[julie_m]

There was a similar device on an overnight ferry I travelled on once (Hoek van Holland to Harwich) with a choice of three or four radio stations (one of which was silent during my crossing) and an option for an alarm, I think either 1 or 2 hours before docking.

I didn't examine it too closely, but I suspect it would have consisted of little more than a 100V line transformer switched across different cores of a multi-core cable.