In article ,
Sam Goldwasser wrote:
This can't be done unless you have a neutral at the switch position.
When the switch is on there is no potential difference between the
terminals therefore no power source.

It could be battery or supercap powered.

Might be difficult to fit all this inside a switch backing box?

Or, if the box is grounded, there will be line voltage between the load
side and the box in a correctly wired normal (not 3-way) circuit.

I hope you're not recommending this? In the UK it would be forbidden - not
least because it unbalances the line and neutral.

--
*Gun Control: Use both hands.

Dave Plowman London SW
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