PIR-Based Motion Detector: (1) Operation: Strictly speaking, individual PIR sensors do not detect motion; rather, they detect abrupt changes in temperature at a given point. As an object, such as a human, passes in front of the background, such as a wall, the temperature at that point will rise from room temperature to body temperature, and then back again. This quick change triggers the detection. Moving objects of identical temperature, however, will trigger a detection because that is the sensors job. It detects motion regardless of temperature. This is also why you can take a piece of paper from a desk that is the same, exact, temperature as everything in the room, attach it to a stick (also the same exact temperature as the room) and wave it slowly in front of a sensor (at any range within the limit of the sensor) and it will activate the sensor). Mcgyver2k (talk) 09: 02, 18 April 2013 (UTC). PIRs can be equipped with more than one internal sensing element so that, with the appropriate electronics, it can detect the apparent direction of movement. As an object passes in front of adjacent sensors in turn, this implies the direction of movement. This may be used by on-board electronics to reduce false alarms, i. e. , by requiring adjacent sensors to trip in succession. It may also be used to signal the direction of movement to a monitoring apparatus. PIRs come in many configurations for a wide variety of applications. The most common models have numerous Fresnel lenses or mirror segments, an effective range of about ten metres (thirty feet), and a field of view less than 180 degrees. Models with wider fields of view, including 360 degrees, are availabletypically designed to mount on a ceiling. Some larger PIRs are made with single segment mirrors and can sense changes in infrared energy over one hundred feet away from the PIR. There are also PIRs designed with reversible orientation mirrors which allow either broad coverage (110 wide) or very narrow "curtain" coverage, or with individually selectable segments to "shape" the coverage