How does a motion sensor detect motion?
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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.
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