How Does a Motion Sensor Work?

Motion sensors are a part of many different mechanisms you encounter every day, from automatic doors at supermarkets to security systems that might protect your home. There are many different types of motion sensors, and while they all perform the same basic function, each has its advantages and disadvantages.

Outdoor Motion Light
Image of motion sensor and light.
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Active Sensors

Active sensors may use infrared energy, radar waves or other methods to sweep the area they cover to detect movement. Once activated, an active sensor sends out pulses of energy and times the echoes those pulses create when the energy reflects off of nearby objects. If someone walks into the scanned area, the echo time will change, indicating that someone is moving through the area. When the sensor detects movement beyond its tolerance limits, it will trigger.

Passive Infrared Sensors

Passive infrared sensors work by measuring the infrared energy of the surrounding environment. All living things give off heat, and these sensors can detect that heat. When a human or animal enters an area covered by passive infrared sensors, the increase in infrared energy tells the sensor that someone or something is moving through the area. Users can calibrate these sensors to trigger only at certain levels of infrared heat, in order to prevent them from going off in the presence of birds or small animals.

Beam and Photoelectric Sensors

These motion sensors rely on a focused beam of energy traveling between an emitter and a sensor unit. Whenever something moves between the emitter and sensor, it interrupts this beam and triggers the motion detector. In some cases, this beam may be invisible infrared energy, while less expensive units may use visible light to detect motion. Some passive sensors rely on ambient light to detect movement, only triggering when something passes near enough to the sensor to reduce the amount of light available.

Advantages and Disadvantages

Active sensors are usually the most effective motion detectors, although they require more energy to run and require calibration to prevent false positive results. The sensors that trigger automatic doors in supermarkets and retail stores are often active sensors. Passive infrared allows you to cover a much larger area with motion sensors and requires less energy, but a slow increase in temperature may or may not trigger a passive detector. These units are common parts of security systems, especially in remote areas. Beam sensors are often the cheapest and most simple form of motion detector, and often cover doors and windows in home security systems. However, they are prone to false positives, and an intruder has to step directly into the beam to trigger one of these units.