The first project that you'll be working on is a simple RC car, like the one shown here:

This particular car is an Xmods car, sold by Radio Shack, also available at other retail and online outlets. You can certainly use other RC cars as well. The advantage of this particular set is that the inputs to the drivetrain and steering are very easy to access.

The following is the car, exposing the center control mechanism:

There are two connections that you will want direct access to. The first is the drive motor, and the second is the steering mechanism. For this particular model of RC car, the drive mechanism is in the rear. What you are normally looking for is two wires that will directly drive the DC motor of the car. On this system, there is a connector in the rear of the car, it looks as shown in the following image:

In the main control section of the car, you can see that there is a connector that plugs in these two wires in order to control the speed of the car, as shown here:

Remove this plug and these wires; you'll use Raspberry Pi and a motor controller to provide the voltage to the drive system of the car. The motor will run faster or slower based on the level of voltage that is applied to these wires and the polarity of the voltage will determine the direction. Raspberry Pi will need to provide a positive or negative 6 volt signal to control the speed and direction of the car.

You'll also need to replace the control signals that go to the front of the car for the steering. This is a bit more difficult. The following is the connector that goes to the front of the car:

The five-pin connector that comes from the control module is shown in the following:

The trick is to determine how the wires control the steering. One way to determine this is by opening up the unit, the following is how it looks from inside:

As you can see in the previous image, the blue and yellow wires are attached to a DC motor and the orange, brown, and red wires are attached to another control circuit. The motor will drive the wheels left or right, the polarity of the voltage will determine the direction, and its magnitude will cause the wheels to turn more or less sharply. The orange, brown, and red wires are interesting as their purpose is a bit difficult to discover. To do this, you can hook up a voltmeter and an oscilloscope. The orange and brown wires are straightforward, they are 3.5 volt and GND, respectively. The red wire is a control wire, the signal is a Pulse Width Modulation (PWM) signal, a square wave at 330 Hz and 10 percent duty cycle, and it is an enable control signal. Without the signal, the turning mechanism is not engaged.

Now that you understand the signals that are used in the original system to control the car, you can replicate those with Raspberry Pi. To control the steering, Raspberry Pi needs to provide a 3.3 volt DC signal, a GND signal, a 330 Hz, a 3.3 volt PWM signal, and the +/- 6 volt drive signal to the turning mechanism. To make these available, you can use the existing cables, solder some additional cable length, and use some shrink-wrap tubing to create a new connector with the connector that is available in the car:

You'll also need the access to the rear wheel compartment of your car to drive the two rear wheels. The following is how the access will look:

Also, you'll need to connect the battery power to Raspberry Pi, here is the modified connection to get the battery power from the car:

To control the car, you'll need to provide each of the control signals. The +/- 6 volt signals cannot be sourced directly by Raspberry Pi. You'll need some sort of motor controller to source the signal to control the rear wheel drive of the car and turning mechanism of the car. The simplest way to provide these signals is to use a motor shield, an additional piece of hardware that installs at the top of Raspberry Pi and can source the voltage and current to power both of these mechanisms. The RaspiRobot Board V2 is available online and can provide these signals. Here is a picture:

The specifics on the board can be found at http://www.monkmakes.com/?page_id=698. The board will provide two key signals to your RC car, the drive signal and the turn signal. You'll need one more additional signal, the PWM signal that enables the steering control. The following are the steps to connect Raspberry Pi to the board:

Now the hardware is connected.

The hardware is ready, now you can access all this functionality from Raspberry Pi. First, install the library associated with the control board, found at http://www.monkmakes.com/?page_id=698. Perform the following steps:

Now you'll create some Python code that will allow you to access both the drive motor and the steering motor. The code will look similar to the following:

The specifics on the code are as follows:

Now you can run the program by typing sudo python xmod.py. LED 1 on the control board should turn on, the rear wheels should move in the forward direction, and the steering should turn. This confirms that you have connected everything correctly. To make this a bit more interesting, you can add more dynamic control of the motors by adding some control code. The following is the first part of the python code:

Before you start, you may want to copy your python code in a new file, you can call it xmodControl.py. In this code you'll have some additional import statements, which will allow you to sense key presses from the keyboard without hitting the enter key. This will make the real-time interface seem more real time. The getch() function senses the actual key press.

The rest of this code will look similar to the previous program. Now the second part of this code is as follows:

The second part of the code is a while loop that takes the input and translates it into commands for your RC car, going forward and backward and turning right and left. This program is quite simple, you'll almost certainly want to add more commands that provide more ways to control the speed and direction.

You can now control your RC Car, but you certainly want to do this without any connected cables. This section will show you how to add a wireless LAN device so that you can control your car remotely. The first step in doing this is to install a Wireless LAN device. There are several possible ways to do this; however, the one that works well, with full documentation, is described at https://learn.adafruit.com/setting-up-a-raspberry-pi-as-a-wifi-access-point/overview.

You should now be able to connect to your Raspberry Pi via the Wireless Access Point. Once you've created the wireless access point, you can login via a VNC connection, this way you can add a USB webcam to your car to make it even easier to control. To do this, first download an application that can support a VNC connection. You can get this on your Raspberry Pi using an application called vncserver. You'll need to install a version of this on your Raspberry Pi by typing sudo apt-get install tightvncserver in a terminal window on your Raspberry Pi.

TightVNC Server is an application that will allow you to remotely view your complete graphical desktop. Once you have it installed, you can do the following:

You can now access all the capabilities of your system; however, they may be slower if you are doing a graphics-intense data transfer. To avoid having to type vncserver each time you boot your Raspberry Pi, use the instructions given at http://www.havetheknowhow.com/Configure-the-server/Run-VNC-on-boot.html.

Vncserver is also available via Linux. You can use an application called Remote Desktop Viewer to view the remote Raspberry Pi GUI system. If you have not installed this application, install it using the update software application based on the type of Linux system you have. Once you have the software, perform the following steps:

You can see the graphical screen of Raspberry Pi. Now you are ready to observe the output of a USB webcam connected to your car. This is quite straightforward, simply plug in a USB webcam and download a video viewer. One such video viewer that works well is luvcview. To install this, type sudo apt-get install luvcview.

With all these tools installed, you can now run vncview, bring up a luvcview window so you can see what your RC Car is seeing and control it remotely by running the xcmodControl.py program that you wrote earlier. The screen will look similar to the following:

There are a lot of additions that you can make to your Raspberry Pi controlled car, such as adding the joystick control or more autonomy. However, let's move on to the next project.

Now you know how to work with Raspberry Pi to add its capability to an existing piece of hardware, in this case, an RC Car. In the next chapter, you'll learn how to add Raspberry Pi to a toy robot that can walk, and make it talk and listen to the voice commands.