Those of you who use a mobile phone with a touch-screen may have wondered why items on the touch-screen do not trigger when you hold the phone to your ear while answering a call. Well, designers of mobile phones with touch-screen have built-in a feature that prevents a situation such as “My ear took that stupid picture, not me.” The savior in this situation is the tiny sensor placed close to the speaker of the phone, and this proximity sensor prevents touch-screen activity when anything comes very close to the speaker. That is what happens when your ear touches the screen as you are on a call, but does not generate any touch events.
So, what sort of proximity sensors do the phones use? Well, in most cases, it is an optical sensor or a light sensing device. The sensor senses the ambient light intensity and provides a “near” or “far” output. When nothing is covering the sensor, the ambient light falling on it makes it give out a “far” reading, and keeps the touch-screen active.
When you are on a call, your ear covers the sensor, obstructing the device to see ambient light. Its output changes to “near” and the phone ignores any activity from the touch-screen, until the sensor changes its state. Of course, the mobile phone considers more complications such as what happens when the ambient light falls very low, but we will discuss more on different types of proximity sensors instead.
Different types of proximity sensors detect nearby objects. Usually, the proximity sensor is used to activate an electrical circuit when an object either makes contact with it or comes within a certain distance of the sensor. The sensing mechanism differentiates the types of sensors and these can be Inductive, Capacitive, Acoustic, Piezoelectric and Infra-Red.
You may have seen doors that open automatically when you step up to them. When you are close to the door, the weight of your body changes the output of a piezoelectric sensor placed under the floor near the door triggering a mechanism to open the door.
Cars avoid bumping into walls while backing. The proximity sensor (a transmitter and sensor pair) used here works acoustically. A pair is fitted on the backside of the car. The transmitter generates a high frequency sound signal and the sensor measures the time difference of the signal bounced back from the wall. The time difference reduces as the car approaches the wall, telling the driver when to stop.
Computer screens inside ATM kiosks and the screen on your mobile are examples of capacitive proximity sensors. When you put a finger or a style on the screen, the device detects the change in the capacitance of the screen. The device measures the capacitance change in two directions, horizontal and vertical, or in x and y directions, to pinpoint the exact location of your finger and operate the function directly underneath.
When a security guard checks you out with a wand, or you walk through a metal detector door, the guard may ask you to remove your watch, coins from your pocket and in many cases, even your belt. The reason is the wand or the door has an inductive proximity sensor that will trigger in the presence of metals (mostly made of iron or steel).
Finally, the fire detector in your home or office is a classic example of a proximity sensor working on Infrared principles. Level of infrared activity beyond a threshold will trigger the alarm, and bring the fire brigade rushing.
