This PIR (Passive InfraRed) detector is designed to replace a switch where there is no neutral. That makes it much more complicated to derive power, since it has to be able to power itself while the load in series with it is on.

The circuitry is quite clever. While the load is off and the unit has full mains voltage across it, it uses a high voltage and low current discrete voltage regulator to create a supply voltage for the PIR circuitry. When the load is on, the unit has a slight delay in the point the triac switches the load, so it can grab some power, which is diverted directly to the power supply capacitor via a diode.

The PIR control chip si notable for having an integrated voltage regulator for stable operation of the sensor and also to give a consistent voltage for the dividers used for setting sensitivity, delay and light sensing level.

I had a very frustrating evening trying to test the adjustment of the delay, which is not documented well in the vague datasheet. I ran out of time due to having a flight the next day, and should probably do another test when I get back, adding the decoupling capacitors shown on the datasheet. They may make the delay more stable.

The schematic has a lot of interesting stuff in it. It's quite a clever system they use for deriving power with no neutral.

Here's an AliX link to a typical listing for these. (Not a sponsor.)
https://vi.aliexpress.com/item/1005006581419905.html


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