Or you could run AC round the loop and isolate it with an AC-input optoisolator. Using a separate transformer winding for the look keeps it completely separate from the circuitry.

 

Or use a separate wall adapter around the loop.

 

A lot of old network cards have isolated DC to DC converters on them so you could get one of them off an old board (or buy a new one) to have a fully isolated loop powered off the same supply as the alarm controller.