Anyway, old habits die hard. I guess I can't analyze a circuit even partially and then not go into the lab and build it. Since I had the parts available...
In industrial settings where things have to work, battery powered emergency lights are operated by a simple relay. With ac available the relay that controls power to the light stays open. When power is lost, the relay fails closed supplying battery power to the light. Simple and reliable. Instead of powering a light, the closed contact could be used to provide on/off info to the arduino.
Relays are relatively large and expensive, though;
Please enter a valid email to subscribe
We need to confirm your email address.
To complete the subscription, please click the link in the
email we just sent you.
Thank you for subscribing!
via Egeo 16