[ ANDROID ] Bluetooth Controlled Model Car

Bluetooth Controlled Model Car

15.06.09

Ever since seeing the first bluetooth controlled RC car I wanted to make one. For those who haven't seen it, it used a serial port Bluetooth module (about 200 USD), a "Mini SSC II" serial servo controller board (roughly 50 USD) and an old RC car. . Needless to say it was unnecessarily expensive, which really put me off. So years passed until I suddenly stumbled across some really cheap GP-GC021 bluetooth modules on ebay. (Update: Since then I've found an even cheaper unit which is superior. See the RF-BT0417C) What made them so special was that they were directly TTL compatible UART modules. Not only would I save vast amounts of money, but also effort by not having to use RS-232 voltage levels. I decided to use the RC car from my previous RF control endeavors, but also add servo steering so it would be more fun and practical to use. Given my previous work on the COM laser turret, controlling motors over a serial protocol was no difficulty, and the firm- and software required was already written. The only new thing required would be implementing servo control over a serial protocol.

I've simply connected the UART of the bluetooth module to the built-in UART module in ATTiny2313 so they can communicate directly with one-another. A portion of PORTB is used to control the various functions in the model car. The ATtiny2313 firmware goes through a loop which consists of reading the UART, and then setting the outputs and servo position. First all the outputs are set, except the servo signal. A simple bit pattern where each of the six most significant bits represent a value. A one is true, 0 false, where descending from MSB we have : | Headlights | Brakelights | Forward | Reverse | Left | Right | Unused | Unused |. Based on the value of the Left or Right bits the position variable is set to a preset value representing far left or far right. I used preset values due to my DIY servo steering not using the exact center of the servo rotor, and some unevenness in the mechanics. After the position of the servo is set, the remaining bits are simply sent to PORTB, lighting lights or starting the engine. After this a function is called which positions the servo. Due to the way I wrote the program, ie not using interrupts, the car can only respond every 15ms. The firmware was written in MikroC Pro from Mikro Elektronica. I've been toying with the idea of making some better firmware using AVR-GCC, which is my compiler of choice now. After programming for my MIDI and ROV project, my grasp AVR programming is much better. Adding sensors, PWM control for the motor, and better servo control are some of the things I'd do.

 

I've received a bunch of emails asking how I soldered the bluetooth module to the circuit board. Basically you have two options here because of the weird footprint it has: You can either design a special breakout PCB for it, which I'll do if I use one of these modules again. OR you can do what I did, which is solder it to the topside of some veroboard, by using old component leads. There are only ten connections to be made, so it's not a big deal.

In addition to firmware for the ATtiny2313 some software is needed for the controlling bluetooth device. I wrote a small program in Visual basic, which can control the car through any serial port, real or virtual. This allows the use of USB bluetooth modules which have a serial port profile. I've added the ability for users to control the car using the keyboard, which allows for far easier control than even high end RC controllers can give IMO. Standard gaming layout is used: W for UP, S for down, A for left and D for RIGHT. I've also written some apps for various phone models, which are described below.

Download the complete package, including schematic, firmware, software, cell phone app and source code. Android source files are also included.


The Java App
Controlling the car over bluetooth on a PC is great, but cell phones also have bluetooth. During the COM Laser Turret project I dabbled in java and fooled around trying to send serial commands over bluetooth to the PC. Eventually I found this tutorial on interfacing with GPS modules over bluetooth. So I took the source code, and stripped it down until I understood the basics, and had an application that would search for, and connect to bluetooth devices. Implementing a control menu for the car and sending serial commands was rather easy after that. The bluetooth program works perfectly on my k750i, not at all on my father's k810i and will only receive commands on my mother's N73. So your mileage will vary unfortunately. Some firmware types may not even allow bluetooth access from java apps. The java app itself was written in Netbeans, which is free and open source. I have included the source files and hopefully commented sufficiently for the code to be understandable.

Update 6.11.10
Satyamfifa found a fix for the Java app, making it work on his phone: "the j2me code was not working on my Nokia 2700 classic. i added flush() after write() and it started to work". I no longer have Netbeans, so if you want to use this app on your phone, try the fix mentioned here and recompile the app yourself. Hopefully it will solve the problem.

The Android App - 08.06.10
Since the last update I've purchased an HTC Hero, which is awesome. Their Dev Guide proved real useful, and makes learning how to program for Android a breeze. Anyway, to really prove the worth of my Android phone, it had to be able to control this car. The first program I made was a little soundboard app, this taught me the basics, and from here the only real hurdle was the bluetooth communication itself. In the end I had to use one of the bluetooth examples (Bluetooth Chat), because why change perfection? The app uses the phone's internal accelerometers for steering, and two large buttons for forward and reverse. Much easier to use than the k750i.
As for the app itself, finding bluetooth devices is done using a slightly modified version of the "DeviceListActivity.java" from the bluetooth chat example. This simply finds paired devices, and ones in the vicinity, and displays them in a menu. The on-screen buttons simply control the data byte sent to the car. Again, I hope I've commented enough for it to be useful. All the needed source files are available in the zipped download above. The ready to install program is on the Android Market, simply search for "BluCar" and it should pop up. Alternatively, it's also in the complete project package above. Know that use of bluetooth requires Android 2.01 or better.

Adding Servo Steering
The construction of a new servo steering system was a major part of this project, but not directly related to microcontrollers or bluetooth. Since it might be of use to someone who decides to build this project, I'll include some info on what I did. The original steering system of the car was made so the car would drive straight forward, and when in reverse turn to the left. This way only a two channel remote is needed to provide steering, which saves complexity of the electronics. And that was important considering that the original circuit contained only discrete components! This was lucky for me in the sense that it would only require some simple modification to provide full directional steering. First is an image of the original steering system and the servo I planned on using (see left). I found an ebay auction for four small 9G hobby servos, which seem quite adequate for a small indoor RC car. Since I didn't want to damage the car too badly in case I failed to make the servo steering work, I fabricated a new steering bracket using ABS plastic, taken from a CD-ROM drive. The plastic was almost as thick as the original stuff, and seems identical otherwise. I cut out the piece using the original as a template to ensure the wheels would stay aligned with one-another. The piece was shaped, and a thick pin (leg from a 5A diode) was epoxied to it to serve as a guide. The servo itself was epoxied in the space behind the wheels, where it fit nicely after some work with my Dremel. I'm afraid the one I put in might fail some day, and with all the epoxy I used to fasten it I might not get it out. Remember to allow for servo replacement, as they do wear out eventually. The "arm" type servo horn was fastened to the servo after being shortened, and the holes cut into a long track. As is evident in the picture below, as the servo rotates, the pin will be forced to move horizontally either left or right.

 

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