Passive infrared sensor
Passive InfraRed sensors (PIR's) are electronic devices which are used in some security alarm systems to detect motion of an infrared emitting source, usually a human body. All objects, living or not, whose temperature is anything above absolute zero (-273.15°C or -459.67°F) emit infrared 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 it's housing. The heart of the sensor is a solid state 'chip', approximately 1/4 inch square, mounted on a printed circuit board which also contains the necessary electronics required to interpret the signals from this chip. The printed circuit board is contained in a 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 translucent to visible light) plastic sheet which may or may not have Fresnel lenses molded into it. This plastic sheet prevents the intrusion of dust and insects while the Fresnel lenses focus the infrared energy onto the surface of the chip.
Some PIR's 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 it's 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.
An intruder entering the protected area is detected when the infrared energy emitted from his 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 it's contacts, thereby activating the detection input on the alarm control panel. Conversly, if an intruder were to try to defeat a PIR perhaps by holding some sort of thermal sheild between himself and the PIR, a correspoding 'cold' spot moving across the face of the chip will also cause the relay to de-energize.
Manufacturers recommend careful placement of their product to prevent false alarms. They suggest mounting the PIRs in such a way that the PIR cannot 'see' out of a window. Although the frequency of infrared radiation to which the chips are sensitive does not penetrate glass very well, a strong infrared source (a vehicle headlight, sunlight reflecting from a vehicle window) can overload the chip with enough infrared energy to fool the electronics. A person passing on the other side of the glass however would not be 'seen' by the PIR.
They also recommended that the PIR not be placed in such a position that an HVAC vent would blow hot or cold air onto the surface of the plastic which covers the housing's window. Although air has very low emissivity (emits very small amounts of infrared energy), the air blowing on the plastic window cover could change the plastic's temperature enough to once again fool the electronics.
PIR's come in many configurations for a wide varity of applications. The most common used in home security systems has numerous Fresnel lenses or mirror segments and has an effective range of about thirty feet. Some larger PIR's are fabricated with single segment mirrors and can sense changes in infrared energy over one hundred feet away from the PIR. There are also PIR's designed with reversable orientation mirrors which allow either broad coverage (110° wide) or very narrow 'curtain' coverage.