Elevon mixing is the correct mix to use if it is now correctly implemented on the FlySky transmitter.

Sabertooth 5v lines: this isn't what's causing your main problem, but running 5 volt feeds from both of the Sabertooth ESCs to the receiver can cause some glitchy behavior. I wanted to fix your main trouble without inundating you with tasks, but since we're already on the topic: disconnect the red receiver pigtail wire from one (and only one) of the Sabertooth ESCs. Fold it back and wrap it with electrical tape. Problem avoided.

Charging the battery: it was a good idea to charge the battery. A low battery can cause all sorts of difficult to diagnose trouble. Nice call.

Now the main fix. We can do this two ways:

  1. We can re-route the motor power wires to have each ESC control motors on both sides of the 'bot; or
  2. We can re-route the receiver signal wires so that each ESC gets only signals for the side of the 'bot it controls.

Since I don't know how large a pain it would be for you to re-run the motor power wires, I'm going with #2. You've already got plenty of receiver pigtail length to accomplish that fix. If you'd rather implement solution #1 write back and I'll walk you thru that process -- but for now let's just get your 'bot running.

Take a look at the new receiver wiring diagram below:

Correct receiver wiring for ESCs controlling right/left sides of the robot
There is no need to split the channel 1 and channel 2 signal leads and run both signals to both ESCs. The left side ESC will get signal and ground from channel 1 (no 5v connection) and the right side ESC will get signal, ground, and 5v feed from channel 2. Note that the S1 and S2 connections on each ESC are 'jumpered' so that they both get the same signal input.

Prop the 'bot up off its wheels and power it up.

  • Give it a little forward throttle. Check that the two wheels on each of the sides of the bot are spinning in the same direction.

    • If one wheel spins one way and the other wheel on the same side spins the other way: switch the motor power leads for the backward spinning motor at the ESC power output terminals.
    • If the wheels on each side of the robot spin the same direction (both forward or both backward), move on.

  • Now go back to our earlier post (a couple posts down) and work thru the steps to get both sides to spin forward on forward throttle, and turn correctly in response to response to turn commands.

I went thru that pretty fast. If something doesn't make sense just drop me a note.

Q: Dear Mark,

By now, I have read at least 85% of your forum, which has become like a bible to me. Thank you for this, and for keeping Aaron's dream alive. I'm sure it hurts in an unimaginable way, no parent should ever experience it and it is so easy to give up, but you did not.

What's cool about what you guys have done, combat robotics and robotics in general is the cumulative nature of the learning experience, the more you learn, the more there is to learn and your site is like a never ending stream of fresh info.

I built a 2 wheeler bot with 2 CIM motors with 16:1 P80 gearboxes and 12" wheels, 2 Victor SP speed controllers. This will eventually become a self balancing ride-able scooter for my kids (after I figure out my IMU / Kalman filter issues), but for now I added some casters (kids call them 'training wheels' lol) and it is an R/C experimentation platform to figure out how things work in combat robotics. My kids want a battlebot and I decided to do it, but I'm taking steps and experimenting with some smaller scale stuff, like this 2 wheeler and a 4 wheel hockey bot that's in the works (thanks to the Tentacle calculator, which is an invaluable tool).

I have 2 questions:

  1. For my 2 wheeler with Victor SPs, I tried using the IMX-1 mixer (from Robot Logic). I like it a lot, it is very simple (just plug and play) the steering feels good and the reversing channel works like a charm. I use a Spektrum DX6i, just because that's what I have. The issue I'm having is the fact that I have no idea how to fine tune this system. Basically what I'm noticing is that one of my motors is a bit slower. I also notice that when I push the Elevator stick all the way up, the LEDs on the Victor on the faster motor light steady, which tells me it is at its max, while on the slower Victor, the LEDs are rapidly blinking and never reach the steady max state. I tested the Victors separately (without the IMX-1) and I can reach full speed at full stick throw without issues, the motors have no binding, everything feels pretty equal. When I try to calibrate them with the IMX-1 board, I get a consistent result, one motor is always slower. I know I can probably offset the Victors separately “by eye” then plug the mixer and at some point I might achieve similar speeds, but this is very annoying. Is there anything I'm missing, any other trivial way to trim things with this system? I'm thinking in the heat of the battle, it's slightly awkward to not being able to make fine adjustments. Maybe something I can do within the DX6i? I can't wrap my brain around it because any adjustment I try in the TX, makes a mess after it gets mixed through the board.

  2. Kind of related to 1., I read most (if not all) posts related to transmitters and I noticed you have a preference for the Futaba T6J as an entry level (~$200 and under) TX for combat. Right now, this is what I do to get mixing done through my DX6i [when not using the IMX-1 mixer]:

    ELEVON > ACT (makes 50% throws)

    Then I apply the D/R & EXPO settings as you recommend in your posts.

    For the most part it is very usable, but I can't figure out a way to add flipping/reversing (for driving inverted or with the rear) and there's that diamond shaped throw pattern best described in the IMX-1 mixer's manual:

    "Unlike our mixer, some RC transmitters and mixers use an “add and clamp” algorithm that reduces the controllability of your robot. This "simple" way to do tank/elevon mixing will cause your motor outputs to clamp to their maximum everywhere outside an inscribed "diamond" in your joystick's range of motion. This means that when your stick is anywhere within a triangle in each corner of its range, your robot is trying to spin as fast as it can. Our algorithm gives you smooth proportional control all the way to the corners of your stick's travel. The only time your robot spins at its maximum speed is when you jam the stick all the way to the corner."

    I drove with and without the IMX-1 and there is a noticeable difference, the IMX-1 algorithm is indeed nicer. But I don’t like the lack of fine tuning options, I think doing all the mixing and flipping inside the TX is much more elegant and simple (and less points of failure in combat).

    My question is: Will the Futaba T6J allow more control over the mixing compared to the DX6i? Will it allow me to assign a button/switch for inverted mix (inverted/backwards driving)? I looked all over and could not find any info on how to program the 6J for combat. Does your guide for the 6XAP (or the older Battlebots-era guides for the 9C series) apply to the 6J at all?

Sorry for the many words, so many questions to ask... Thank you in advance for your time and for your great service to all the noobs.

New York, USA

A: [Mark J.] Thank you for your kind comments about 'Ask Aaron'.

Question #1 has a fairly simple solution, once we find the right component to adjust:

  • The IMX-1 mixer - A very nice unit, and as you note it's completely 'plug and play'. Its simplicity is a strength, but it also prevents any fine tuning of the outputs. Nothing to be done there.

  • The DX6i transmitter - It has lots of tweeks, but nothing that can balance the throttle output after it's already gone thru the mixer. That leaves us with...

  • The Victor SP speed controllers - Take a look at page 6 of the Victor SP Quick Start Guide and follow the calibration procedure for each of the two speed controllers with the mixer in place. After calibration both of the Victors should give full throttle in both forward and reverse.

Question #2 [a cluster of three questions] is a little trickier to answer. As you've noticed, the Elevon mixing in the Spectrum transmitters is not optimized for robot use. You've implemented the standard work-around in order to get full-range PWM output. Unfortunately, this fix takes up not only the mix option but also both of the programmable mixes!

Among the reasons I prefer Futaba radio systems is their superior mixing implementation. With a Futaba transmitter you do not need a 'work-around' to get full PWM output, and their mixing does not use the “add and clamp” algorithm -- you have full response all the way out to the far corners of the stick travel, just as you do with the IMX-1 mixer. The 6J transmitter also has 'sub-trims' which allow trim adjustments on individual receiver outputs rather than just on the transmitter stick inputs. This is very handy for adjustments to individual channel outputs after transmitter mixing has taken place: if one motor is a little slow, or if it 'creeps' at neutral.

More good news: programming the Futaba 6J for robot use is nearly identical to programming the Futaba 6XAPs. The menu layout is a little different, and a couple of the function abbreviations have changed, but the Team Run Amok Futaba Programming Guide should walk you thru the 6J with very little trouble.

Now the bad news -- I've never found a method to assign an invert function to a switch on any of the commonly used R/C transmitters. Full-function hobby R/C systems are built for model aircraft, and there isn't an instance where an aircraft needs to reverse the response direction of a channel in flight. Even using the programmable mixes won't get the job done.

My advice: re-calibrate the Victor SPs to work properly with your IMX-1 mixer and continue to use its invert function. You'll need an on-board mixer anyway when you get around to experimenting with piezo gyros.

Q: How to operate a wireless Bluetooth control using dpdt switch??? Not joystick [India]

A: [Mark J.] Assuming:

  1. you're replacing a potentiometer-based joystick; and
  2. you want low-medium-high resistance to signal motor reverse-off-forward;

a Double Pole Double Throw (DPDT) switch has the wrong logic for this 'sequential switching' -- it cannot be used.

What you need is a Single Pole Triple Throw (SP3T) switch, also known as an "on-on-on" switch. The upper diagram on the right shows the circuit to replace a 10K ohm potentiometer for simple reverse-off-forward control.

SP3T switches are difficult to find, particularly in a 'momentary contact' style that returns to center when released. A single-axis joystick controller would be much easier to source, and probably cheaper.

Alternately, you can convert your controller to two momentary Single Pole Single Throw (SPST) push button switches. The lower diagram on the right shows the circuit for twin push buttons. The upper button (normally closed) gives a motor forward command when pressed; the lower button (normally open) gives a motor reverse command when pressed.

Extra credit quiz: what signal is sent if both push buttons are pressed?

Progressive switching with SP3T switch

Progressive switching with two SPST push button switches

Q: Hi Mark, Sage here. Hope you're doing well.

I have bought the iMAX Quattro B6 charger. Although the site states 60W per port, instead it's 50W per port. Soon I'm gonna buy 6S 5000mah LiPos. (Although I'd realistically need about 2000mah worth of battery capacity in a match, I'll just double it to be on the safe side. I read it in the Riobotz Combots Tutorial. Plus 25% reserve for unexpected situations.)

What bothers me is that my charger might take too long to charge my batteries. The manual states: "For 50W charge power, current is regulated accordingly. Eg. 11.1v battery, charge current approx. 4.5A. Eg. 22.2v battery, charge current approx. 2.2A." The current is user selectable but I kinda have a bad feeling that it won't allow me to increase it past 2.2A. That's gonna take >2hrs to fully charge my battery!

Is the charger too puny for my application or will I be fine with it? If the charger is puny, I thought of some alternate ways to make it work:

  • First, I can use 2 small battery packs instead of a big one. Like a pair of 3S 5000mah batteries in series or 6S 2500mah batteries in parallel. I'll also buy another charger. In this way I have 8 charging ports and 10 batteries in total, 2 in my bot and 8 on charge. And so I'm effectively doubling the power pumped into the batteries from 50 to 100 watts as I'm utilizing 2 ports at once, which halves the charging time to just above an hour for one set (2 batteries).

  • Second, instead of five 6S 5000mah batteries, I can buy 7 or 8 and just hope I don't run out of charged packs and stick with just a single charger.
What do u think Mark? Which way should I go? [India]

A: [Mark J.] I know you're asking about your battery charger, but there are a couple of assumptions hidden in your question that I think I'd better address before I get to the charger...

  • Mounting LiPoly batteries properly is tricky -- they're a little 'squishy'. If you just strap the pack to the chassis with a couple narrow zip-ties, a hard impact will cause a lot of localized pressure that can crush the battery enough to cause an internal short. Shorted LiPolys burn! Ideally, the battery pack should be padded and securely enclosed within a rigid container.

  • Good LiPoly mounting does not make for quick and easy battery swaps. If you're planning on swapping packs between matches you'd better place a lot of thought into making those swaps quick and foolproof. If your robot took a lot of damage and you need to scramble to get it ready for the next match you don't need any extra trouble when swapping out the battery.
Now, back to the charger:

You're worrying way too much about battery power. Your calculations are telling you that you only need 2000 mAh for a full match, you added 25% to that for good measure, and then doubled the whole thing! If you only pull down the charge on a battery pack by 2000 mAh in a match, your charger can top that off in an hour. If you get in a rush, you can easily run two matches back-to-back on a fully charged 5000 mAh pack. You've got enough battery power to run 10 full matches without any charging at all!

Your current charger is fine, and I think five battery packs is overkill. Given that there's a lot of time between matches in the early rounds of a tournament, your current charger with just three 5000 mAh packs should get you thru a tournament just fine.

Q: Can a Gyro sensor could be used as a substitute by attaching it to the robot's wheel so that the sensor spins around as the wheel turns? [Houston, Texas]

A: [Mark J.] A substitute for what, exactly?

There is some general confusion about the operation and uses of R/C peizo gyros. Our Gyro Guide answers many of the questions about how and why to use a peizo gyro in a combat robot.

The gyro is mounted on the robot chassis and is connected to the R/C receiver. It monitors the turning motion of the robot and compares it to the turning signal output from the receiver. If the turning motion differs from the turning signal from the receiver, the gyro adjusts the turning signal to correct for the difference before passing it on to the motor controllers.

Mounting the gyro to a wheel would only let it monitor the rate at which the wheel was rotating, not if the robot was turning at the rate the radio was commanding. It would also make it hard to wire the gyro to the receiver and motor controllers!

Q: Hi Mark... bang bang guy here. I have read the transmitter requirements in the archives, and I have become more confused as to which remote to buy. I have seen many transmitters on the net but I am terribly confused! Can you please suggest some which have been used in combat and are trusted and are also not too expensive?

I would like to use the left vertical axis for controlling both the motors together (like the bot going straight and reverse) and the right stick for steering. I will definitely need a inverting function 'coz my bot will be invertible. I also have a weapon to control.

There is also one more confusion. As in bang control, we can make two types of turns:

  • one is 360 degree turn - the left and right drive wheels running a same speed but opposite direction;
  • the other is a half turn, in which only one side drive moves and other is stationary.

I understand how we can achieve the 360 turn by just operating the steering throttle, and smooth turns by using combination of both the throttle and steering sticks, but how will I be able to make a half turn?

And also, can we get ready-made transmitters with the left vertical control axis already positioned to self-center? If not which transmitter should I buy so that I can get parts easily for that modification? Please help me out as I am buying a transmitter for the first time and I dont want to make mistakes.

Also can you explain the modes, the control which I want comes in which kind of mode? Will I require a mode switching?

Thank you SOOOO much! [India]

A: [Mark J.] Lots of questions -- but that's why I'm here.

Your control preferences are entirely normal for combat robots and can be met by just about any full-function 6 channel radio system.

  • Throttle on the left vertical stick axis and steering on the right horizontal stick axis is my preference as well. It requires 'Elevon Mixing' (sometimes called 'Delta Wing') which is a very common function, and a 'Mode 1' transmitter. Your motor controller inputs will plug into channels 1 and 2 on the receiver.

  • A 'Mode 1' transmitter has the 'Throttle' (channel 3) assigned to the vertical axis on the right stick and the 'Elevator' (channel 2) assigned to the vertical axis on the left stick. A 'Mode 2' transmitter has these reversed. A true Mode 1 transmitter will have the vertical axis on the left stick spring-centered - you won't need to worry about changing it.

  • If you purchase a transmitter with 'Mode Switching' capability either the right or left stick may have the vertical axis spring-centered. It's fairly simple to open the transmitter case and swap the spring centering mechanism from one stick to the other as needed.

  • Avionic RCB6i transmitterI know that the Avionic RCB6i radio is available to Indian builders at a good price. Features include:

    • mode switching;
    • Delta Wing mixing;
    • position-settable failsafes on all channels;
    • 3-position switch on channel 5 to operate a reversible weapon controller forward/off/reverse;
    • 2-position switch on channel 6 to operate the invert function on the 'RageBridge' controller;
    • all of the most needed combat control functions (servo reverse, dual-rates, digital trims, model memory); and
    • a user manual that makes sense.

  • Another possible radio would be the Orange T-Six transmitter and matching Orange R620 receiver. Inexpensive and widely used in combat robots, the Orange radio has more features than the Avionic but lacks the position-settable failsafe controls and useful 3-way switch on channel 5 that the Avionic offers. For your purpose, I'd buy the Avionic.

Once you get used to proportional control of your robot you won't need to worry about the type of maneuvers you had to resort to with your 'Bang Bang' system. If you really need a 'half turn' it can be accomplished by combining equal amounts of throttle and turn input. The turn input will command one motor forward and one motor backward, and the throttle input will balance out the backward motor command with an equal forward command leaving that motor motionless.

Q: Which transmitter would u suggest to buy for first timers ? regular aviation style or the gun style ( piston style) used for rc cars. also can you tell me how is the control on the piston style controller, my bot runs on skid drive. I am more into wedgebots. I also run one weapon bot .

A: [Mark J.] Unless you have a VERY strong personal preference for pistol-grip style transmitters there is no reason to consider their use for a combat robot. Pistol grip transmitters in general do not have the Transmitter Functions needed to properly and safely control a combat robot, and the few that do are quite expensive. Read thru our Radio Guides to learn about radio selection and programming, and purchase a full-featured twin-stick style radio system.

If absolutely necessary a pistol grip transmitter can be used to control a skid-steer robot by using the channel mixing function found on many speed controllers. Channel mixing takes the output from the transmitter throttle channel and splits it between the two motor control channels of the controller to direct them both forward/reverse as needed. The transmitter steering output is likewise split between the two motor control channels to provide differential power to the motors on either side of the 'bot for turning. But seriously, you don't want to use a pistol-grip transmitter for your combat robot.

Q: Hi Aaron, how I can build the remote control for a battlebot? ¿Tienes algún esquema del circuito? [Ecuador]

A: [Mark J.] I VERY STRONGLY recommend that you NOT attempt to build your own radio control for a combat robot. Commercial radio systems are rugged, reliable, loaded with features, and inexpensive. You aren't going to achieve any of those attributes by building your own system. Even a $25 Chinese R/C system would be far preferable to a home-brew radio. Don't do it!

Q: when did 2.4ghz radios become popular? [Havertown, Pennsylvania]

A: [Mark J.] February 3rd, 2008 at 10:24 AM pacific standard time.

Q: that's very specific.

A: That's when RoboGames decided that tracking transmitter frequency crystals was too much bother and mandated spread-spectrum radios for their combat events. The 2.4 gHz radios would have taken over eventually, but RoboGames pushed the issue.

About that same time RoboGames also decided that the single most popular large combat class - Hobbyweight robots - was too much bother and dropped it. Superheavys and Feathers followed. Power corrupts.

Q: hello i am preparing for techfest i need urgent guidance about how to use 2.4 ghz transmitter to move weapon and locomotion [India]

A: [Mark J.] If you're in need of urgent guidance you would be well advised to search the FAQ and the Ask Aaron archives to see if your question has been previously answered rather than wait for me to get back to you. In this case, it has been discussed and answered in great detail.

  • Start by reading thru the FAQ. Pay particular attention to question #19, but read the whole thing -- if you don't know how to rig your R/C transmitter to control your motors there's a whole lot more you're gonna need to learn about.

  • Many Indian robot builders use solenoids to control their robot motors, so you'll want to read the special Solenoid FAQ to see if that option will work for you.

  • There are many, many, many questions on this topic answered in the Ask Aaron archives.

    • The Motors & Controllers archive has answers to many questions on speed controller selection for drivetrains.

    • The Robot Weapons archive has answers about weapon motor controller selection.

    • The Radio Archive will supply details on programming your transmitter for intuitive control of the drive and weapon motors, as well as help on battery selection.
That should get you a start.
Q: Hi aaron, excuse me I have a question... Is this battery enough for 2 motors banebots Rs550 for build a sumo robot? This robot weigh 3kg without force magnet. [link to cheap Hong Kong 5000 mAh 4S Lipo battery removed] [Equador]

A: [Mark J.] Scroll down to the next post for an answer to battery capacity required for a 3kg sumo robot powered by RS-550 motors. The 5000mah battery you found has at least five times the capacity you need for a sumo match. To give a more precise answer I would need to know the motor gearing and wheel diameter of your robot.

Advice: since you don't need a battery this large, spend some of the money you're saving to buy a better quality Lipo -- something from a different distributor.

Q: Hello,

I am using a duratrax 550 motor to drive a 3 kg robot 6WD. The motor will be geared 21:1 and the wheel size is 36mm in diameter. I am having trouble with the selection of the correct battery (lipo?) for it. Would appreciate it if i am given some advice on this matter. Many thanks [Singapore]

A: [Mark J.] A single Duratrax 550 motor? I'm going to assume two motors, as one motor would be an odd design for a 6 wheel drive robot. I'm also going to assume this robot will compete in the 3kg sumo division.

Two RS-550 motors is a whole lot of power for a 3kg robot. Acceleration performance will be excellent, and the motors will be very lightly loaded so they won't consume much power. The Tentacle Torque/Amp Calculator says the motors will consume only a bit over 5 amps total under heavy pushing before the wheels break traction and limit greater current flow. Total power consumption for a three minute long match (very rare in sumo) would be about 300 mAh. Any small three-cell LiPoly pack with 400 or more mAh capacity should be fine for your purpose.

Q: I see builders asking a lot of questions here about weapons and motors and once in a while a question about wheels. Are there questions that builders should be asking but aren't? [Western US]

A: [Mark J.] Weapons give the opportunity to deal damage if you can apply them to your opponent, and your motorized drivetrain gives the ability to to move toward your opponent -- but the it's the interface between the driver and the machine that allows the robot to be responsive and well controlled. We get very few questions about tweeking the control interface to make the robot driveable.

I see plenty of examples of combat robots with poorly set-up transmitters. Machines that wander about like lost sheep -- unable to reliably point their nose at their opponent, incapable of driving across the arena in a straight line, and spinning around uselessly when attempting a simple turn. Many builders don't even know what functions are avaiable on their transmitters that might be useful to them.

If you're interested in using the full capability of your computerized transmitter or want to know which radio system to buy in the first place, Team Run Amok has a few guides on the subject:

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 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 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; 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. Radio Reception Guide

Robot haiku:

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

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 doesn´t blink and stay fix, why? [Aveiro, Portugal]

A: You might benefit from reading this post on 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 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

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?


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, 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.

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 mayselect 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 DATAINPUT 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.
'Flexy Flier'...
'Hammerhead' combat robot.
...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 s