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What are PIR Sensors?

By M.J. Casey
Updated May 16, 2024
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PIR (Passive Infrared Radiation) sensors perceive motion by detecting the heat generated by a moving human body. Passive means the device simply responds to the radiation received rather than actively sending a signal or scanning an area. Through refinement of the incoming heat signal, PIR sensors are very sophisticated devices with many uses.

Sensitive pyroelectric materials such as gallium nitride generate a small electric charge when exposed to infrared radiation. These materials are similar to photoelectric materials used in solar cells, but respond to heat rather than light. Other materials used are cesium nitrate and lithium tantalite.

No electricity is used and no radiation is emitted from PIR sensors, so in essence they cannot be detected unless seen. They must be connected to an electrical system to send the signal once activated, however. The electrical system may be battery operated and still undetectable. This silent behavior makes PIR sensors ideal for many applications, such as protecting valuables or restricting access to dangerous areas.

Early in development, random heat radiation such as from sunlight or a stationary heat source might cause false positive responses. By adding filters in front of the device, these erroneous signals are usually stopped. Infrared filters stop heat radiation that is not in the same wavelength range as that emitted from the human body.

A Fresnel lens, which consists of a series of concentric grooves on a transparent surface, enables a wider field of detection and breaks the incoming radiation into zones. By activating more area of the pyroelectric material, the Fresnel lens increases the sensitivity of PIR sensors. The detection of heat change from one zone to the next ensures that the source of the heat is moving. The signal generated from the PIR sensors is sent to a comparator that checks for established patterns that are to be ignored.

Common applications for PIR sensors include controlling room lights and door openers. They are ideal for detection of someone entering a secure area and have been combined with signal-activated cameras to capture images of the intruder. Advanced PIR sensors allow user programming to control devices as diverse as vending machines, walk-by displays, and escalators. Their obvious use in security has broadened to include energy and advertising markets. In combination with other sensors and camera systems, PIR sensors may be used in traffic control systems monitoring traffic flow, hazardous conditions, and traffic law violators.

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