How microwave body detectors work. With RF section schematic.

When I first took a microwave triggered lamp apart in a video I joked about the hidden chip being a standard PIR body sensor chip - and was then aghast to find that it WAS. Which meant that all the microwave signal generation and movement detection was being done by a single transistor and some cunningly designed PCB tracks.
I'll openly admit that microwave circuitry is such a specialist area where the only people who truly understand it are those who work with that type of circuitry all the time. It's not just a specialist area of electronics, but a specialist subsection of RF design where simple things like tracks stop behaving like conventional conductors.
Now I understand this circuitry better it all makes a lot more sense. It's got strong similarities to the Colpitts oscillator FM micro transmitters I used to make for the 88-108MHz frequencies - not 300MHz as I mentioned in the video. (Well not deliberately.)
The chip being used on this breathtakingly cheap and versatile module is a customised BISS0001 with a rarely used pin repurposed as the input to an internal 3.3V voltage regulator. That removes the need for a separate regulator on the PCB.
Here's a search link to find these modules on eBay. Target price less than a US dollar each:-
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