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Turn your Raspberry Pi into a radio controller for RC toys.
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control-specs Merge pull request #12 from lavanoid/master Mar 30, 2015
.gitignore Save config on server instead of in web storage Nov 12, 2017
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Makefile Remove references to scons and libjansson Feb 6, 2018
README.md Remove references to scons and libjansson Feb 6, 2018
control.html
host_files.py
mailbox.c Support for Raspberry Pi 2 and 3 May 15, 2016
mailbox.h Add option for testing on x64 Jul 14, 2017
pi_pcm.c
watch.html Improve saving of parameters Sep 17, 2018

README.md

pi-rc

pi-rc is a program that turns your Raspberry Pi into a radio controller for RC cars without any additional hardware. With it, you should be able to drive just about any cheap toy-grade RC car programmatically with your Raspberry Pi.

Note that running this program turns your Pi into a rather powerful radio transmitter. Do not use this code unless you understand what it does, what frequencies it might interfere with, what frequencies are allowed, what is legal in your country, etc. I assume no responsibility for your actions or for any problems running this program may cause.

Getting started

pi-rc should work with most toy-grade RC cars, i.e. ones that only drive forward, backward, left, and right. To get started, place a jumper cable on GPIO pin 4 (that's pin 7 on the revision B board, see this diagram to act as an antenna and compile the pi_pcm program on your Raspberry Pi and run it as root:

make
sudo -s
./pi_pcm

Every RC car I've seen uses a different set of command codes, so you'll need to use another program that iterates through possible command codes and watches the RC car to see if it responds. Once it does, it will save the image and the command that caused the car to move.

To run this search, you'll need a computer with a webcam or an Android phone. iPhones won't work because Safari doesn't allow access to the camera. Run ./pi_pcm -v on the Raspberry Pi so that it can start broadcasting commands. Also run python3 host_files.py on the Raspberry Pi, and open a browser either on the Pi or on a separate computer and visit https://:4443/watch.html. Firefox and Chrome have been tested and confirmed as working, but Safari will not work.

Place the car in an area where you can control the lighting and avoid changes in ambient lighting, such as a closet, and turn the car on. If your car is darkly colored, try to place it against a white background, for example by putting a piece of paper behind it. Point the webcam at it. In the web page, set the frequency to the car's frequency. Most toy-grade RC cars in the US run in the 27 or 49 MHz band, which should be printed on the car. Most cars run in a specific frequency in the 27 or 49 MHz band, such as 27.255; if you don't know the car's exact frequency, just pick the middle value in that range, e.g. 49.860 MHz for 49 MHz. Click "Start Monitoring". The web page will start sending different commands and checking for movement.

Once the browser sees the car move, the image from the webcam will freeze and the parameters of the command that it just broadcast will be preserved.

Now that you have the basic command structure, you can search for the specific commands to make it drive. Visit https://:4443/control.html. This page will prompt you for the values found from the search, and then ask you repeatedly to enter different signal bursts. Try entering values from 1-100 and see how the car reacts. Certain values should make the car drive some combination of forward/reverse + left/right.

Once you have all of the signals, you can save the information in a JSON file and use control.html to control the car.

Signals

How Stuff Works has a good explanation of what the command signals for toy RC cars look like. One thing to note is that the Raspberry Pi cannot reliably stop broadcasting for a certain amount of time, so it instead broadcasts at a different frequency to simulate the pauses.

Programming

That's fun and all, but the real fun in my mind comes from buying an external battery pack for the Raspberry Pi, taping it to the top of the car, and turning it into an autonomous vehicle. I've tried to make it easy to send commands to the controller program.

The pi_pcm program listens for JSON-formatted commands over TCP port 12345 by default. You can also have it listen over UDP by using the -u option.

Messages are formatted as an array of objects with the following fields defined:

frequency The frequency that the car runs at.
dead_frequency Another frequency to simulate a "pause" in broadcasting. If you are in the United States and your car is running in the 27 MHz band, 49.890 MHz should be a safe signal here, and for cars in 49 MHz, try 26.995. DO NOT use frequencies outside of the normal range for toy RC cars, and make sure you comply with all regulations in your country.
burst_us The length of a signal burst in microseconds.
spacing The length of a signal spacing in microseconds.
repeats The number of times to broadcast the message.

A typical command might look like:

[
    {
        "frequency": 26.995,
        "dead_frequency": 49.830,
        "burst_us": 1200,
        "spacing_us": 400,
        "repeats": 4
    },
    {
        "frequency": 26.995,
        "dead_frequency": 49.830,
        "burst_us": 400,
        "spacing_us": 400,
        "repeats": 40
    }
]

where the first object is the synchronization signal and the second is the command signal. The Pi will continue broadcasting a given command until a new command is received. If you want to stop the car from moving, try sending a signal repeat that doesn't correspond to any action from the RC car.

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