Re: testing for 24vac

Probably the safest bet would be to use an opto isolator. These are
simple devices, that work like a switch. The input to the opto on the
24Vac side is an LED which gets turned on when the voltage is present.
The output side is a transistor that gets turned on by the led. To
make this work you would need a few simple components in addition to
the opto. You would need to put a current limiting resistor in series
with the 24Vac input and use a pullup resistor to your local power (I
will assume +5v for this example) and then put a small cap to ground to
filter out the glitches that occur when the AC input goes to zero.
Basically the resistor and capacitor are in series between your power
and ground with one side of the opto output connected between the two
devices and the other leg at ground.. When no Vac is present, you will
see your +5v at the component junction and when the Vac is present,
this signal will go low (or pretty close to it - use a 1k or 10k
pullup).

.

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