July, 2017, CTT headlight article

[lionelsoni]

This article, on page 55, describes using LEDs as directional headlights in Lionel NW2 locomotives.  There is a simpler way, using the original incandescent lamps.  Just wire each lamp's center terminal to one of the brush holders, leaving the lamp socket shells connected to the locomotive frame.  It's not perfect, because the lamp that should be off will actually glow dimly; but you may not notice it, and the modification to the locomotive is minimal and reversible.  This, like the scheme described in the article, works for almost any postwar Lionel locomotive with a 3-position e-unit.

The wiring diagram in the article is inscrutable--at least I couldn't scrute it.  Fortunately, the connections that the author intended are simply between the un-grounded field winding and one of the brush holders.

The author seems to have gotten away with ignoring the reverse-voltage limit specified for the LED, which is usually about 5 volts.  This can work if the 1 kilohm ballast resistor (not a "pull-up" resistor--that's something else) delivers little enough current so as not to damage the LED.  But I would put a diode in series to be safe (1N914, 1N4149, 1N4001, e.g.).

With the diode, it becomes possible to put a small electrolytic capacitor in parallel with only the LED, to very slightly delay the turn-on and turn-off.  This greatly improves the realism in my opinion, compared to the almost instantaneous on-off of the LED.  The capacitance is not critical, but very roughly in the 100 microfarad ballpark.

For headlights, I use the LEDs that have been dyed to make their color temperature resemble incandescents.  Ordinary LEDs are very bluish, because the manufacturer can boast a better efficacy number that way; but their color is very unlike a traditional locomotive headlight.

You can reshape an LED to resemble a headlight better by cutting off much of the hemispherical tip, filing what remains to a flatter bulge, and then polishing with toothpaste.  As long as you don't cut into the innards embedded in the plastic, the LED will not be harmed electrically.

[JTrains]

+1 for the idea of using a cap to slow the illumination of the LED.