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I'm attempting to build a system to help my Father-in-law monitor the power to his house. Here's the details of his problem: He lives in the woods where power is somewhat unreliable so he has a whole house generator. He travels frequently and he'd like a system to let him know when he's A) lost external power to the house B) lost external power and is on generator power and C) lost generator power.

My current solution which detects B and C: an arduino Uno with a Adafruit PowerBoost 1000 and a Adafruit Fona 808. The PowerBoost is connected to an outlet with on Generator power, so I can monitor whether my device has AC power and whether it's on battery power. I use the Fona for notification via SMS.

My initial configuration to detect A was to put a simple 5VDC A/C Adapter into an outlet that wasn't on the Generator circuit and have that power a relay. If the relay contact is closed, I have external AC power. I'm concerned about the reliability of a relay being closed for several days/months/years at a time... is my concern valid? Is there a simple/more elegant solution to this part?

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If you pass a few mA through the contact, and it is a signal type with sealed contacts it should be reliable. I'm not a big fan of reed relays.

You could also use an optoisolator rather than a relay. You'd need a couple resistors (for the input and a pullup on the output).

There are also modules made to accept an AC input and provide a logic level output, probably $15 or so. That would get rid of the adapter and the relay (Photo from Digikey).

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