Be sure to check out the comments at the bottom. The circuit needs work. In particular, I'd definitely recommend going with the programmable zener. As is the circuit is very temperature sensitive; even now when the ride in is a relatively balmy 27*F, the voltage goes up from 12.5 to 12.9 volts. In my freezer it went to 13.25. Still well within the operating range of the light, but there's no excuse for building a circuit that can't hold better than that.