Inside (and hacking) a 1 dollar radar motion sensor

10maurycy10 recently bought some cheap RCWL-0516 microwave motion sensors, mostly 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?

First, I found a datasheet for the the large SOIC-16 chip. It turns out the BISS0001 is an infrared motion sensor chip? How does that work?

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.

At first glance, the whole thing is just a single transistor oscillator working at a frequency of 3.18 GHz,ut it’s actually 2 oscillators in one, the microwave oscillation is pulsed at around 20 MHz.

See how the module was hacked in the post here.


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