Two prong LEDs.

[Potrail2378]

Hi, I was recently gifted 10 red/white 3vdc tower bi-colour LEDs. They work OK, one voltage direction is white, the other voltage direction is red. I was intending to use them in a converted-to-dcc class 37. Only trouble is that there are only two terminals, but three wires - blue, white and yellow. Is it possible to wire these up so I have bi-directional lights at both ends of the loco? Thanks in advance.

[96RAF]

Two legged bi-colour leds work by reversing the applied voltage. These can be installed in a DC loco as you reverse the voltage to reverse the loco. A resistor is required in each leg. A DC loco if fitted with filament bulbs will likely have diodes to light only the forward end bulb in the direction of travel. Bi-colour leds are essentially back to back diodes so provide their own logic.

To incorporate single lens bi-colour leds in a DCC loco it is more normal to use three legged common anode leds. The blue wire goes to the middle leg, the white wire goes to the front white leg and rear red leg, the yellow wire goes to the front red leg and rear white leg.

Resistors must be fitted in series with the colour legs as a minimum, but an extra resistor can be used in the power leg to drop the brightness.

Chris is likely to provide one if his excellent standard wiring diagrams for you shortly.

[Chrissaf]

For the reasons Rob has outlined, it would be easier to wire up half of them as white colour and the other half as red colour [in other words use them as if they were normal single colour LEDs]. Typically, this would be a total of eight LEDs. Two white at front, two red at front, two white at rear and two red at rear. To integrate these with the blue, white and yellow wires of a decoder is done like this.

[ColinB]

I think you can do it. From the blue wire take two 1 kilo ohm resistors connect the other end of each resistor to each lead of the led. Now connect the white wire to the junction of one of the led leads and the resistor and the yellow wire to the junction of the other led lead and the resistor. I think that should work.

[96RAF]

Like this Colin?

Reverse the white and yellow connections at the other end of the loco.

Not convinced that would work without breadboarding it.

[ColinB]

Yes, exactly. No, I am not sure but it looks logically correct. I don't have any two pin bi colour leds or else I would try it out.

[Chrissaf]

I have just BreadBoarded it with two red LEDs back to back [as I too, didn't have a bi-colour one to try]. With two red LEDs it did indeed work. It would still need to be confirmed as workable with a decoder, because of the 'floating voltage' nature of the white and yellow wires. In my BB experiment I was using a bench DC power supply as the 12 volt power source.