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Comment by existencebox | Hacker News Reader

I have Rpis controlling my various plant-grow-lamps around the house; the relays they switch are set to fail-open so if they fall over I can go back to manual control; so you probably assume correctly. Now if the relays themselves get sad I'd probably be in for a bad time but fixing that seems like a reduction problem? (I speak as someone clueless about actual EE beyond treating it like legos)

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Most relays are rated for a "number of switches", usually in the few hundreds-of-thousands for both mechanical and SSDs. If you switch twice a day (on/off), you've got a few hundred years.

Note, however, that switching voltages/currents higher than rated will likely shorten the life of your relay.

I don't know what kind of relays you're using, but most relays tend to fail in a closed position, so it's worth considering what happens if a light is turned on continuously. Also, hardware or software failure in the RPi could turn on all of the lights in your house, e.g., if the same software running on every unit encounters a bug.

And for those cases I decided to place wall switches in parallel. As automation fails you can still operate all lights with classic, wall switch, way.

Also, solution for jamming relays are SSRs. In practice there were issues only with two biggest light zones with multiple lamps switched at the same time.

I've had SSR's stuck shut too.

Yeah normally open relays with standard switches wired in parallel.

Just don't connect one phase to the relay and different phase to the switch.

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