Relay to drive low voltage from high voltage

Hi, Another often-used solution is a optoisolator chip driven from the 115/230V and it's output to Arduino.

There are a couple shown here: http://arduino-info.wikispaces.com/Popular-ICs

See: how to sense 220v input -solved- I hope - Raspberry Pi Forums for an example. There are better circuits using a capacitor in the input circuit; I'll look.