The Opto-coupler Plug is a small board containing a dual opto-coupler. It can be used to detect the presence of low DC voltages (a few dozen volts) and currents (milliamps) without creating a direct electrical connection to any other circuits.
Both input circuits consist of a built-in LED which must be driven with ≈ 3 .. 55 mA of current. Connect the two included 1kΩ resistors in series to use this board with 3 .. 12 V DC sources. Chose other resistor values for higher or lower voltages.
In theory, this plug could be used to detect AC mains power, when part of a suitable circuit. In practice, you should only consider this if you know exactly what you're doing, and have taken all the necessary safety precautions. High voltages can be lethal - proceed at your own risk!
The PCB only variant contains just the board for the Opto-coupler Plug. No parts, no headers.
As connectors you can use a Plug Headers set with 10 male and 10 female 6-pin headers, which makes it easy to connect multiple plugs and JeeNodes in a “standard” way. Or you can solder the board directly in place for more permanent use.
The documentation and full design details are freely available.
Update - the latest v2 board can now also be used as dual opto-coupled output interface. Each channel drives the built-in photo transistor to act as a (polarized) switch. Limit the output current to ~40 mA max.