Sending signal over long distance wire
Relays already make great current loop devices. Your issue though is the voltage drop along the wires.

The solution to that is to drive it with a constant current / current limiter circuit and use a larger supply at the relay to cover the voltage drop.







simulate this circuit &ndash; Schematic created using CircuitLab

Note, in the schematic above I added a smaller relay on the logic side to provide full isolation. 

Alternatively, you could flip all that around.







simulate this circuit

Of course these methods rely on the relay current being small. If need be you could add a smaller, lower current, 5V relay to drive the large 12V one.