Chapter 4. The Light and Proximity Sensors

In this chapter, we will learn about proximity and light sensors, and we will develop a small application using them. We will also learn about the concepts of wake locks and wakeup and non-wakeup sensors. We will understand the hardware sensor FIFO queue and what happens to sensors when the application processor goes into suspended mode.

You will learn the following things in this chapter:

• Understanding the light and proximity sensors.
• Understanding requirements for the automatic torchlight and screen brightness app.
• How to use the proximity sensor in the phone and turn phone's flashlight off and on.
• How to use the phone's light sensor and adjust the screen brightness.
• What are wake locks and how should we use them?
• What are wakeup and non-wakeup sensors, and what is the hardware FIFO sensor queue?

Understanding the light and proximity sensors

The light sensor is a part of the environmental sensors, and the proximity sensor is a part of the positional sensors for Android. Both light and proximity sensors can be found in almost every Android device today. The light sensor is responsible for measuring the illuminance in lux. Illuminance is the amount of light striking a surface. It is also known as incident light, where the "incident" is the beam of light actually landing on the surface.

The proximity sensor measures the proximity of any object near the screen. There are two types of proximity sensor found on Android devices.

• The first type of the proximity sensor provides the absolute distance in centimetres between the object and the phone. There are very few phones which support this type of proximity sensor.
• The second type of sensor gives only two values in form of integers, which represents the proximity of the object from the phone. When the object is near, then the value is 0, while...

The automatic torch light and screen brightness app requirements

As a learning assignment for this chapter, we will be developing a small application that will make use of the light and proximity sensor in the phone to turn on and turn off the flash light and adjust the screen brightness. This app will be running in the foreground Android activity and will start processing the sensor values on onResume() and will stop on onPause(). We will have the separate activity for each proximity sensor and light sensor, and both will work independently. The following are the high-level requirements for the automatic torch light application:

1. Create an Android activity to process the proximity sensor values.
2. Whenever any object comes close to the phone (the proximity sensor gives the near value), turn on the flashlight, and whenever that object goes away from the phone (the proximity sensor gives the far value), then turn off the flashlight.
3. Create an Android activity to process the light sensor values...

Time for action – turning the torch light on and off using the proximity sensor

In this section, we will be learning how to use the proximity sensor to turn the camera flash light on and off. As discussed earlier, most proximity sensors return the absolute distance in cm, but some return only the near and far values. The near value is 0 and the far value is the maximum range of the proximity sensor. There are a lot of common use cases for proximity sensors, such as to turn off the screen while the user is on a call, or to detect if the phone is inside the pocket or outside. For our example, we will be turning the camera flashlight on whenever any object comes near the phone screen and turning it off when the object goes far from the phone screen. The proximity sensor has on-change reporting mode, the details of reporting modes are explained in Chapter 1, Sensor Fundamentals. It is fired as soon as the proximity of the object near the phone changes.

The following code shows how to use the...

Time for action – adjusting the screen brightness using the light sensor

One of the most common use cases for the light sensor is to adjust the screen brightness according to the external lighting conditions. The maximum range of the light sensor might be different on different Android devices, but most of them support from 0 lux to several thousand lux. Lux is the standard unit for measuring the luminance of the light falling on a surface. For our example, we will use a range from 0 to 100 lux, as normal indoor lighting falls within this range. But for sunlight and strong lights the range can go up to 1,000 lux or more. In the sample app, we will increase the screen brightness, when the indoor lighting goes low, and similarly we will decrease the screen brightness when it goes high.

1. We followed the standard steps to get values from the sensor. We select the sensor type to the TYPE_LIGHT in the getDefaultSensor() method of SensorManager. We also called the custom initScreenBrightness() method...

Wake locks, wakeup sensors, and the FIFO queue

All Android applications run on a dedicated Application Processor (AP), which is a part of the main CPU of the phone. This application processor is designed in such a way that it goes into the suspended mode when the user is not interacting with the phone. In this suspended mode, it reduces the power consumption by 10 times or more, but this freezes all the applications in the background. To work around this problem, the Android platform provides a solution using wake locks. If an application has to perform some important operation in the background and doesn't want the application processor to go into suspended mode, then it has to request a wake lock from the system's power service. Once the important operation is completed, it should release the wake lock. Wake lock can be obtained using the PowerManager object, which is provided by the system power service. The newWakeLock() method of PowerManager provides the object of wake lock. This newWakeLock...

Summary

In this chapter, we looked at the two new proximity and light sensors and developed a small app using them. We also learned how to turn on and turn off the flashlight using the proximity sensor and adjust the screen brightness using the light sensor. We understood how to wake up the application processor when it's in suspended mode using wake locks. We looked at the wakeup and non-wake up sensors and their FIFO queues.

In the next chapter, we will learn about motion sensors (accelerometer, gyroscope, linear acceleration, gravity, and significant motion) and position sensors (magnetometer and orientation). We will also explore the newly introduced fingerprint sensor.