What is a Motion Sensor?

Fundamentals
There are two types of motion sensors: active motion sensors and passive motion sensors. (Getty Images)

A motion sensor (or motion detector) is an electronic device that is designed to detect and measure movement. Motion sensors are used primarily in home and business security systems, but they can also be found in phones, paper towel dispensers, game consoles, and virtual reality systems. Unlike many other types of sensors (which can be handheld and isolated), motion sensors are typically embedded systems with three major components: a sensor unit, an embedded computer, and hardware (or the mechanical component). These three parts vary in size and configuration, as motion sensors can be customized to perform highly specific functions. For example, motion sensors can be used to activate floodlights, trigger audible alarms, activate switches, and even alert the police.  

There are two types of motion sensors: active motion sensors and passive motion sensors. Active sensors have both a transmitter and a receiver. This type of sensor detects motion by measuring changes in the amount of sound or radiation reflecting back into the receiver. When an object interrupts or alters the sensor’s field, an electric pulse is sent to the embedded computer, which in turn interacts with the mechanical component. The most common type of active motion detector uses ultrasonic sensor technology; these motion sensors emit sound waves to detect the presence of objects. There are also microwave sensors (which emit microwave radiation), and tomographic sensors (which transmit and receive radio waves).  

Active ultrasonic motion sensor.
Active ultrasonic motion sensor. (https://ultrasonicsensor.wordpress.com/)

Unlike an active motion sensor, a passive motion sensor does not have a transmitter. Instead of measuring a constant reflection, the sensor detects motion based on a perceived increase of radiation in its environment. The most widely used type of passive motion sensor in home security systems is the passive infrared (PIR) sensor. The PIR sensor is designed to detect the infrared radiation emitted naturally from the human body. The receiver is contained in a filter that only allows infrared to pass through it. When a person walks into the PIR sensor’s field of detection, the difference in radiation creates a positive charge within the receiver; this perceived change causes the sensing unit to send electrical data to the embedded computer and hardware component.

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