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I'm not a professional electronics person. For some time I'm playing with the thought of adding to the existing non-smart electrical infrastructure some kind of smart traits. All lamp switches get only phase (live) line 220VAC (or 110VAC). This poses a challenge to create a DC power source to power a micro-controller at the location of the switch. This power supply should provide very low power. Less than 0.5W. It should be small enough to fit into the switch box with a micro-controller and some not too much more electronics.

I wonder if there is already a solution for this? I thought perhaps the power supply could be connected in serial to the switch and perhaps will reduce the voltage going to the load by a few volts. When the switch is open, the power supply will get its source from the phase and will close loop through the load assuming that the load has low enough resistance characteristics. Something like what is shown in the following diagram:

enter image description here

The switch in the diagram will eventually be some semiconductor switching component controlled by the micro-controller. But for the matter of the discussion, it can be a regular mechanical switch. The load can consume up to 16 Amps (or twice in case of 110VAC).

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This is often done by "2 wire" PIR or light controllers, but it has a two potentially significant issues.

Firstly the switched load has to let through the low current when "off" This works with tungsten bulbs, but causes issues with many Led or low energy bulbs.

Secondly you have the potential safety problems, when an "off" bulb holder still has the current going through you device. From personal experience I know this is enough to "sting"!

I would generally avoid this kind of circuit unless absolutely necessary.

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