What's passive infraRed ( PIR ) for motion sensors

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The most frequent use of the PIR sensor is as an 'area' sensor. Whether it is to detect 'someone moving in the front yard', or 'someone moving in the bathroom', or 'someone moving through a doorway', or even 'someone opened the beer cooler', it is all technically the same sensor and logic.

There is a simple electronic device which is sensitive to 'heat', or rather the infrared light that is emitted by warm or hot objects (like humans).