How does the PIR sensor works?

Passive infra-red (PIR) motion detectors used in security systems are basically heat radiation detectors, sensing the motion of a heat source (infra-red radiation) to detect an intruder. They have a photo-diode that is sensitive to the heat radiation frequencies emitted by the human body, and a special lens that looks at 20 - 30 narrow fields within the protected space. They 'see' the temperature of the background, and any rapid change to this temperature is considered to be a suitable trigger. A person (or animal) moving in the space will move in and out of the fields of view, giving infra-red images to the detector in a rapid 'now you see me, now you don't' pattern.

This generates a series of pulses in the photo-diode as the heat source moves into and out of the fields of view of the PIR lens system. Even if part of your body is at the same temperature as the background, other parts will be at different temperatures, and will probably be detected. (Your head is hotter, your hands are cooler.) Slow changes in the infra-red signature of the room are ignored, as are single pulses, which may be environmental changes (air conditioners) or electrical noise (RFI).

Most modern PIR detectors count the number of times a target appears and disappears from their view, and if it happens (say) 3 times in 5 seconds, they will 'trip' and give an alarm signal. Other detectors analyse the pattern of changes to the heat signal to see if it's human, animal or environmental.

These detectors don't emit any light, they simply look for other sources of heat radiation (hence, 'passive'.) Other devices, such as microwave detectors or ultrasonic detectors, emit a signal and look for the 'doppler effect' frequency shift that a moving target will cause in the reflected signal. Like 'break-beam' detectors, they are 'active' detectors.