Inside an outdoor PIR sensor switch.

Описание: I've taken all the floodlights to bits, so I might as well take one of the PIR sensors to bits too. Mainly to see how "waterproof" it is and how the mains side of things is handled.

It took me by surprise with a very odd bridge rectifier arrangement that serves both to rectify the supply and also to clamp the capacitor limited supply down to about 24V. It took me a moment to get my head round what it was actually doing. It's basically using two back-to-back zeners to shunt the AC supply to 24V while also rectifying it! The stand-by dissipation in the circuit is around 1W and I'm guessing that a lot of it is actually in those zeners, so I suppose by sharing the load between two operating on alternate cycles keeps them cooler.
Other than that it generates a smooth 5V supply from the main 24v one using a resistor and 5V zener, and then uses a common LP8072C PIR controller chip with associated circuitry to switch a 24V relay.
The circuit appears to have a design oddity. There is a small snubber network (quenches sparks and interference across switch contacts) but instead of connecting it across the relay contacts they have connected it across the load.