One of the most frequent questions we’re asked is “what’s a proximity sensor? what is it used for in a smartphone?”. If I’m in the right mood I generally answer that, if you put your device on the windshield of your car, it will help you mantaining a safe distance from the preceeding car.

Obviously I immediately say I’m just joking, because I don’t want anyone’s life on my coscience. Nonetheless technically a car proximity sensor works just the same as the one in your phone.

What is it?

It’s a device emitting electromagnetic radiations, like a radar, and can receive back reflected waves. Clearly, the way these waves are reflected depends on the obstacle they hit, and in particular its distance.

Right on this principle, hence based on reflected signal characteristics (it’s strength in particular), the proximity sensor can know when something is close to it.

It’s worth noting that not all the proximity sensors work this way, but in our phones the “EM radiations” are just infrared light (like that of tv remote controller). If you have a webcam, in the dark, it’s possible to see the sensor light during a call. Webcams can partially see infrared lights.

Drawing of proximity sensor for smartphones

What is it for?

Capacitive touchscreens are very sensible, because they don’t work based on pressure like resistive ones, but only when touched by some conductive material, like a finger. But you’ll soon find out that many parts of the body are conductive.

It comes out that even an ear can activate the touchscreen, and during a phone call this can become quite troublesome.

The solution commonly adopted is to put a proximity sensor next to the smartphone speaker, in the most natural position that could tell we’re talking on the phone. In that case, the touchscreen gets disabled preventing unwanted consequences.

Actually this solution was also exploited to turn off the display during calls to save some battery charge.

That’s interesting, is there anything else to know?

As explained, this sensor is close to the speaker, which is the part of the phone that is more likely close to body during a call. In some cases, if your smartphone manual doesn’t tell you, you might find it because display protective films leave a space right in front of the sensor not to interfere with its waves.

However, if you try to put a finger over that spot (and if you’ve read this far, you have already tried, I know!) you’ll find nothing happens. This is because during normal use we’re supposed to have full control of the touchscreen, and if proximity sensor was active the display could turn off in wrong moments.

In fact this safety system turns on only during phone calls, so if you want you can call somebody and find where the sensor is, and what distance it considers close, that is to which point the display doesn’t turn off. Have fun!