Re @jim-p 's post #7, I was wondering in particular why you needed a latching relay and an Arduino? Not sure what triggering you are using, but I'm sure the Arduino could do the toggling (latching/unlatching).
If so a circuit of the familiar sort that @ShermanP suggested, like this:
But I assume you want the Arduino for more than driving a relay? Otherwise a few basic components could do so cheaply.