Detection Devices (Light, Electromagnetic, and Motion Sensors)

An optical sensor (光学センサー) is a sensor that detects whether light is falling on it and how strong that light is, then converts this into an electrical signal.

A sensor is an input-side component that reads a physical quantity such as light, sound, temperature, or movement and turns it into an electrical signal a computer can work with. An optical sensor converts the amount of light reaching its light-receiving surface into a current or voltage level, so it can tell a device whether it is bright or dark, or whether something is blocking the light. When a smartphone brightens or dims its screen to suit the room, for example, the work is done by an ambient light sensor, one type of optical sensor.

AnalogyThink of a lookout posted in a room who notices when the light changes and reports it.

1 past questions on this term 光学センサー →

An infrared sensor (赤外線センサー) detects infrared light, which has a longer wavelength than visible light and cannot be seen by the human eye.

Any object with a temperature gives off infrared light that depends on how warm it is. An infrared sensor reads the amount of this infrared and how it changes, so it can tell when people come and go, or how warm an object is, even in the dark. It is found in the motion detectors of lights that switch on as someone walks up, and also in the part of a TV that picks up the infrared signal sent by the remote control.

AnalogyIt is like skin so sensitive to heat that it can feel a person's warmth even in total darkness.

How to remember"Infra" is Latin for "below": infrared sits just below red in frequency, past the edge of what the eye can see.

A magnetic sensor (磁気センサー) detects the strength and direction of the magnetic field around it.

A magnetic field is the region around a magnet or an electric current where magnetic force acts. The Earth produces a weak field of its own, so measuring its direction with a magnetic sensor reveals which way is north; this is how a smartphone's electronic compass shows your heading in a map app. Fix a magnet to a door, and the same kind of sensor can also tell whether the door is open or shut from whether the field is present.

AnalogyIt is a pocket compass that reports north with an electrical signal instead of a needle.

An accelerometer (加速度センサー) detects how quickly an object's speed changes, which is its acceleration.

Acceleration is the amount by which speed changes per unit of time. Besides changes in speed such as starting off or stopping suddenly, an accelerometer also senses the direction of gravity, which acts even when the device is still, so it can tell how the device is tilted. It is what lets a smartphone switch between portrait and landscape, and what lets a pedometer count the jolts of each step.

AnalogyIt is like a train passenger who knows the train is braking because their body is pulled forward.

How to rememberThe word itself says it: accelerating means changing speed. Cruise at a steady speed and the reading stays the same.

A gyroscope sensor (ジャイロセンサー) detects how fast an object is rotating, which is its angular velocity.

Angular velocity is how many degrees an object's orientation changes per unit of time. A gyroscope sensor measures which axis a device is turning around and how fast, so it can follow small changes in orientation. It keeps camera images steady and drones level, and it is often paired with an accelerometer to work out a device's orientation.

AnalogyIt is like sitting in a swivel chair with your eyes closed and still feeling it when someone spins you around.

How to remember"Gyro" comes from the Greek word for "turning"; the name traces back to the gyroscope, a device that relies on a spinning top keeping its axis pointed the same way.