At a basic level, PIR sensors operate using positive differential change. Two IR-sensitive slots sit in front of the lens, with their field of “vision” determining the range of detection. While ‘idle’, the ambient level of infrared detected in the room (naturally emitted by all objects) is the same on both slots. The minute an object, person or animal moves through the detecting area, it is sensed by one of the slots before the other, causing a positive differential as it passes by, and a negative differential change as it leaves the area.
These differentials are what the PIR sensor uses to detect movement