I have a customer that sent me the Lionel Legacy Milwaukee Road Northern to make the MARS light operate in prototypical fashion. This proved to be a bit more complicated than it first looked. The object is to only have the MARS light lit when the locomotive is stopped and the next movement is reverse. At first I thought I could just monitor the cab light and reverse light outputs of the RCMC and if both were on, fire up the MARS light. I had checked several steam RCMC boards, and both of those lights were active. However, when time came to install my little custom board, turns out the folks at Lionel has repurposed the reverse light to continually light the gauges in the cab, that idea when out the window! Back to the drawing board! 
The existing MARS light run anytime the locomotive is in forward, but it's a pulsating light that wanes and then intensifies. That gives me a method to know if I'm in reverse and stopped, the cab light is on and the MARS light is off. Another problem encountered is the lighting for the RCMC has no common ground, so I needed to totally isolate my sensing of the two inputs from powering my MARS simulator board. I also had to add a filter so that the pulsating light always got detected.
I got to thinking that maybe I could add a little flexibility to the board in case it could be useful in other situations. So, there is a jumper option to invert the sense of one of the inputs, originally it was hard wired to sense the off state. The opto-isolated inputs will sense anything from around about 2 volts up to 18 volts, and they have filtering for PWM signals so I still have a steady output voltage.
Combining all those requirements into a PCB I created this.
It's powered from track power, and senses two inputs, obviously for the first task, they were LED outputs on the RCMC board. The output is 12VDC at 50ma or less, perfect for the MARS simulator or a relay if you need to switch more power.
< Click on graphic to expand >
I would have taken a picture of the actual board, but I only built one for the project and it's already installed. I didn't think to take a picture until I decided to post this description.
