I did an LED install on an old Athearn BB several years ago, but with advancing age I can't remember how I connected the wires. Here's what I do remember:
I believe I use a 3mm LED with a 1K ohm resistor wired in behind it. I either painted the sides of the round tube of the LED flat black or used black shrink tubing to cut light bleed. I mounted the LED in the headlight opening with some superglue- used very sparingly- and then where do I solder the two wires?
I have wired the Athearn engine using the 5-wire method but I can't recall where I attach the LED wires.
Help!
Cedarwoodron
For DC, it needs to go to the track pickups. One side is the frame of the loco, the other is the tab sticking up from the trucks. If the polarity is right, the LED will light only when going forward. If it's backwards, the LED will light going in reverse.
The old holder for the light bulb is riveted to the frame, unless you've cut that off. But often it's not tight and can flicker. The other side was the extension of the original metal tab that went under the two truck towers and across the top of the motor.
NMRA standard says the engineer's side (right) rail is positive for the loco to move in the forward direction, so typically the towers on the truck should be the + side for the LED to act as a headlight.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Keep this link in Your files.......just about everything you need to know about working on Athearn BB engines, including the 5 wire system to refresh Your memory. I tap into the pick-up leads:
http://www.mcor-nmra.org/Publications/Articles/Athearn_TuneUp.php
Take Care!
Frank
The link that Frank gives is great. I've given it out many times.
I use these for headlights, rear headlight, and ditch lights:
https://www.modeltrainsoftware.com/smd-chip-leds.html
They are a bit pricey, as I buy the lights set up for 7-19 volts, AC/DC/DCC, and about $3.25 a piece, but I don't have to mess with resistors, the wires a very small, like magnetic wire, and easy to hide, and deal with. One wire to the "+" side and the other to the "-" side, doesn't matter which wire, and your good to go.
Mike.
My You Tube