How Motion Sensors Work

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Motion sensors, along with cameras and lighting systems, have become one of the most recognizable parts of the best home security systems. They are also used for a variety of everyday purposes outside of home security. However, you may have wondered how motion sensors work from a mechanical perspective.

Read on to learn how motion sensors work and the different types of sensors available.

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What is a motion sensor?

A motion sensor or motion detector is a device that sends an electrical signal when it gets triggered by motion. This signal can be synced to turn on a light, activate a simple alarm or be part of a home security system. Motion sensors are usually placed near entry points and outside homes to detect potential intruders and stop unwelcome guests.

A motion sensor can be used to activate a security camera, which then immediately starts recording, or simply to turn on a light when you approach the front door. Its variety of uses in home security systems and relatively low cost have made them a common element of even the simplest of security systems.

The first motion detection device was invented by attorney Samuel Bagno in 1947 in New York. Its patent claims that it would activate by “interference with mechanical vibrations in air or other fluid using active vibration detection systems using ultrasonic detection means using Doppler shift detection circuits”.

Other uses for motion sensors

Motion sensors are used widely and for a variety of purposes unrelated to home security, including automatic door openers in public buildings and activating public smart lighting systems that conserve energy when not in use.

Types of motion sensors

Motion sensors that are part of a security system are classified as having either active or passive detection systems. Active sensors fall into three categories: optical, microwave and acoustic.

Active sensors

• Optical sensor: An optical sensor works by detecting and measuring light intensity, turning it into electrical signals that can be read by the motion sensor, usually in conjunction with a light transmitter. The sensor then detects any changes in light intensity, which activates the electrical trigger inside the device.
• Microwave sensor: A microwave sensor uses high frequency radio waves to sense motion. The advantage of a microwave sensor is that it has range in every direction, emitting waves by a transmitter which are then bounced back to a receiver that detects any changes.
• Acoustic sensor: As the name suggests, an acoustic sensor measures sound waves in the surrounding environment, detecting any sudden changes. The device produces an ultrasonic sound wave which humans cannot hear that then bounces back to the sensor. These work best for intruder detection in otherwise quiet environments.

Passive sensors

Passive sensors don’t emit anything, only detecting changes in how infrared radiation hits its passive infrared (PIR) sensor. These are also sometimes called passive infrared detectors (PID). By detecting the radiation emitted by bodies, it can detect the presence of people, animals and some objects. Most low-cost sensors are passive sensors.

Dual-technology sensors

Some sensors may use multiple technologies at once for a more accurate reading, using a passive PIR sensor on top of a microwave sensor that only activates after the PIR sensor is tripped. This can potentially save energy when compared to operating a transmitter nonstop and gives more accurate readings by using two sensors to detect motion.

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