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An open appeal for a simple way to control 2-colour, 2-lead LEDs. I am using these LEDs as the 'mimic' lights for my model train layout point control. Working on 12v DC, the point motor is wired to provide a +12v feed to either of 2 connections when the motor switches from one position to the other. I usually use two separate LEDs (1 red and 1 green) wired to a common -12v DC and, I have no problems with the basic circuitry required. However, for these 2-lead LEDs to work, I have to reverse the polarity in order to change the colour output.

I am sure someone out there has a simple solution to this and, I would be glad of any assistance. I can supply any further information you may need.

SamGibson
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GSSR
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1 Answers1

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This should do it. If only A is high then D1 is illuminated via R2. If only B is high then D2 is illuminated via R1. If both are low or high then both LEDs are off.

You can alter R1 and R2 individually to control the brightness of each color.

schematic

simulate this circuit – Schematic created using CircuitLab

It's possible it will work acceptably well with a single series resistor and the bipolar LED connected between A and B, but that depends a lot on what actually comes out of the open terminal and I'm not sure about that. Might be worth a try, but try it on a spare LED with something like 10K first.

Edit: I've added R3 because it's possible A goes negative when B goes positive due to self-generated voltage. And vice-versa.

Spehro Pefhany
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  • Spehro: Thanks for your help. I’ll definitely try this and, report back. I assume 1N4006 is the diode type? – GSSR Feb 12 '20 at 11:55
  • Yes, any kind of 1N4004, 1N4005, 1N4006, 1N4007 will be similar. Higher number represents a higher voltage rating, but they're all about the same price and probably only two real types at the factory. – Spehro Pefhany Feb 12 '20 at 11:56
  • Oh, OK, so for 12v circuit, a 50v diode is fine? – GSSR Feb 12 '20 at 12:06
  • Probably 1N4148 is okay, but there might be an inductive spike so higher voltage is safer. – Spehro Pefhany Feb 12 '20 at 12:07
  • I appreciate your advice and, as a novice, I have learnt something new today. Thank you – GSSR Feb 12 '20 at 12:21
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    Spehero: Today, I have produced a prototype circuit on my breadboard and, as suggested by you, all worked. I just need to play about with resistors to control the LED output brightness. I have ordered components to complete the 22 boards I need for my modelling. Again, my thanks for your speedy solution. Gary – GSSR Feb 13 '20 at 19:32