Android Things GPIO pins are used to interact with external devices. GPIO stands for General Purpose Input Output and it is an interface to read the state of an external device. In this article, we will discover how we can use Android Things GPIO pins to control DC motors, allowing us to build a remote-controlled car. At the end of this article, you will build an Android Things car that moves in all directions and you can control it using your smartphone or your browser.

Android Things provides a set of APIs we can use to interact with two-state devices like buttons or LEDs using Android Things GPIO. Using Android Things GPIO API, we can simply read the state of a pin or set its value. In more details, in this article, we will explore how to use Android Things GPIO to control motors. Maybe you already know, there are other ways to control motors. Using Android Things GPIO pins we can turn it on or off but we cannot control the motor velocity. GPIO pins have only two states: on or high and off or low. If we want to have more control over the motors applying a proportional control we can use Android Things PWM pins.

Anyway, in this context, we want to only control the motors and turn them on or off so we can move our Android Things remote controlled car in all directions. The final result is shown in the picture below:

Android Things GPIO pins

Before describing how to build an Android Things remote car, it is useful to give an overview of how to use GPIO pins. The first step to control an Android Things GPIO pin is getting the reference to the PeripheralManagerService:

PeripheralManagerService service = new PeripheralManagerService();

The next step is opening the connection to the pin:

pin = service.openGpio(pin_name);

To know which pins are GPIO, according to your Android Things board, you can refer to the Android Things pinout. Once the connection is open we can set the value of the pin using these commands:

pin.setDirection(Gpio.DIRECTION_OUT_INITIALLY_LOW);
pin.setValue(true); // High

For more on how to use Android Things GPIO pins you can refer to my book "Android Things Projects."

Before digging into the project details you should read my other articles about Android Things:

• How to install Android Things
• How to build an Android Things project

How to Control a Motor Using Android Things

Usually, we connect the device directly to the Android Things board. Anyway, when we use motors this is not possible because a motor may require much more current than a GPIO pin can provide. In this case, we have to provide an external power source and use the Android Things GPIO pins to control the motor. Moreover, we want to control the motor rotation direction. For these reasons, it is advisable to use a simple motor driver that simplifies our work.

In this project, we will use L298N, a simple driver that can control two motors and their directions:

This driver can control motors using PWM too, but, in this project, we will not use these features. Using two Android Things GPIO pins for each motor, we can control its rotation direction or stop it.

Let us see how to connect this driver to our Android Things board. The schema below shows how to connect the GPIO pins to the L298N and the motors:

Even if the schema seems a little bit complex, it is very simple: this project uses four different Android Things GPIO pins:

Right motor:

• BCM17
• BCM27

Left motor:

• BCM23
• BCM24

If you are using a different board than Raspberry Pi3, you have to change the pin names.

At this time, we can create a simple Java class that controls the motors:

public class MotorController {

    PeripheralManagerService service = new PeripheralManagerService();
    private Gpio pin1Motor1;
    private Gpio pin2Motor1;

    private Gpio pin1Motor2;
    private Gpio pin2Motor2;

    public MotorController() {
        try {
            // Right
            pin1Motor1 = service.openGpio("BCM17");
            pin2Motor1 = service.openGpio("BCM27");

            // Left
            pin1Motor2 = service.openGpio("BCM23");
            pin2Motor2 = service.openGpio("BCM24");

            pin1Motor1.setDirection(Gpio.DIRECTION_OUT_INITIALLY_LOW);
            pin2Motor1.setDirection(Gpio.DIRECTION_OUT_INITIALLY_LOW);
            pin1Motor2.setDirection(Gpio.DIRECTION_OUT_INITIALLY_LOW);
            pin2Motor2.setDirection(Gpio.DIRECTION_OUT_INITIALLY_LOW);

        } catch (IOException e) {
            e.printStackTrace();
        }
    }


    public void forward() {
        setPinValues(true, false, false, true);
    }

    public void backward() {
        setPinValues(false, true, true, false);
    }

    public void stop(){
        setPinValues(false, false,false,false);
    }

    public void turnLeft() {
        setPinValues(false, false, false, true);
    }

    public void turnRight() {
        setPinValues(true, false, false, false);
    }

    private void setPinValues(boolean p11, boolean p12, 
                              boolean p21, boolean p22 ) {

        try {
            pin1Motor1.setValue(p11);
            pin2Motor1.setValue(p12);
            pin1Motor2.setValue(p21);
            pin2Motor2.setValue(p22);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

}

This class accomplishes these tasks:

1. It gets a reference to the PeripheralManagerService.
2. It opens the GPIO pins.
3. It sets the directions and the initial value.

Moreover, it defines four different methods that control how the car will move:

• Forward
• Backward
• Turn left
• Turn right

All these movements can be controlled turning on or off each pin defined above.

That's all. Now, it is time to implement how we will control the car. There are several options we can implement for this purpose. We could use a simple Web server that has an HTML interface or we can use Android Nearby API, for example, or even a Bluetooth connection.

In this tutorial, we will use a simple Web Interface.

How to Implement an Android Things HTTP Interface

As said before, we will implement an HTTP interface so we can use it to control the Android Things remote car. To implement a simple HTTP Web server, we can use NanoHTTPD that is a simple and light-weight HTTP server. To do this, it is necessary to modify the build.gradle file by adding:

compile 'org.nanohttpd:nanohttpd:2.2.0'

Now let us create a new class called RobotHTTPServer that handles the incoming HTTP requests:

public class RobotHttpServer extends NanoHTTPD {
 public RobotHttpServer(int port, 
                        Context context, 
                        CommandListener listener) {
        super(port);
        this.context = context;
        this.listener = listener;
        Log.d(TAG, "Starting Server");
        try {
            start();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

   @Override
    public Response serve(IHTTPSession session) {
        Map<String, String> params = session.getParms();

        String control = params.get("control");
        String action = params.get("btn");

        Log.d(TAG, "Serve - Control ["+control+"] - Action ["+action+"]");

        if (action != null && !"".equals(action))
          listener.onCommand(action);

        return newFixedLengthResponse(readHTMLFile().toString());
    }
..
}

The HTML page is very simple and it is made by 5 buttons that represent the four directions and the stop button.

We will add the HTML page to the assets/ directory. The last part is defining a CommandListener that is the callback function that is invoked everytime the HTTP server receives a command:

 public static interface CommandListener { public void onCommand(String command); }

Assembling the App to Control the Android Things Remote Car

The last step is assembling everything and gluing these classes so that we can finally build the Android Things remote controlled car. To this purpose, it is necessary to create a MainActivity:

public class MainActivity extends Activity {

    private String TAG = getClass().getName();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Log.d(TAG, "OnCreate...");

        final MotorController mc = new MotorController();
        RobotHttpServer server = new RobotHttpServer(8090, this,
                  new RobotHttpServer.CommandListener() {
            @Override
            public void onCommand(String command) {
                Log.d(TAG, "Command received ["+command+"]");
                if (command.equals("F"))
                    mc.forward();
                else if (command.equals("B"))
                    mc.backward();
                else if (command.equals("S"))
                    mc.stop();
                else if (command.equals("L"))
                    mc.turnLeft();
                else if (command.equals("R"))
                    mc.turnRight();
            }
        });
    }
}

As you can see, the code is very simple, everytime the CommandListener receives a new command it calls a method of the class that handles the motor to control the motors.

This simple project can be further expanded. We could add a set of new feature like Vision, Machine learning, and so on. For this reason, we have used Android Things instead of Arduino or an ESP8266.

You now know how to interact with Android Things GPIO pins and how to turn them on or off. Moreover, you learned how to use motors. All these information you have acquired can be used to build your first Android Things remote controlled car.

Now you can play with your toy!