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I have a system outdoors (~200W) that is powered from the house with a battery backup on UPS. When the UPS switches over to battery power, there is no way to currently generate a notification that the main power is lost.

The system has a LTE connection and a Raspberry Pi. I'm wondering if there's a relay that could sit between the AC power, and use the GPIO of the Pi to send a notification that the UPS has switched over?

What kind of relay should I be looking for? What things do I need to consider?

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  • \$\begingroup\$ are you certain that the ups does not have a monitoring port? \$\endgroup\$
    – jsotola
    Apr 23, 2020 at 2:19

3 Answers 3

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You need a relay with a 120VAC coil (if in US or Canada, 240VAC elsewhere) and low current SPDT contacts. The relay coil would be connected in parallel with the UPS AC input.

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Sounds like you just have to detect the loss of AC power, and nothing to do with the UPS, so an AC-adapter plugged into the input power and driving a signal relay with gold contacts (or just driving a transistor) would work.

You could use an AC-coil relay, but then the wiring would be at mains voltage, and typically the contacts are not appropriate for signal levels.

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An AC mains-operated relay, with low coil consumption (around 1.2VA) and a set of gold-plated SPDT contacts, would suffice.

Here's the schematic.

enter image description here

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