"Dave Plowman (News)" writes:

In article ,
M.Joshi wrote:
I am trying to design an audible/visual indicator that beeps every 5
minutes when a light switch is turned on.

The complete circuit has to fit behind a light switch plate and so a
transformer cannot be used.

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.

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.

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