A passive infrared sensor is often used in motion detectors. Motion is detected when an infrared source with one temperature, such as a human, passes in front of an infrared source with another temperature, such as a wall. The sensor does not dectect the “heat” from the object passing in front of it. Instead it detects that the object breaks the field which the sensor has determined as the “normal” state. Any object, even one exactly the same temperature as the surrounding objects will cause the PIR to activate if it moves in the field of the sensors.
The term passive means that the PIR device does not emit an infrared beam but merely passively accepts incoming infrared radiation.