Directional Constant-intensity Lighting for DC Locomotives

This article describes how to add constant-intensity directional LED lighting to a DC locomotive. Constant-intensity means the LED is at full brightness standing still or at any speed. Directional means the LED is only lit in the direction of travel. This will work on locos with headlights on one end (such as an F7) or both ends.

Parts required:

First, an explanation about this particular LED. This LED is a "12 volt" type, which means it will have full brilliance at 12 volts DC. There is already a current-limiting resistor build directly into the LED for this. However, it will quickly fail if more than 12 volts is applied. Standard DC power packs can easily supply 16 volts or more, so we need to add some more resistance to make it work. Experimentation has found 1K is a good value. As you bring up the throttle, the LED will come up to full brilliance just before the loco begins to move. This is very convenient, especially for photos: you can take headlight-on photos of the loco while it is standing still. The light also has a slight blue tint to it.

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Wiring diagram

Build LED assembly I make a feeler gauge of two popsicle sticks taped together to find the correct spacing for the LEDs. Here is the assembly. Test the assembly by touching the leads to a 9 volt battery. Only one LED should light. Now reverse the leads and the other LED should light. Don't worry about keeping track of which one is which for now.

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Assembly

Attach to loco Remove any existing lamp. You may wish to cut the wires close to the lamp, so you can reuse them to attach to the LED assembly. You can also just solder the assembly across the motor terminals. Then test directionality by placing the loco mechanism (without the shell) on the track. Orient the LED assembly in approximate position and power it up. Try both directions. If the LEDs are lighting backwards, just twist the entire assembly around the other way.

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Test run

Add receptacles to shell We add a short piece of PEX 1/4" water tubing as a receptacle for the LEDs. This allows the shell to be completely removed for service without unsoldering anything. A 5mm LED will slide nicely into this tubing. Usually 1/2" of tubing is about right, but it may need to be shorter or longer depending on other obstructions. It is soft plastic and cuts with a utility knife. Position it up against the headlight opening and glue it in place with hot-melt glue or similar.

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PEX tubing receptables

Insert LEDs Bring the shell close and slide the LEDs into the PEX tubing receptacles. They should stay in place on their own. Here we have covered up the weight with black tape so prevent anything from shorting out on it.

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LEDs slid into PEX receptacles

Reassemble Now it should work!

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Moving forward - backing up!


Home • Revised Feb 21 2016