I am trying to detect whether a signal is high or low from an ESU LokSound 5 decoder for HO locomotives. My idea was to use one of the auxiliary pins used to drive an LED (AUX 3 in my case).
The problem is that these decoders are common positive rather than common negative, which means I can't directly connect AUX 3 to a digital pin on the Arduino and expect it to work flawlessly since it outputs a negative voltage rather than positive.
I'm trying my best to not use an op amp, as the locomotive I'm making this accessory for is already cramped. It sort of works if I directly connect it, but it is not smooth at all. I've tried using the built-in pull-up resistor, I've tried using an external pull-down resistor, I've tried using a resistor in line and I can't seem to figure out how to solve this without resorting to using an op-amp.