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Protecting arduino while datalogging relay function events

I’m looking into datalogging events from koyo timer (KT-V4S-D) which is already in use and can’t be changed. I’d be using an arduino and SD card which is straightforward. The question is what si the best approach for reading the events.

The NO side of the output relay has a 120VAC LED attached and I was considering having the arduino pin with pullup resistor connected to the NC side of the relay and reading when it goes High (ie. the timer is activated). I am concerned about being on same relay as 120VAC, but should I worry since they are not connected at same time? would a diode be a reasonable protection solution?

Another option would be tying into and reading the momentary switch that connect “start input” (terminal 3) and 0V (terminal 5). Maybe that is the better choice, but I am still curious about if it would be safe on the relay.



It’s not really clear to me what you are trying to do, but if you can post a drawing of your proposed connections, we might be able to tell you whether it looks reasonable.