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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: 3, Insightful) by VLM on Thursday July 04 2024, @02:58PM
The author would likely greatly enjoy getting a ham radio license and messing around with microwave stuff "for real".
One of the strangest counter-productive regulatory effects I've seen over the decades is wifi using ISM bands means consumer-ish product mfgrs no longer use ISM bands to avoid interfering with "important" signals like wifi. So mfgrs no longer see FCC certification nor do they use legal ISM bands. So, I'm not entirely surprised the author claims this runs at 3.1 GHz or whatever nonsense instead of somewhere legal.