I had a project to add a special control to the Legacy Milwaukee Road Northern locomotive to prototypically control the MARS light. The desired operation was only when the locomotive was stopped and was in reverse. For some Legacy steamers, this would just be monitor the cab light and the reverse light output. However, for this particular locomotive, Lionel had usurped the reverse light output to light the cab gauges, so I didn't have that option. I opted to monitor the cab light, which allowed me to know when the locomotive was stopped, and the existing MARS light output, that told me the locomotive's next movement direction. If the MARS light was on, the locomotive was in forward, if it was off, it was in the reverse. So, to achieve the desired operation, I wanted to trigger it when the cab light was on and the MARS light was off.
A complication was the fact that the lighting outputs don't share a common with a power source I could use to actually power the new MARS light board. The solution was to opto-isolate the two logic inputs so that they had no electrical connection to the output side of the board. The board also had to invert the sense of the MARS light as I wanted the output to be on when the MARS light was not operating. Another issue to deal with is I wanted the inputs to be flexible for maximum utility. So, they are designed to trigger on any input voltage from 2 volts to 18 volts and AC or DC. All of that was incorporated into the project. This worked out great and I completed that project.
Almost immediately another project came up to add a prototypical MARS light to a pair of E8 diesels, and my little board seemed to be a good fit. In this case, I monitor the headlight and the CAB light, the MARS light only come on when the headlight is active and the CAB light is off. Again, I needed one channel to be inverse logic to accomplish the task.
WOW, this is turning out to be a pretty useful board I thought. With a couple of changes, I think it may be a more universal solution to adding custom lighting. I added a jumper option to invert one of the input channels logic, and the Universal MARS Light Trigger Board was born. Actually, this can be used for more task than operating a MARS or beacon, those just happened to be the first obvious uses. Another cool capability is to be able to monitor the drive motor voltage and know when you're moving or stopped for virtually any locomotive. Adding a diode and you can have directional motion sensing. The possibilities seem almost endless.
Without further ado, here's my project.
The actual PCB board.
Annotated 3D shot with signal ID's.
This is my first cut at a document describing the functions and suggested applications.
