A pyroelectric sensor, also PIR sensor, is a semiconductor sensor used to detect temperature changes. PIR sensors are based on the eponymous pyroelectricity, a property of some piezoelectric semiconductor crystals. In this case, a temperature change .DELTA.T leads to a measurable change in the electrical voltage at the sensor output. Like other temperature sensors, PIR sensors do not react to a specific, constant temperature level, but only to the change in temperature. There are analog and digital pyroelectric sensors.Digital PIR sensors have a much higher immunity to interference, due to their structure. Analog PIR sensors, however, are far cheaper. Further differences are nonexistent. A simple optic is placed over the active sensor element to improve detection of changes and range. Typically, a spherically or cylindrically curved HDPE Fresnel lens is used. Due to the design of this lens, on the one hand, the spatial effect of the sensor can be influenced and adapted to the respective application, on the other hand, the steps of the Fresnel lens in combination with the two sensor elements result in a spatial fanning of the sensitivity ranges, which the detection of spatial changes of heat sources through steeper transitions improved. The Fresnel lens may be subdivided into individual sector elements and into a plurality of partial lenses.

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