Model Train-related Notes (a.k.a. “blog”) -- these are personal notes and musings on the subject of model train control, automation, electronics, or whatever I find interesting.

2019-09-21 - Searchlight Car conversion to LED

Category Train

I’ve just finished this project:

The car itself is a Model Power HO Scale “Safety First #624 Searchlight”. It’s a depressed flat car with a nice & sturdy die cast metal construction. It’s really well done.

Jim got it for Randall and when we tried to use it, somehow trains started crawling when pulling the car.

The issue was quickly obvious: the car initially used a miniature 12V light bulb, and was simply drawing too much current on the DCC power bus, with no current limiting resistor. The transparent plastic cover was removed easily. Inside the bulb was screwed and was easy to remove. However the bulb socket was held by some kind of glue and was a lot harder to remove.

I replaced the bulb by a 3.5mm white LED, and added a 22 kΩ resistor under the car. I decided to keep my changes extremely simple --  I’m not using any kind of rectifier bridge, which means the light is technically only lit during about half of the DCC cycle, and I’m not trying to use any kind of capacitor to prevent flickering.

In daylight the LED plastic casing neatly focuses the light towards the front of the bulb. That’s perfect when the LED is encased in a panel, but here it is counterproductive as no light gets diffused on the side on the aluminized paper that acts as a deflector. Consequently it was producing a piercing narrow beam. To compensate for that, I coated the top of the LED with a dab of white glue. It dries leaving a translucent coating that diffuses the light a bit.

Detail on the LED under the searchlight cover, showing the coating on top of the LED to act as a diffuser:

The bottom of the car with the resistor and the wires held with kapton tape:

It is now installed at Randall in one of the currently unused tracks next to the Stockton town. I’ve made a little vignette with it. It looks nice and visible, without being too bright:


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