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 it's window. 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 sensed when the infrared energy being emitted from his body changes the pattern of hot spots on the chip's surface. As the intruder moves so does the hot spot on the surface of the chip. The electronics connected to the chip recognize this moving hot spot as an intruder and de-energize the relay, operating it's contacts thereby activating the detection input on the alarm control panel.

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. Some PIR's are designed with reversable orientation mirrors which allow either broad coverage (110° wide) or very narrow 'curtain' coverage.