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I am looking at getting some multicolor LEDs, which have Red Green and Blue inside of them. I'm doing this for space saving, rather than using 3 different LEDs--I am using just one.

What I want to do is to use a multicolor bulb to produce all sorts of colors, mixing the channels (if you call it that) so I can the full color rainbow. I need to be able to use a single rheostat to do that, rather than using 3 independently so I can twist just one to change the colors up and down. If I need to use a small transistor, that's ok (I'd prefer to keep it simpler if possible) but I need the ability to use a variable controller knob to tune the colors.

I can't seem to find any circuit design that can do this 1) for a multicolor LED, and especially 2) for a variable control knob in it. Sure, I can do this with an Arduino but I'm trying to use this in way more simply.

I'm not set on using PWM or not using it, I just want the ability to set it up and be able to 'tune' it with a knob. If I can get away without using a transistor or having to program it, that would be ideal.

Thanks!

G

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  • \$\begingroup\$ I can imagine this being done with a couple of comparators/op amps, ala a lm3914/5/6 but it wouldn't give you a a full range of colors, be overly large, wirey complicated mess. A microcontroller like the attiny or msp430 would be easier, smaller, neater. Hell I could link a nice msp430 launch pad one later. \$\endgroup\$ – Passerby Oct 28 '14 at 16:03
  • \$\begingroup\$ Difficulty level of doing this with a rheostat and basic components: 8. Difficulty level of doing this with a MCU and push rotary encoder: -3. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 28 '14 at 23:13
  • \$\begingroup\$ @IgnacioVazquez-Abrams why bother with an encoder when a adc + pot is easier? \$\endgroup\$ – Passerby Oct 29 '14 at 0:27
  • \$\begingroup\$ @Passerby: Because the pot won't let you click to pick a different component to vary. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 29 '14 at 0:29
  • \$\begingroup\$ @IgnacioVazquez-Abrams a push button would be better, but that's only if you need individual control, instead of just cycling through a set. \$\endgroup\$ – Passerby Oct 29 '14 at 0:31
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In general you'd need three knobs to set the three LED light levels. You could make a one turn knob potentiometer, read the voltage, and turn the voltage into some combo of LED's with a look up table. But it will most certainly involve programming and PWM.

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  • \$\begingroup\$ I was thinking something along these lines, too. Conceptually, the color could start at red, with the knob turned left, then move toward green in the middle, and blue on the right. I don't see a good way to make it work without adding the smarts of a micro-controller in between the potentiometer and LED module. \$\endgroup\$ – mkeith Oct 29 '14 at 0:37
  • \$\begingroup\$ @mkeith, I guess if we had a whole range of LED's that covered the visible spectrum, we could scan across them, turning on only two at a time. The "trick" with three (RGB) led's is that you can fool the eye into seeing yellow. If you put the three led's into a spectrometer there would be no yellow light. \$\endgroup\$ – George Herold Oct 29 '14 at 0:48
  • \$\begingroup\$ I'd also not mind doing it with 3 leds rather than one, if it could still get the sweep. I'd just put them in a chamber and use a prism to emit one light stream. I was able to do this with 3 rheostats but for easier mixing I want one control \$\endgroup\$ – GWinters Oct 30 '14 at 1:51
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schematic

simulate this circuit – Schematic created using CircuitLab

this might just work if you sized your resistors and zeners properly. it would cause one led to turn on at a time, then stay on without changing too much brightness while the next one turned on if you got the transistors to be saturated just at the point when the pot reaches the zener voltage of the next colour.

it might not work at all - just an idea

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  • \$\begingroup\$ Almost what I meant when I mentioned opamps/comparators like the lm3914, which would give a few colors. Nice setup. \$\endgroup\$ – Passerby Oct 29 '14 at 0:28
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    \$\begingroup\$ thanks Passerby. If you were going to use opamps/comparators etc you could do proper pwm brightness control (and potentially visually linear) and possibly get a full colour sweep using window comparators. \$\endgroup\$ – user57093 Oct 29 '14 at 19:30
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In order to create a full palette of colors, the intensity of each color must be manipulated independently from the others - from zero through some maximum for each color - so I think your proposition is impossible.

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