How does a motion sensor detect motion?
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Your basic motion sensor is a passive infrared sensor. It contains an IR sensor that’s just watching (passive) and not sending out signal (active)
There’s a fresnel style lens over top of this sensor which causes something warm moving through the field of view to have sharp change from really bright to really dim that are easy to detect compared to a slow steady change across the field. There’s a circuit watching for abrupt changes in the IR level received and it treats that as motion
Fresnel lenses are generally a smooth curve on one side but a series of triangle shaped steps on the other. In a light house it helps turn the circular light into a narrow beam, but if you feed a moving light source into the triangles side it’ll seem to jump about an change intensity
There are active ultrasonic sensors that are good for detecting people who have been redditing too long in the stall. They send out an occasional burst of sound too high to hear and compare it to previous echos to see if anything has changed
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