Dave Plowman (News) wrote:
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.





There's almost always a neutral in the switch box, occasionally I see an
arrangement with power coming into the light box and a separate switch
drop but it's fairly rare. Also it'd be possible to power the reminder
with a battery in that case, it wouldn't have to draw much power. If the
lamp is incandescent you could even use a rechargeable battery or memory
backup capacitor to power it and "steal" power to charge the battery
when the switch is off.