How does a motion sensor detect motion?

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How does a motion sensor detect motion?

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It’s basically camera, and there’s a program already in it, that when certain pixels changes/moves, it sends signal. You can also think of it like the computer keeps comparing images every second, and if there’s change, there’s movement.

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

Besides the infrared camera described in another answer, there’s a type of motion sensor that works like a radar. It makes a sound outside the human hearing range, and listens to how the sound returns to the device. So, if something moves, then the sound received is significantly different.

There are also motion sensors using radar, usually on automatic sliding doors etc.

Those use the doppler effect: A signal sent from a moving object (or reflected from one) changes in frequency. You probably know this effect from the horn of a passing car: The tone is higher when it drives towards you and lower when it is driving away.

The sensor sends electromagnetic radiation at 5.8 GHz (5800000000 Hz). Some of this signal is reflected back from the person to the sensor and now has (because the person is moving towards the sensor) a frequency of e.g. 5800000030 Hz for someone walking or 5800000100 Hz for someone running.

While those high frequencies are rather hard (=expensive) to measure, there is a trick: The sent signal can be combined (mixed) with the received signal, resulting (besides other things that do not matter here) in the difference of the frequencies (30 Hz for the walking person). This frequency is easy and cheap to detect.

A car parked in front of the door (or the opposite wall) reflects the signal back without any frequency changes. When combined with the sent signal the result is nothing (because there is no frequency difference). So the sensor is blind to non-moving things.

basically by shooting laser beams in an area, and then if anyone moves in front of it, it will detect the distance of the beams having changed

that Xbox Kinect scene from those dumb paranormal activity movies is a pretty good laymans explanation

So which sensor type can be defeated by walking slowly past it with a bed sheet held in front of you?