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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).

Bottom view of the motherboard that the decoder connects to

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.

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    i think that you are misunderstanding how the circuit is wired ... one side of a lamp is connected to the +V ... the other side of the lamp is connected to an open collector AUX output ... the output gets pulled low to light a lamp .... connect arduino ground to decoder ground ... connect the AUX3 to arduino data pin ... do not connect lamp to AUX3 ... enable arduino internal pullup resistor .... AUX output acts like a switch that connects AUX to ground – jsotola Apr 11 at 23:58

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