Hello everyone, I’m trying to achieve a simple (but not for me 
) problem.
I have an air conditioner with a simple wall mount control panel at 12v, I have reverse engineered it and I’m replacing it with my custom made one controllable from remote. The wall circuit has 7 wires that arrives from the main unit. So far I have been able to do everything, turning it on/off, changing the fan speed and the power.
I’m missing just one piece. 2 of the 7 wires are just signal wires, and they are simply connected to some LEDs in the wall circuit to show if it is currently heating (red led) or cooling (green led). What I want is to replace the LEDs with digital reads on 2 pins on my Core, so that I can read in which mode the air conditioner is operating.
The problem is about how the two wires works. One of them seems to be a 12v, connected to the led and then to the common ground (circuit 1 in the picture). I’m thinking about replacing it with the circuit in picture 2, that should work on paper, so driving a 5v circuit through a NPN commanded by the 12v circuit.
The other one (circuit 3 in picture) seems to be a Ground, connected to the led and then to the 12v that powers the whole wall circuit. This is the opposite of the circuit 1, so I’m missing how to do the same thing, since circuit 4 of course doesn’t work.
Keep in mind that in the picture the wires arriving from the wall are the one with the switch on their side.
So my questions:
a) Do you think this is the right approach, also for circuit 2? I want to avoid a simple voltage divider because I’m not 100% sure of the input voltage (seems to have no spikes, but it is a black box).
b) How should I make the “circuit 4”?
