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DCC wiring tips
Remember that "common" for DCC lights on decoders is almost always positive, not negative. (This is because it's easier to make an electronic component that switches a "pin" to ground, as opposed to supplying voltage.
Driving LEDs
Limit current:
Use a resistor. Calculate the resistor based on the voltage from your common to ground, and subtract the nominal voltage of the LED and use the "remaining" voltage and Ohms law to calculate the current.
You should have the nominal working voltage and the maximum current in hand. If you don't know, then use 3 volts and 20 milliamperes.
Driving bicolor LEDs:
There are 2 different kind of LEDs that have 2 colors, one where there is a common and an individual lead for each color. In this case, you need a common anode LED and most you will find will be common cathode. (Again the same reasoning as why DCC function "outputs" are really inputs to ground.
If you have a common cathod LED, I suggest you get a common anode replacement.
The other type is where there are only 2 leads to the LED and the 2 LEDs are connected in parallel, and in reverse polarity:
