Question

What do you mean by PIR sensor ?

Answer 1

When the sensor is idle, both slots detect the same amount of IR, the ambient amount radiated from the room or walls or outdoors.

When a warm body like a human or animal passes by, it first intercepts one half of the PIR sensor, which causes a positive differential change between the two halves.

Answer 2

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Passive Infrared

PIR sensors allow you to sense motion, almost always used to detect whether a human has moved in or out of the sensors range. They are small, inexpensive, low-power, easy to use and don't wear out.  They are often referred to as PIR, "Passive Infrared", "Pyroelectric", or "IR motion" sensors.

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