I am making a small color organ just for proof of concept fun. I am using 3 passive low, band, and high pass filters with their outputs connected to an envelope detector and a BJT that drives each LED. My main question is do you think this will work or do you have other suggestions to improve this design? Please disregard all component values. They were automatically added and I haven't done any value calculations yet. Assume the signal provided is amplified already and the 1N4004 diodes off the transistor are the LEDs.enter image description here

Also do you think it is better to use passive or active filters in this case? I only plan on driving one or two LEDs per filter.

Thanks for any help!

  • \$\begingroup\$ I don't think it will work at all. There are no LEDs! Use the schematic button on the editor bar. That way you can fix errors and we can copy and edit it in our answers. Double-click the components and enter the right values. \$\endgroup\$ – Transistor Feb 11 '16 at 22:21
  • \$\begingroup\$ The LEDs are going where the 1N4004 diodes off the transistor. I don't have LTspice on this computer so I'm stuck with this online thing. Just assume they are the LEDs. Would it the general schematic work? \$\endgroup\$ – Gigaxalus Feb 11 '16 at 22:26
  • \$\begingroup\$ You don't need LTspice. Use the built-in schematic button on the editor toolbar on this site. Nobody can say if it will work if you only supply nonsense component values and voltage levels. \$\endgroup\$ – Transistor Feb 11 '16 at 22:42
  • \$\begingroup\$ A comment on the schematic itself: you've got some junction dots where they shouldn't be, and a whole lot of missing dots. Additionally, spacing out your components a bit would greatly improve readability. \$\endgroup\$ – uint128_t Feb 12 '16 at 1:04


  • 1V won't light any led.
  • You have NPN transistors on the led high side.

Everything else seems okay I guess.

The envelope detectors seem unneeded, most color organs I've seen don't use one. Probably not an issue but response will be affected, as it will be less sensitive. It depends on how flashy you want it really.

And the specific values of the parts are completely important to how the band pass filters and envelope detector works, so you need to figure those out.

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  • \$\begingroup\$ I am aware that all the values are important and of the voltages needed to light the LEDs, that was just the default value it took. All I am looking for is a rough idea of where I am going with the project. If this direction will work, I will invest time into figuring all the values out. As far as no other color organ using an envelope detector: jameco.com/Jameco/workshop/diy/ledcolorschematic-lg.gif \$\endgroup\$ – Gigaxalus Feb 11 '16 at 22:31
  • \$\begingroup\$ Also I was not aware LEDs had to be on the high side. Thanks! \$\endgroup\$ – Gigaxalus Feb 11 '16 at 22:33
  • \$\begingroup\$ I was under the impression that the envelope detector would smooth out the frequency from the filter providing a better control signal for the transistor. Are the frequencies just too fast to even matter? \$\endgroup\$ – Gigaxalus Feb 11 '16 at 22:39
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    \$\begingroup\$ @Gigaxalus Leds don't have to be on the high side, but your (NPN) transistors should be on the low side (emitter tied to ground) if you want them to actually do anything. The transistor only turns on if current can flow into the base and out of the emitter. Current cannot flow into the base if the emitter is at a higher potential than the base... \$\endgroup\$ – jms Feb 11 '16 at 23:53
  • \$\begingroup\$ The transistor is an npn, that's why the leds are on the high side. The schematic you choose uses op amps instead of passive rc band pass filters, so maybe that's why they use envelope detectors. \$\endgroup\$ – Passerby Feb 11 '16 at 23:57

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