How sensors make smartphones smart

They are your guide when you are lost, your camera for snapping selfies, your digital wallet, DJ and games console, and they can even be your personal assistant. Oh, and you can also call people with them.

Smartphones today are now so smart that many of us cannot imagine working, resting or playing without them. And a lot of these smart functions stem from some seriously smart sensors.

There is a dizzying array of microscopic sensors inside the latest smartphones. Even a bog-standard mobile will have at least 10 sensors packing enough punch to allow anyone to transform their device into a powerful mobile physics lab.

Common sensors

Common sensors that you'll find in most smartphones are:

  • accelerometer
  • gyroscope
  • magnetometer (compass)
  • microphone
  • proximity sensor
  • GPS
  • ambient light sensor
  • touchscreen
  • barometer
  • thermometer (internal) 

Here's how five of the most useful smartphone sensors work.

Accelerometer

This measures acceleration (the rate of change of velocity) and, if you know the mass, they can be used to calculate the resultant force on an object.

Though there are many different types of smartphone accelerometer, they all consist of a tiny electronic device etched onto a silicon chip known as a micro-electromechanical system (MEMS).

Components of a MEMS accelerometer respond to acceleration mechanically – a movement or a stress. This induces a voltage that is used to gauge motion and orientation.

Measuring acceleration in three dimensions, your phone can interpret accelerometer readings to switch off when dropped or tell if you are walking, running, driving or flying.

Gyroscope

Stabilising the International Space Station are four huge gyroscopes, almost as large as the station’s astronauts. In contrast, your smartphone’s electronic gyroscope is the width of a human hair.

Unrecognisable to the scientists who first developed and used gyroscopes in the 18th and 19th centuries, smartphone gyroscopes are MEMS devices like accelerometers, but instead of responding to acceleration, the mechanical parts of the device respond to the phone’s rotation.

When combined, smartphone accelerometers and gyroscopes provide the motion and orientation information required for everything from auto-rotating your phone to enabling mobile augmented reality experiences.

Microphone

Arguably your smartphone’s most important sensor, given its role in performing the phone’s original singular task of calling people, the microphone is embedded in a pinhole just below the screen.

Most smartphone microphones today are MEMS devices, essentially consisting of a charged diaphragm, a pre-amplifier and an analogue-to-digital converter.

When you make a sound, air pressure moves the diaphragm, which alters the voltage across the membrane.

This voltage is boosted by the pre-amplifier and finally converted to a digital signal that the smartphone can use.

Magnetometer

Your smartphone contains three MEMS-based magnetic field sensors, fixed perpendicular to each other, to find the direction of Magnetic North.

These sensors are mainly used in navigation applications, starting up when you open Google Maps, for example. But some apps wield these magnetic measurements more creatively, allowing you to use your phone as a metal detector.

Physicists use sophisticated magnetometers to measure space weather and events like coronal mass ejections, which send strong magnetic fields from the Sun out into the cosmos, scrambling technology here on Earth. Though challenging, you can analyse your smartphone’s raw magnetometer data to spot these solar events too.

Proximity sensor

Consisting of an infrared (IR) diode and an IR radiation detector, your proximity sensor will be located at the top of the screen, near the receiver.

It emits a beam of IR light, some of which bounces back if an object is about 10 cm or less from the device.

When the IR radiation signal registers that the beam has been reflected back, this information can be used by the phone to automatically turn off the device’s screen, as it typically means that the phone is close to your ear or in your pocket. This helps prevent unintended screen taps and needlessly draining your battery.

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