An alarm system is a complex system that uses several sensors to keep our home safe. At the heart of these types of systems, there are sensors that are able to detect motion. In other words, these sensors can detect if an object is moving in their detection area. When this happens, they notify this event. In this chapter, we will create a real-life project that uses these sensors to detect motion and notifies the event to the smartphones. At the end of this project, we will be able to detect if someone is entering our home without our authorization. Once you have built this project, you can expand it, adding more sensors so that you can monitor several rooms. Moreover, this project can be used as a starting point and can be expanded, adding new features as we will see in the following sections. The following figure describes how this Android Things...

In this last step, we will learn how to close the connection with a GPIO pin. This is an important step because in this way, we free the resources and remove all the listeners we added to the GPIO pins.

An Android Things app has a life cycle very similar to an Android app. The place where we implement these actions is the Activity onDestroy method. In this method we have to:

• Remove all the listeners attached to the GPIO pins
• Close the connection to the GPIO pins

So, open MainActivity.java again and look for the onDestroy method and modify it:

@Override
protected void onDestroy()
  { super.onDestroy(); Log.d(TAG, "onDestroy");
  if (gpioPin != null) {
    gpioPin.unregisterGpioCallback(sensorCallback);
      try {
    gpioPin.close(); gpioPin = null;
      }
    catch(Exception e) {}
  }
}

There are two important aspects that we have not covered:

• How to select the pin to connect the peripherals to
• How to identify the pin name

Regarding the first aspect, in Raspberry Pi 3 and Intel Edison, but in general for all the boards, the pins do not provide the same features. In other words, we cannot connect the peripherals to a pin by choosing it randomly. We have to select the pin according to the peripheral specifications. In this context it is important to know the pinout of the boards so that we can identify the right pins for our peripherals.

The second aspect is relevant when we want to develop an Android Things app that will run on different boards. As we said in the previous chapter, from the code point of view, this is not a problem because the Java language at the base of Android Things SDK guarantees us that we can run...

Now we are ready to implement the last part of this project: the notification system. In the next paragraphs, we will describe how to send a notification to the user smartphone when motion is detected. As the messaging system, this IoT project uses Google Firebase. This is a cloud platform developed by Google providing several interesting services. We will use the Notification service.

There are several ways we can send a notification from the Android Things app to a user smartphone. To keep things simple, we will use topic. You can imagine a topic like a channel. After a device subscribes to a topic, it will receive all the messages published to this channel. In our project, the user smartphone behaves like a subscriber receiving messages from the channel, while the Android Things app behaves like a publisher publishing the messages.

Now...

In order to receive the notifications, we have to install an Android companion app on our smartphone. Just to simplify the system, this Android app will:

1. Subscribe to the channel used by the Android Things app to send notifications.
2. Implement a service to listen to the incoming notification.
3. Show the notification to the user.

If you do not know how to receive notifications in Android you can visit https://firebase.google.com/docs/android/setup to know more. The source code of the Android companion app is provided with this book's source code. The app interface is very simple because we simply have to subscribe to the topic and wait for the incoming notifications.

To install the app, just open the project using Android Studio and connect your smartphone to your PC/Mac. Add the google-services.json file to your app module. This file is the same one that...