120VAC to normally closed building alarm

Hi, 

I need to take 120VAC signals from a PLC to generate Normally closed building alarms.

This PLC monitors and controls two redundant diesel generators that run after a commercial power outage.

I've already identified the PLC AC inputs that I need (commercial ac fail, generator 1 running, generator 2 running, generator 1 alarm, generator 2 alarm) butthese are 120Vac signals which can't be used in arduino.

 

I was thinking of converting each 120Vac signal to 5Vdc using cell phone chargers and then using them as inputs to an arduino that would trigger external relays to create the normally closed building alarms but I was wondering if there was a simpler way to do it.

Is there an IC that will convert 120Vac to a normally closed signal? Or to a 5Vdc output?

 

Thanks,

Alex. 

Standard warning - if you are not comfortable with and qualified to work with AC mains power then you should just purchase a commercial unit or look into using a magnetic current sensor that does not require an electrical connection to the AC mains wires.

Get an opto-isolator with darlington output. Put another (standard) led in parallel with the led in the opto-isolator and make sure they are in opposite directions (anode to cathode, cathode to anode). Put that in series with a small ceramic capacitor, probably about 1uf, 200V and apply AC power. The external led should illuminate and the darlington side of the opto-isolator can be used to drive an arduino input. If you google arduino opto-isolator you will find lots of references on how to connect the opto-isolator to the arduino.