Diving Deep Into the RCWL-0516 Sub-$1 Microwave Radar — and Turning It Into an S-Band Transmitter

Pseudonymous engineer "10maurycy10" has been digging deep into the inner workings of an RCWL-0516 microwave radar sensor, to find out how such a handy device can be sold for under a dollar — and figured out how to turn it into an S-band transmitter instead.

"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," 10maurycy10 explains. "Getting one working was quite easy, I just connected the VIN pin to five volts, GND to ground, and added a 1µF decoupling capacitor on the 3V3 pin. When someone moves within ~5 meters [around 16 feet], the OUT pin goes up to 3 volts for 3 seconds. So it works, but how?"

The compact radar modules, popular for their low cost, are driven by a chip marked as a BISS0001 — which is designed to detection motion using an infrared sensor. The RCWL-0516, though, uses microwave radar sensing — which works, 10maurycy10 explains, because the resulting signals are roughly compatible. "Because a motion sensor doesn’t care about the exact speed," the engineer writes, "all the chip has to do is look for millivolt-level changes: all the hard work is already done."

Analyzing the radio side of the board, 10maurycy10 found what appeared to be a single-transistor 3.18GHz oscillator — but which, when connected to an oscilloscope, revealed itself to also offer a 20MHz pulse. "What’s happening is that as the oscillator runs, it changes the 33pF capacitors, raising the emitter voltage until oscillator can’t run any more," 10maurycy10 concludes. "At this point, the 220 Ohm resistor discharges the capacitor, restarting the oscillation in just a few nanoseconds."

That turns the device into a super-regenerative receiver, with motion detected by the interference between echoes from static and moving objects and relies on signals arriving during the oscillation's start-up period. "This sort of radar-by-wishful-thinking approach is probably why the sensor has very inconsistent performance," the engineer supposes. "Indoors it works very nicely, with up to five meters of range, but outdoors with no convenient static returns it often doesn’t work at all."

Having figured out how the radar operates, 10maurycy10's last trick was to modify it — disabling the 20MHz pulse to create an S-band microwave transmitter instead. The transmitter can be driven by on-off keying, the engineer explains, and can still operate as a radar — though with dramatically reduced sensitivity. Combining it with an unmodified module improves things again, trading sensitivity for the ability to work as a speed sensor.

The full analysis is written up on 10maurycy10's blog.