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A PIR sensor detects movement by measuring changes in infrared (IR) energy — essentially body heat — in its field of view. Humans and animals emit infrared radiation. When a warm body moves across the sensor’s coverage area, the sensor picks up a sudden change in IR levels, triggering: An alarm A camera recording A notification A light activation.

The most common objection to acquiring a burglar alarm is that, faced with a wailing siren from an absent neighbour, most of us will just...