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Passive InfraRed sensors (PIRs) are electronic devices which are used in some security alarm systems to detect motion of an infrared emitting source, usually a human body. more...

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All objects, living or not, whose temperature is anything above absolute zero (−273.15°C or −459.67°F) emit infrared radiation; see black body radiation. This radiation (energy) 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 the PIR does not emit any energy of any type but merely sits 'passive' accepting infrared energy through the 'window' in its housing. The heart of the sensor is a solid state 'chip', approximately 1/4 inch square made from a pyroelectric material. This chip is then mounted on a printed circuit board which also contains the necessary electronics required to interpret the signals from the chip. The printed circuit board is contained in the housing which is then mounted in a location where the chip can 'see' the area to be 'protected'. The aforementioned window in the housing allows infrared energy to reach the chip. The window is covered with an infrared-transparent (but only translucent to visible light) plastic sheet which may or may not have Fresnel lenses moulded into it. This plastic sheet prevents the intrusion of dust and insects while the Fresnel lenses, if present, focus the infrared energy onto the surface of the chip.

Some PIRs use a plastic segmented parabolic mirror or mirrors to focus the infrared energy onto the surface of the chip. Their plastic window cover has no Fresnel lenses molded into it. In either case, the PIR can be thought of as a kind of infrared 'camera' which remembers the amount of infrared energy falling on its surface, focused there by the mirrors or the Fresnel lenses. It might help to think of these focused points as 'hot spots' on the surface of the chip. Once power is applied to the PIR the electronics in the PIR shortly settle into a quiescent state and energize a small relay. This relay controls a set of electrical contacts which are usually connected to the detection input of an alarm control panel.

The actual sensor on the chip is made from natural or artificial pyroelectric materials, usually in the form of a thin film, out of gallium nitride (GaN), caesium nitrate (CsNO₃), polyvinyl fluorides, derivatives of phenylpyrazine, and cobalt phthalocyanine. (See pyroelectric crystals.) Lithium tantalate (LiTaO₃) is a crystal exhibiting both piezoelectric and pyroelectric properties.

An intruder entering the protected area is detected when the infrared energy emitted from the intruder's body is focused by a Fresnel lens or a mirror segment and overlaps a section on the chip which had previously been looking at some much cooler part of the protected area. That portion of the chip is now much warmer than when the intruder wasn't there. As the intruder moves, so does the hot spot on the surface of the chip. This moving hot spot causes the electronics connected to the chip to de-energize the relay, operating its contacts, thereby activating the detection input on the alarm control panel. Conversely, if an intruder were to try to defeat a PIR perhaps by holding some sort of thermal shield between himself and the PIR, a corresponding 'cold' spot moving across the face of the chip will also cause the relay to de-energize — unless the thermal shield has the same temperature as the objects behind it.