Passive infrared sensor (PIR), how it works?

A Passive InfraRed sensor (PIR sensor) is an electronic device that measures infrared (IR) light radiating from objects in its field of view. PIR sensors are often used in the construction of PIR-based motion detectors (see below). Apparent 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.[1]

All objects emit what is known as black body radiation. It ilios usually infrared radiation that is invisible to the human eye but can be detected by electronic devices designed for such a purpose. The term passive in this instance means that the PIR device does not emit an infrared beam but merely passively accepts incoming infrared radiation.

In a PIR-based motion detector (usually called a PID, for Passive Infrared Detector), the PIR sensor is typically mounted on a printed circuit board containing the necessary electronics required to interpret the signals from the pyroelectric sensor chip. The complete assembly is contained within a housing mounted in a location where the sensor can view the area to be monitored. Infrared energy is able to reach the pyroelectric sensor through the window because the plastic used is transparent to infrared radiation (but only translucent to visible light). This plastic sheet also prevents the intrusion of dust and/or insects from obscuring the sensor's field of view, and in the case of insects, from generating false alarms.

A few mechanisms have been used to focus the distant infrared energy onto the sensor surface. The window may have multiple Fresnel lenses molded into it.
Alternatively, some PIDs are manufactured with internal plastic, segmented parabolic mirrors to focus the infrared energy. Where mirrors are used, the plastic window cover has no Fresnel lenses molded into it. This filtering window may be used to limit the wavelengths to 8-14 micrometers which is closest to the infrared radiation emitted by humans (9.4 micrometers being the strongest).