The problem I'm trying to solve: I have a vehicle that has 3 wires for the tail/brake light. When the vehicle is on there is 12 V+ between the red and black wires and there is 7 V+ between the red and orange wires. When you hit the brakes the 7 V+ between the red and orange wires steps up to 10 V+.
So I need to create a circuit that will turn on a set of LED lights when the voltage goes above 9 V. Currently, my circuit consists of a voltage divider (changing 12 V down to 9 V), an LM324N op-amp, and an FQI50N06 MOSFET (schematic shows IRF540).
For some reason, on the op-amp, when there is no input, the output is on, when the non-inverting input is higher than the inverting input the output is on, and when the inverting input is higher than the non-inverting input the output is off.
So in my mind, this circuit is working, except when there is no input, then the output should be off.
EDIT: Something that just popped into my head as soon as I posted this: I wonder if putting a diode on the output would prevent the output from being on when there is no input.
Please see the attached schematic: