How the Pir Alarm Works

Written by tony breedlove
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PIR (Passive InfraRed) alarms are used to detect the heat, or heat energy, dissipated by a person moving within the detectors' field of view. The alarms also will detect dogs, cats, raccoons, or any warm blooded creature. PIR alarms are also called "motion detectors" and "passive infrared detectors." They are called "passive" because they do not project any type of beam. They are often used in security systems and are excellent for detecting intruders.

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What is Infrared?

Infrared is radiation that is above the wavelength of visible light. Humans cannot see infrared. A PIR system detects the infrared heat energy, then trips a switch to set off an alarm or turn on a light.

Fresnel Lens

PIR alarms are composed of three basic parts. On the outside is the Fresnel lens, which is an opaque white plastic window. The detector chip sits behind the lens. The Fresnel lens is much like the lenses used in old lighthouses, which allowed relatively small lights to be seen for many miles. The Fresnel lens amplifies the infrared heat signature being dissipated by a person passing by and shines that amplified signal onto the PIR chip.

PIR Chip and Alarm

The PIR detector chip is a small electronic component that is sensitive to infrared energy. In addition to amplifying the energy and shining it on the surface of the PIR chip, the lens also splits or breaks up the infrared signal. (That process can be compared to watching a car at night as it drives past a line of poles. Each time the car passes a pole, its headlights appear to flash). The PIR chip sees the flashes and creates a tiny voltage spike, which is sent to a circuit and sets off an alarm or turns on a light.


PIR alarms are used outdoors to activate security lights. Also, you can purchase a battery powered PIR transmitter and receiver pair at any large home improvement store; the PIR detector/transmitter is placed outside and the receiver is placed inside. When someone passes by the PIR detector outside, the receiver inside will chime.

Because PIR detectors detect any warm-blooded creature, most can be adjusted to limit false alarms created by stray animals.

PIR alarms also are used in indoor alarm systems in businesses and homes. PIR alarms (called motion detectors) connected to alarm systems cannot be adjusted, but there are "pet immune" detectors. Those are designated "30#" for animals weighing less than 13.6kg. and "70#" for animals less than 31.8kg.

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