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LED Color Organ using OpAmps

Wheelie016

Nov 20, 2012
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I want to build a LED Color Organ using OpAmps and some passive filters. I was just wondering if the circuit I have "designed" would work. I'm fairly new to OpAmps, but I have learned a lot from the good ole interweb. I'll add a schematic as soon as I can, but until then I'll describe it as best I can.

First I'll have the audio signal go through a low pass filter with a cut-off frequency of 361 Hz. The component values are 200 ohms for the resistor and then 2.2 uf for the cap. Then the signal will pass through a LM386 powered by 5v and in a non-inverting format with a gain of 3.1 using 10k and 4k7 resistors on the negative feedback loop. After that it just goes through a standard diode and resistor in series to a transistor that lights the LED's.

So given that terrible description, would you think it work? Or am I at least in the right direction? Advice would be much appreciated. And again I plan on adding a pic as soon as I draw a decent one.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Adding a schematic will help a great deal.

You may wish to have a capacitor after the diode on the output of your amplifier to sustain the lights rather than turning them on and off with each half cycle, but we can talk about that more when you show us your circuit
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Click the "go advanced" option just below the quick reply box. You get more tools, one of them is an attachment tool. Use that to attach images.
 

Wheelie016

Nov 20, 2012
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Ah, okay thanks. And here it is, I think/hope.

And btw, I am assuming my input voltage will be roughly 1.5 volts because I would assume the ipod and most mp3 players use 3v. So I halved that in order for the volume to be half way.
 

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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Several problems:

1) single ended supply
2) no input capacitor
3) half output voltage will occur at just under full volume
4) no filtering

This might be a better starting point.
 

Wheelie016

Nov 20, 2012
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Well that's better than nothing I suppose.

1.By single sided supply, do you mean that I have no negative voltage?
2.How would I choose what input capacitor to use?
3.So 1.5 volts is basically full volume voltage?
4. Ah, Stupid me. I forgot to draw it on there.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Well that's better than nothing I suppose.

1.By single sided supply, do you mean that I have no negative voltage?

yes. Now it *may* not be necessary, but it's generally a bad idea to use the negative supply rail as a reference point.

Also the amplifier needs something to stop your input floating al over the place.

2.How would I choose what input capacitor to use?

It may form part of your filter. Especially for a high pass filter.

For a low pass filter it may interfere with it, so the best answer is "judiciously"

(use a value about 10 x larger than the capacitor in your low pass filter and you'd be OK in most cases.)

3.So 1.5 volts is basically full volume voltage?

No, every time the voltage doubles you hear what seems to be a similar increment in the sound level. So the normal output level may be a couple of hundred millivolts or less. It will have peaks to larger values.

4. Ah, Stupid me. I forgot to draw it on there.

Draw it in and show me.
 

Wheelie016

Nov 20, 2012
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I guess what I'll do for the power is use to 9v batteries in series and make a center tap between them and use that as ground.

And here is the drawing of the input capacitor and the low pass filter.
 

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(*steve*)

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I guess what I'll do for the power is use to 9v batteries in series and make a center tap between them and use that as ground.

That would be a good safe start. Just don't try to use them to drive your load :)

And here is the drawing of the input capacitor and the low pass filter.

At first glance the resistor values look small, however it's possibly not too inappropriate given that your source is designed to drive a relatively low impedance load.

You've actually drawn a bandpass filter here, what is the frequency range you've calculated for it?
 

Wheelie016

Nov 20, 2012
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Gotcha. And the High-Pass filter is ~14 Hz and Low-Pass is ~361 Hz. Btw, I'm placing the filter before the OpAmp, just in case that matters.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Try it and see how it goes. It would be more typical to use higher value resistors and lower value capacitors (say by a factor of 10) but in this case it probably won't make much difference.

You understand what those frequencies mean? That's where the amplitude is cut to 1/2. At double the frequenct (for a low pass filter) it will be cut by another 50%, and double that by another 50%, and so on.

I think you'll probably want a level control, and if you've got multiple filters, that needs to be before the filters. Because they have a low impedance (the filters), this may be a little tricky. If you're content to adjust the volume on the iPod, or to have a level control for each band (after the filter and before the op-amp) then this won't be a problem.

I suggest you build this up (perhaps show me a schematic of the whole circuit so far so I can check it) and see how it goes. If you're drawing a schematic, just be sure to show the power supply connections to the op-amp as well. Since these are typically not shown, people sometimes fail to connect them.
 

Wheelie016

Nov 20, 2012
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When it comes to level control, do you mean have a single variable resistor before all the filters, or have separate ones in between each band pass and opamp? And if I had to do this, what size? or will it matter seeing has how I can change it? I'll just spitball and say 10k seeing as how I have plenty of those. :D

And I'll get on to drawing it up ASAP.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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If you're breadboarding this, just set it up as is and see how it goes.
 
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