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https://10maurycy10.github.io/projects/motion_sensor_hacking/
I recently got some cheap RCWL-0516 microwave motion sensors, mostly because I was wondering how China managed to make a radar for under a dollar:
Getting one working was quite easy, I just connected the VIN pin to 5 volts, GND to ground, and added a 1 uF decoupling capacitor on the 3V3 pin. When someone moves within ~5 meters, the OUT pin goes up to 3 volts for 3 seconds.
So it works, but how?
Generally, motion and speed sensing (doppler) radars work by sending out a continuous carrier and mixing the received signal with the transmitted carrier to create a low frequency IF signal. If reflections are coming from a moving object, the received signal will slowly drift in and out of phase with the transmitted signal, creating a beat frequency at just a few hertz. Because a motion sensor doesn't care about the exact speed, all the chip has to do is look for millivolt-level changes: all the hard work is already done.
(Score: 1) by rst on Thursday July 04 2024, @09:05PM
I wonder how different it really is from something like this: https://www.dsc.com/?n=products&o=view&id=1015 [dsc.com]
(Commercial motion sensor with PIR and microwave combined.)
The approvals documents say that this runs at over 10 GHz and of course this has some extra circuitry for the PIR and communication with the panel, and I'm sure having that fancy plastic case with the silk screened logo adds a lot to the price, but if you ignore those things for the moment, I wonder just how different it really is.