How would one go about using a smoke unit (approximately 125 mA draw) on DCC with Button control
The rating of the T1 (and all of our other motor decoder) lighting functions are 100 mA, and I would not push that beyond its rating. If you want to the run the smoke unit I would advise tying to function wires together and attaching them to both to the smoke unit. That means if you are using the purple and green wire you should attach both of those wires to one side, and the blue wire to the other side. Once the wires are attached check on our Simple Programming guide for remapping both function wires to the same button. NOTE: make sure both functions are set to turn on constant bright in both directions to avoid a directional smoke unit. It is possible to use the white and yellow function wires but they too will have to be set to come on in both directions to avoid burning out one or both of the lighting transistors.
Another way to run to a smoke unit is to give it its out motor decoder or use an FL2. The FL2 lighting function only decoder has much heftier transistors (and much bigger transistors) than the rest of our decoder line so those functions (rated at 200 mA) could handle a smoke unit. The other option (if space allows) is to put a motor decoder (T1, M1, MC2 etc. etc.) and use the motor drive to service only the smoke unit. Although this method would work well if you have the space for a second decoder the FL2 is much cheaper and would be far less superfluous so that would be my first choice.