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Inside a 1 Dollar Radar Motion Sensor

posted by janrinok on Thursday July 04 2024, @09:34AM   Printer-friendly

owl writes:

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.

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  • (Score: 3, Interesting) by bussdriver on Friday July 05 2024, @09:49PM (1 child)

    by bussdriver (6876) on Friday July 05 2024, @09:49PM (#1363215)

    Camera motion detection takes electricity. it would be nice to combine these so the camera is off until actual interesting motion happens at which point an AI tracker could ignore cats etc.

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  • (Score: 2) by RS3 on Saturday July 06 2024, @12:16AM

    by RS3 (6367) on Saturday July 06 2024, @12:16AM (#1363235)

    Reasonable point. I'm not sure how much power the camera system consumes. It may be significant. Ones I've seen, including the one I have, maybe use 20 watts or so. Mine doesn't say on it, but I'm pretty savvy with electrical things. The biggest power consumer in mine is a 1 TB spinning rust HD. It's very quiet, but I'm pretty sure it spins constantly. They could optimize power consumption by spinning it down when not needed. I'm sure better ones do.

    Again, to be clear, I agree with your point.

    I would make the point that there will be some latency if you power up the camera system. You might wish to capture some video before the motion event.

    I'll also check- it may be that some (most?) camera systems are in sleep mode until needing to record.