PIR stand for Passive Infrared. A PIR works by detecting changes in ambient IR emissions. There are 1, 2 and 4 windows PIR sensor chips. The more windows you have, the high the resolution the scanning and fewer false positives. There is a clever trick that I came up with a few years ago to detect approximate distance of an object of known size. I accomplished this by using 2 to 4 PIR units, all 4 window units, and set them at a particular angle from one an other. This allowed me to use the sensors as eyes with depth perception. I stupidly did not patent the idea... and now someone else took the idea and is making the devices for the military.
One advantage of using a passive sensor is that is does not require much energy to function. This allows for long battery life in mobile devices or just for convenient placement. One serious draw back for this sensor is the calibration time. The device measures the a change in IR energy, so it must have a baseline. This sensor makes a motion detector when installed in an area where the environment is consistent. This is why it is use as a detector in security systems and for turning on lights.
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