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I'm looking for a simple (hopefully !) circuit which would allow a variable resistor to control the brightness of three different coloured LED's. As the variable resesistance is changed in value first one, then the second, then third LED would fade out in brightness.

It doesn't matter if there's a bit of crossover so that LED2 starts fading before LED1 is completely out.

I thought at first this was simply a case of wiring the LEDs in parallel at a point after the power had been through the variable resistor but I read that wiring up LED's in parallel had difficulties so I thought I would ask.

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  • \$\begingroup\$ How even a fading do you need? \$\endgroup\$ Aug 9, 2014 at 4:10

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A differential pair something like the following could cross-fade two LEDs:

schematic

simulate this circuit – Schematic created using CircuitLab

This is because the emitters are connected so if you turn Q1 on, Q2 will turn off (because the Q2 Vbe will drop because the voltage at the emitter of Q2 will go up).

So you position the pot at ground and then adjust the R3 R4 bias until the D1 LED gives the desired brightness. Then as you turn up the pot they should cross fade I think. But this circuit is just a back of a napkin scribble. I would simulate it first. Then you might take what you learn to figure out how to control 3 LEDs somehow. Then build.

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  • \$\begingroup\$ thank you for your answer which is very useful. It's more complicated than I had initially thought so I'm glad I asked ! I was unaware of CircuitLab which looks like a good resource, I will do as you say and fiddle with the your suggestion in there. Thanks again. \$\endgroup\$
    – glaucon
    Aug 12, 2014 at 22:32
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I do not understand your question very much. For example, if the LEDs are in parallel, you could use a POT, in series with them, to limit the current and, therefore, its brightness. The POT value may vary depending on how you want the fade.

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ This will fade all three at the same time. The question is about fading them in sequence. \$\endgroup\$ Aug 9, 2014 at 5:46
  • \$\begingroup\$ @Jesus - Thanks for your response but I didn't want to fade them all together. Instead I wanted first D1 to go dim; then D2 to go dim; then D3 go to dim. \$\endgroup\$
    – glaucon
    Aug 12, 2014 at 22:28

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