So I have a device (plasma cutter) that sends a switched 120VAC signal (arc OK) to tell when a process is good. It's not a fast signal; just on for a few seconds to minutes then off. Then it can repeat the process.
How do I "step" it down to a usable 5VDC signal that I can use for the Arduino to read as an input?
I've been researching endlessly and it seems like a common solution is the use a wall-wart for a small electronic device. Downside to this is I've read that the wall-warts are unregulated so the voltage can fluctuate.
Others just step down from 120VAC to something like 12VAC then use a bridge rectifier to convert to 5VDC.
Plug pack output feeds an optocoupler to isolate the variable voltage.
Weedpharma
Edit: I have used this and it works well. You simply tie the IO pin to 5v and the opto transistor takes it to ground when 5 volts is applied to the opto input. I used a mobile phone charger. You will probably have a few in your draw!
Simplest I think would be to attach a 120vac light to the signal line. Incandescent, neon, led lamps are readily available and inexpensive. Sense the light with an LDR. Encase both in heat shrink to isolate from other light sources. If this were a critical detection, redundant sensors might be wise. - Scotty
There are NUMEROUS relays available with 120V AC Coils. Use the contact closure to energize the Arduino's input.
A word on relay contacts: Unless you have gold contacts in your relay, use a 470 ohm pullup resistor to 5V on your input and connect the contacts between the input and GND. (Silver Oxide contacts in many relays need 10mA to ensure reliable switching).