Very nice idea and good logic.
I can't help but think the circuit is way over designed with too many components. It doesn't matter to you because you have it built now.
I would have thought a UJT transistor with a simple RC ramp generator but....wait! Yeah another contact on the relay would go a long way to accommodate much simpler circuitry. probably not happening on a latching relay.
Here is something ese you may consider...steal power from the furnace circuit. Hook your common to the one side of the contact and to the other side of the contact make a simple trickle charge circuit with.
furnace connection -> diode -> resistor (say 1K) -> capacitor to common (say
1000uf) -> 9v input.
If the voltage is critical you could insert a 7809 regulator to setady the voltage at 9 volts from the 30 you may get off the trickle voltage stealing circuit or you could use a parallel resistor across the ripple capacitor to drop the voltage down to 9-12 volts. The trickle current has to be light and the cap. big enough to carry the relay coil snaps.
I have discovered the timer input on my HRV system and gor PB to activate it for 20-40-60 by holding it down longer for one, two or three blinks of the feedbak LED on the wall. Short shots of ventilation stuff is nice. You get it when you want it but don't have to find it running your wallet down a month later.
4 minutes (plus or minus). I move an alligator clip depending on season, you can put in a switch. I use it rather than turning the heat/air up and down, for a short comfort adjustment. It runs from a 9 volt battery and has very low standby current, about 80 nanoamps. The battery lasts two or three years. Others can modify for different purposes. If you think of another use, please post it. MikeK
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