How Does a Motion Sensor Work?

The way in which a motion sensor works typically depends on the type of sensor being used, which often depends on the device that uses the sensor. One of the most common types of sensor technology is an active sensor that sends out bursts of energy, which bounces back in a way similar to sonar. There are also passive sensors that do not send out any type of signal, but instead receive energy from their surroundings to detect motion. Some systems can also use a combination of both active and passive technology to create a motion sensor that emits and receives an energy signal.

A motion sensor is an electronic device — often part of a larger system, such as a light or camera — that detects motion to activate the system. There are two basic types of motion sensors: active sensors and passive sensors. Active sensors emit a signal, typically an ultrasonic burst of sound waves, similar to a sonar system, which is reflected by the surroundings; this reflected signal is received by the sensor. When something moves within the area of an active motion sensor, the change in signal that is reflected to the sensor activates the system. This type of sensor is often used for indoor security and automatic garage door openers.


Passive sensors are a type of motion sensor that do not emit a signal, but instead detect infrared radiation around the sensor. When a person or animal moves through the area, heat from the movement is detected by the sensor, which then activates the system to which it is connected. This type of motion sensor is often used for motion-activated lights or cameras in a security system. Passive sensors are typically set to detect only sudden or extreme changes in thermal radiation, which prevents such systems from activating due to changes in surrounding temperature caused by the sun rising or pavement cooling at night.

A motion sensor can also utilize a combination of passive and active technology. This type of system is often used in stores to trigger a sound when someone enters the business. The motion sensor usually consists of two parts: one that emits a laser or similar energy signal, and a second piece that receives the signal. When someone passes through the system, the signal is interrupted and the receiving piece activates a response such as a bell. This type of system is not usually used for security purposes, since the energy beam could be avoided quite easily to keep from activating the system.