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How do PIR units work in electrical circuits work?
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For more on marking an answer as the "Best Answer", please visit our FAQ.The heart of the unit is a little component that is sensitive to infra-red light. In the old days they used to be made of gallium arsenide. This component is a semi-conductor - halfway between a conductor like copper and an insulator like plastic. When exposed to light, the electrical resistance changes. The rest of the circuit connected to this device just amplifies the signal and provides levels of logic to minimise false signals - i.e. two detections (counts) must be made within a period of X seconds for the unit to regard the signal as valid. The curved white plastic screen on the front acts as an optical collector of infra-red light. A bit like a lens.