Which type of sensor is commonly used to detect motion in an area?

A Passive Infrared (PIR) sensor is commonly used to detect motion in an area because it operates by detecting changes in infrared radiation, which is emitted by objects based on their temperature. When a warm body, such as a person or animal, moves in front of the sensor, it causes a change in the infrared energy detected, triggering the alert. This type of sensor is especially effective for indoor and outdoor security applications because it can cover wide areas and is designed to differentiate between moving objects and stationary backgrounds.

Ultrasonic sensors emit sound waves and measure their reflection off objects to detect motion, which can be affected by various environmental factors, such as humidity and temperature, making them less reliable in certain situations. Magnetic contact sensors are primarily used to detect the opening or closing of doors or windows rather than general motion. Vibration sensors detect changes in pressure or movement but are typically used to monitor specific surfaces rather than wide-ranging motion detection, making them less common for general security applications.