120 Ac voltage sensor

[metallica21156]

Im looking to monitor 120 volts ac with a pi. Is there a sensor that can do that or do i need to go the route of using a transformer with a sensor?

[pcmanbob]

It depends what you want to do.

If you want to actually measure the voltage then its not easy.

But if all you want to do is monitor if the 120v AC is present or has failed you could use a simple relay with a 120v AC coil and just use one of the contacts connected to the pi gpio the same as any switch, or you could use a simple 3v power supply plunged in to a socket and then connect that to the pi gpio.

[metallica21156]

i had forgotten about using a relay. Thats probably the easiest for me to do. All i want to know if its present so that would work.