Hi, I would like to setup a buch of 120v relay with my arduino. I want it to be activated by the flip of a switch so I want a sensor that read if the circuit is on or off. Then if it's on i will activate different other relay.
As anyone try somethin similar to this?
Please don't tell me that I just have to use a switch directly on my arduino. This project is just a part of a bigger electronic setup and I don't want to change everithing. It's more like patch 
Thanks
From the scope of the project you just gave, why bother with a ardruino, just have your switch power up the relays and your done.
I love the easy ones.
Unless If you must use a ardruino, maybe get a 5 volt wall wart and have that go to a input pin.. unless If you can’t tie off the 120v power then a induction coil circuit on the ac line might be a possibility. Unless.....
HCPL3700 or similar, I've posted this AC detector circuit lots of times now. DIP or SMD
https://www.digikey.com/products/en/isolators/optoisolators-transistor-photovoltaic-output/903?k=hcpl3700
