Basic electronics help required.

["David Chapman" <david@xxxxxxxxxx>]

It's now so long since I did my HND in electronics I've forgotten how
to do this... ..

Project is for automated curtain control in a home cinema room (and a few
other things)

I have automated switched 240v AC but not control over duration of the
"on" time.  I need to limit that to about 10 seconds. I think I
can do this with a relay, capacitor, resistor and diode.

Circuit would be such that the relay is energised when power is connected
but drops out after 10 seconds. If power is disconnected then connected
again the process repeats.

How I think it would work is: Power is applied,  the relay and diode are
used to charge the capacitor (this is the 10 second timer) as it charges
the capacitor changes the voltage on the relay coil, possbly by bringing
the 0 side up to line voltage and of course at some point the relay drops
out. As it does that a changeover set of contacts in the relay close which
discharges the capacitor so that the circuit is ready to go again when
power re-applied.

It would be simpler to do this with a relay with a 12v coil but that would
mean I'd need a transformer as well, something I'm trying to avoid as I'm
sure there is a simple way to do this all at AC level ?

David C


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