How does a PIR sensor trigger an alarm?

A Passive Infrared (PIR) sensor triggers an alarm primarily by detecting changes in infrared radiation, which is essentially the thermal energy emitted by objects, particularly warm bodies like humans and animals. When a warm body moves within the sensor's field of view, it causes a variation in the infrared radiation levels that the sensor detects. This change in thermal energy is translated into an electrical signal, leading to the alarm being triggered.

The operation of PIR sensors is based on detecting the difference in temperature between the background environment and the moving object. When the sensor identifies a change—such as someone walking through its field—it generates an electric pulse that can signal an alarm system. This is a fundamental characteristic of how PIR sensors function, setting them apart from other sensors that might rely on sound, light, or other forms of energy.