Mobile Fundamentals · Inside a Phone · Lesson 2 of 4

Sensors

The dozen tiny sensors that let a phone feel the world.

A phone that feels

Your phone is full of tiny sensors that measure the world around it. Most are chips just a few millimeters across, and nearly every smartphone carries a set of them.

• The accelerometer senses movement and tilt. It rotates the screen and counts your steps.
• The gyroscope senses rotation, which helps with games and steady video.
• The magnetometer senses magnetic fields and works as a compass.
• The ambient light sensor adjusts screen brightness to the room.
• The proximity sensor detects when something is right in front of the top of the screen. During a call, it turns the screen off when you hold the phone to your ear, so your cheek doesn't tap buttons. Most phones do this by bouncing invisible infrared light off whatever is close.
• The barometer measures air pressure.

These sensors produce data all the time. On both Android and iPhone, apps can usually read the motion sensors without asking you first. Researchers have shown that motion data can sometimes reveal what someone typed on the screen, so sensor data is more sensitive than it looks.

Which floor?

Some sensors are more useful than they look. The barometer measures air pressure, the same thing weather stations track. Air pressure drops slightly as you go higher. By watching tiny changes in pressure, a phone can tell when you've gone up or down, even by a single floor.

That's how the Health app on iPhone counts flights of stairs climbed, and many fitness apps work the same way. Most iPhones since the iPhone 6 have a barometer, and so do many Android phones, though not all.

Height matters in emergencies too. In the US, the FCC set a target for carriers to locate indoor 911 callers vertically to within 3 meters, roughly one floor, so responders know which floor of a building to search. Barometer readings from the phone are one of the main ways to meet it.

For investigators, this kind of data can show whether someone climbed stairs or changed floors at a certain time. That can still work indoors, where GPS is often weak.