I have a thermostat with a built in delay of 5 min after a power outage to protect my air conditioner. The power must be off at least 15 seconds before the thermostat recognizes that the power is off, presumably the time necessary for its caps to discharge.

I am acquiring a standby generator to power the whole house, and it aromatically turns on in 10 seconds, not enough for the thermostat to notice. The thermostat runs by stealing a little power across one or more of three relay coils (Heat, AC, and fan.) One hot wire and 3 relay control connections.

Easy to put a n/o relay on the hot wire and and delay closing it when the power comes on, but that is not elegant. I don't fancy the idea of energizing a relay for ever when I hope I will never need it. And house current is not handy by the thermostat.

So I would like to use a N/C relay, and use the thermostat power, (a super cap should keep everything on long enough) and a pic as a timer.

The ac sensor that has me stumped. I can't connect 24 volts directly to an a/c optoisolator, and I am not comftorable dropping 24v to 2 volts with a resistor. If I were to make an ordinary power supply as would be needed for the pic, the filter caps would keep power on too long just like the thermostat. I have to be careful how much power I steal or I will triger on of the control relays.

Any eligent ideas?