How to convert (almost) any 27 or 49 MHz RC Car into a Robotic car

I am teaching a robotics course in my kids high school (Rambam Mesivta in Lawrence, NY). I was looking for a cost effective way to teach them about the priciples of robotics without spending an arm and a leg, and getting them excited about the prospects of hacking hardware and teach them how to interface to the arduino platform. We studied H-Bridges made using  trasistors and switches, but there was something missing.

After reading this article:

http://www.et.byu.edu/~bmazzeo/LTR/tech.phtml

It suddenly occurred to me that almost any toy RC car might be using this or almost similar architecture. I opened up 4 RC cars that were donated to the project and indeed they all use more or less the same configuration: All these cars have 2 H-Bridges on a small board mounted inside. Almost all the cars these days use a variation on the RX2C toy car chip (link) (i.e. pins 6,7,10,11 are the drivers for H-Bridge) and these boards can accept  logic voltage levels regardless of the battery voltages. The boards that do not use the chip will usually have an equivalent daughter board that fills the same function.  Furthermore- the boards are optimized to drive the motors of the cars optimally. Contrary to the article  they are safe to turn on both sides of the H-Bridge at the same time by mistake - all they do is turn on braking mode.

Almost all RC cars are configures so the rear wheels are running of a power H-Bridge for the forward/reverse motion and one H-Bridge is used to run a servo or servo-like mechanism in the front wheel. There is another configuration in which the right two wheels are connected to one motor and the left two wheels connected to another and this configuration can do some interesting stunts such as turning on the spot and other tricks.

The chip specifies an additional output - "turbo mode" however I am yet to see a car with this function implemented.

If you open you car and do not find the standard configuration - do not despair - figuring out the H-Bridge inputs are easy - and usually are the pins connected to 2.2k resistor.

The steps involved in making this work:
1) remove the RC chip - more on this later.
2) solder 4 wires to the H-Bridge Terminals
3) solder a wire to the negative terminal of battery - to power the arduino
4) solder a wire to the positive side of the switch - to power the arduino..

Here is a video of this mod after we completed the mod in 25 minutes.

Once reassembled, you can control the car from the arduino - and viola! you have a robotic car!
After doing a few cars - i find that the process of conversion of a car takes about 30-40 minutes.s a
Note the cars are sensorless!

VARIANTS:
* RX2C microchip based controller/receiver
* Daughter bvoard based controller/receiver
* Analog (transistor based) H-Bridge - can be used with analogWrite and PWM to control the power
* Relay based H-Bridge for the bigger devices. This can only be turned on and off - if you want to use PWM on there - you will have to turn to a custom H-Bridge design and ditch the board.

NOTE: This mod cancels the RC functionality. I have had thought to hack the RC part to allow two arduinos (or more) to communicate via hacking the transmitter and then connecting the (disconnected) signals lines as an arduino input. thus one would kill a few birds in one stroke: a dual H-Bridge, robotic car, and half duplex RC communications between arduinos. maybe I will tacle this in the future :)

WARNING: Make sure that the switch is off while programming your fully charged vehicle -or you will have  a robotic missile flying off the table at maximum speed and power. Also you can expect some weak movement of the wheels due to the H-Bridge being powered by the USB cable - even when switch is off.

Here is a video of the kids programming their own cars by themselves, and also note the cars are sensorlesss

Step 1 Modifying a car from A-Z

During class I brought a brand new giant monster car I purchased at radio shack and I wanted to show how easy it is to perform this surgery on the car.

These pictures show the beginning of the dismantleation.

Comments

[dflam]

If you like this, please vote for us for the shopbot contest!

[alien200049655]

I'm doing something simular with a newer (digital controlled) "NEW BRIGHT" rc subaru impreza I picked up on clearance from my local Walmart.

I simply removed the existing radio link and signal decoder, and drove the H-bridges Directly from the digital I/O pins of my arduino duemilanove. I had to remove the RF-link because it acted as an energy parasite. once this was done, I got much better function outta my "RcCarduino"
Thats what this type of project is being nick named. ( I take no credit for its inception. )

in a way, the digitally driven cars are easier to work on then the old analog type, and you thought modding your 27Mhz. version was easy. the difference is the digital cars are all surface mount instead of through hole type, so you have to be more careful when soldering.

[dflam]

Thanks for sharing. What do you mean by digital? Almost all RC cars use an RX2C variant. There are surface mount versions and some are through hole versions. I am confused, because my article describes exactly this - connecting the digital outs of the arduino directly to the control line of the H-Bridge. Disabling the radio as a way to prolong battery life is interesting. However I like Doug Paradis's idea about taking out the 2.2k resistor and leaving the chip in place allowing to use the radio to signal the arduino for on-off and synch operations. Doug's system also allows bit banging of information to the robot.

[renegadezombies]

I just did this to an old $3 cheap RC car and it worked perfectly! Good instructable!

[jongscx]

I did the same thing to a "tank style steering" RC vehicle. Nice Writeup!

[anonymouse197]

This is a fantastic project! I have converted my first RC car, and am trying a helicopter now (but understandably struggling with weight issues). Thanks for making this fantastic contribution, I just wish I went to that high-school!

[dflam]

Wow thanks!! I will forward to the principal...

As for helicopters - although weight is an issue - you can use the main rotor assembly with the chip from say 2-3 of them to build a tricopter. Because of the gyroscopic effect of the 2 rotors, it would be easy to control 6 PWM lines and make the thing quite stable! the other thought i had on this was that the LiPo batteries inside are usually 200-240ma batteries, whereas you can use a single 2400 laptop battery on say 3 assemblies giving you 10X the power with 3X the power requirement. also, by stripping the bodies, you can get much more weight to lift and finally you can always up the voltage if you use 2 batteries - and see if you burn out the motors or the board.... :) just a few thoughts... i am sure there will b people who can comment on this.

[engrstephens]

I did this for an upward bound class. I bought 9 identical cars... and found 4 varients of the board. It was pretty impressive.

If you are unsure on the chip.. take a logic probe to one pin. Then activate each function on the remote control. Keep notes. You will find 4 pins are doing all the work. Solder to those chips.

Nice work!

[dflam]

I came up with a simpler method - connect a wire to battery plus and then just touch the pins with it and see what moves. obviously this is inferior to a logical analyzer - but my goal was simplicity.

[engrstephens]

Not a logic analyser.. logic probe. Led + resistor + transistor. A little bit safer if you want to ensure you don't damage the chip.

You can even use the arduino as the logic probe (use d13) or write a simple sketch.

[dflam]

of course assuming u do care about the chip :)

[Computothought]

Super cool... You can get rc cars from Goodwill for about two bucks each (no controller). That makes me think, why not go one step farther and use the guts for controlling other things. Might take an extra bit of electronics though. heading to Goodwill to do some harvesting.

[Doug Paradis]

I have modified RC cars in a similar manner to make robots. I generally do not remove the RX2 chip. Instead I remove the first component coming off RX2 chip on the pins of interest (pins 6, 7, 10, 11 and  the turbo pin if  needed). It is usually a resistor.

I then create a simple daughter board that breaks out the RX2 pins and the H bridges for the micro-controller board. I retain the removed components by placing them on the daughter board. I then connect the daughter board to the main board using the holes where the components originally were.

By doing this I retain the RF functions as triggers for the micro-controller. The signals off the RX2 can be used by the micro-controller for such things as "emergency stop" or "robot pause" which are required in some outdoor robot competitions. They can also be used to trigger different behaviors remotely while still leaving the robot autonomous. An example would be to trigger a camera to take a shot.

The micro-controller is used to send PWM signals to the H-bridges, via the daughter board. This allows you to slow down the converted cars

[dflam]

That is really cool information. Thanks for sharing. I was toying with the idea of doing it that way - however, I wanted something simple the kids can work and learn the principles of robotics fast, i figured 4 wires is enough :) but i think there will be definitely a followup article experimenting with your suggestion. This information is especially true and valuable with regard to hacking RC toy helicopters ;)

And yes we did use PWM on the cars as part of explaining how to operate the motors at various power levels.

[bilalasd]

Really nice project, I was thinking of hacking my rc car too.
Although I was gonna connect the arduino to the controller that way the arduino can control the car wirelessly, without the second arduino.