For the simplest radio shack, or maybe car parts store LVC circuit, i'm wondering.... Using a potentiometer and a relay. If you wired the pot in series with the coil, and perhaps a shunt. I think you might create a magnetic switch that will fail if there is an insufficient sustenance voltage. Wiring it up series on either the negative or positive side may also provide a useful feedback voltage drop. The nice thing about this would be it would draw 0 current until the trigger is pulled.