On Tue, 3 Jan 2006 12:27:55 +0000, M.Joshi
put finger to keyboard and composed:


Franc Zabkar Wrote:
On Thu, 29 Dec 2005 09:02:49 +0000 (GMT), "Dave Plowman (News)"
put finger to keyboard and composed:

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.

What about using a dimmer switch set to about 90% duty cycle? The
circuit could grab some power during the 10% off time.

This is not being built for a blind person - This light switch often
gets left switched on all the time as it controls a light outside.

It already has a neon light on it which seems to dim when the light is
switched on?

The neon is probably wired across the switch contacts. When the switch
is off the neon sees the full mains potential. When the switch is
closed the potential is zero.

If I build the circuitry into a separate box next to the light switch,
is there an easy way to sense when the switch is on/off?

Here's my suggestion:
http://www.users.on.net/~fzabkar/WarnLight.JPG

- Franc Zabkar
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