I have a set of Genie STB garage door safety sensors. I would like to use them to sense a tractor going by. They are 12vdc units. what I can't figure out is how they work. Looking at the wiring diagram shows transmitter and receiver connected to 12 volts. I have the same units on my garage door and they both have 12 volts on them at all times. Does anyone know how to make these work in my application?