I’m working on a GP9, and building my own function decoder with an Arduino to control the couplers and lighting. I’m planning to set it up to cycle between front and rear. For example, hitting F0 will do this:

first press: front headlight on (rear off)

second press: rear headlight on (front off)

third press: both off

fourth press = first press, etc.

I’ll do the same with the number boards and classification lights.

Question: since I’m new, is there any operational disadvantage to this scheme that I’m not considering? I can’t really imagine a situation where I’d want both ends of anything on.

Most G scale control systems automatically switch the lighting with the direction of the locomotive. On the real deal, the engineer has to do this manually, so depending on the situation, he won’t always reverse the lighting during daylight switching. The other situation that pops up is when locomotives are trailing on a train. In this case the trailing locomotive will have its lights on in the opposite direction of travel acting as additional markers. You’ll see this a lot at Conway Scenic when they have locomotives “bracketing” the train.

Rockwall Canyon Jeff said:
Most G scale control systems automatically switch the lighting with the direction of the locomotive.

That’s the behavior that I don’t like, which was the motivation for ripping it all out and starting from scratch.

I now have three functions: headlight, classification lights (white only), and number boards. Each function key cycles between three states (front, rear, off), and none is affected by the direction of movement of the engine. I may set it up so the number boards chase the headlight, but can still be controlled independently so I can leave them on with the headlight off.

I’ll put up a video next week. I’ve just landed in Denver, where I’ll board the California Zephyr tomorrow morning for Emeryville!

I have this all working, in case anyone is interested. Here’s a video showing the three-state functions. Lights cycle front/rear/off.

Here are some photos of what’s inside. I built custom boards for the front and rear lights with LEDs and separate control lines for the headlights, classification lights, and number boards. These lines all go to 10-pin plug that goes onto a header on the Arduino board. The Arduino is also handling the front and rear coupler servos.