Locomotive directional lighting

Question

I would like to have three led lights on the front and rear of a locomotive (garden railways) so 6 leds in total. These are (LED, bi-colour, 5 mm, red + yellow 3 Pins - L-59EYW).

The part I'm struggling with is when the current reverses (direction of locomotive changes) I need the lamps to change, so the rear would go all red and the front all yellow, and vice versa. The voltage is a maximum of 12VDC and and the leds are 2.1V in one direction and 2.4V in the other. Would anyone be able to draw a diagram that I can follow to make this work?

Answer 1

You need to create a bridge rectifier using the common-cathode LED and two other diodes:

^{simulate this circuit – Schematic created using CircuitLab}

The red LED is lit when A is positive with respect to B. The yellow LED is lit in the opposite case. Build 6 of these; hook three of them up one way at one end of the locomotive, and hook the other three up in the other way at the other end.

R1 sets the current through the LEDs; a value of 390 Ω provides about 30 mA, and it needs to be rated for at least 1/2 W. Use a higher value if the LEDs are too bright at this current.

If you haven't done so already, you can do something similar with the headlights:

^{simulate this circuit}