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I am wanting to put have a few LED's flash with the intensity of my music. I don't need a color organ, but just want the LED's to flash in sync with the music. I am amplifying the signal with a LM368 chip and playing music from a 4ohm 3W speaker and it's sound quality is great. The problem is my LED's don't turn on at all..enter image description here

I am using the configuration that many people claim works well but its usually done with a TIP31 transistor and not a 2N3904. I tried inserting an opAmp with a gain of 10 before the base of the transistor with no success.

Does anyone see a problem with my circuit or know of a reason the LED's arn't turning on? Any advice would be appreciated


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You might want to cascade your npn transistor to a pnp that charges a (large) cap with a pulldown resistor; triggering another npn in the orig. location. This increases you component count by 5, but gives you gain from the cascaded transistors to get more switching action, a a longer pulse width from the cap, making it easier for the short-duration soundwaves to trigger long-enough diode flashes to be more visible. (a rheostat/pot for "brightness control" might also be helpful). – Robherc KV5ROB Jan 28 at 21:08
@RobhercKV5ROB Can you explain this more in depth? or a schematic. I think I get the general idea and it makes sense but I want to make sure I know how to correctly cascade if I'm going to try this out, Thanks! – mitch33 Jan 28 at 21:49
I think transistor's solution would be a better/less complex solution so long as you reduce the # of leds, wire them in parallel, or up the +5V source sufficiently, but if you still want to see a diagram, I could work one up and add it in a (5th) answer post (no diagrams/images in comments, unfortunately). – Robherc KV5ROB Jan 28 at 21:57
No need, Ill stick with transistors, Thanks! – mitch33 Jan 28 at 22:00
up vote 5 down vote accepted

Odds are that Q1 is smoked. You forgot to add a current limiting base resistor to limit the current.

You should probably add a reverse diode on the base (after the resistor) to protect the transistor. The diode is recommended because you are feeding the base with an alternating voltage that swings above and below zero volts. When it swings negative the base-emitter junction is reverse biased. It will probably survive given that you are operating on a low voltage but it's good practice anyway.

Test the transistor with your multimeter diode test function. You should get 0.7 V b-e and b-c with + lead on base. You should get high reading when leads reversed.


simulate this circuit – Schematic created using CircuitLab

Figure 1. Modified circuit.

When you get Q1 going again the next problem will be that you'll probably smoke the LEDs. You have no LED current limiting resistor in your schematic. You might get away with it if your supply voltage is low.

Edit: I couldn't read the supply voltage. I now see it's only 5 V. That won't be enough for four LEDs. As others have suggested, try them in parallel pairs.

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Why the down-vote with no comment? This answer is the best of the 4 ... – brhans Jan 28 at 21:17
switching the LEDs from series-connected to parrallel-connected should fix the low supply voltage problem (though it'll put more current strain on your little transistor if you don't upgrade it). – Robherc KV5ROB Jan 28 at 21:19
Thanks, Can you explain to me why the diode is needed? or how it serves as protection for the transistor? – mitch33 Jan 28 at 21:35
@mitch33: See the updated 'reverse diode' paragraph. – transistor Jan 28 at 21:45
Thanks for the explanation, that makes perfect sense. Much appreciated. – mitch33 Jan 28 at 21:46

The drawing shows the LED power supply as 5 volts - that is too low to light four LEDs in series.

The forward voltage across an LED depends on the colour and chemistry - common red LEDs are about 1.9 volts, and other colours are higher, up to 3.2 volts for blue and white.

The supply voltage must be greater than the sum of the LED voltages, plus a volt or so for a current limiting resistor. With a 5 volt supply, you could have two red LEDs in series with a 100 ohm resistor, to give about 10 mA current through the LEDs.

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This is probably the most important problem. Red would light a bit, but blue or white wouldn't even at a third of their typical voltage. – Passerby Jan 28 at 21:56

There are too many LED's even if each one was blue which has ~1.2V drop on it, that would be 4.8V with the transistor having an additional 0.7V drop. The transistor won't turn on. Try it with one led and see if it works. enter image description here

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Blue LEDs are around 3 volts, I believe. Infrared LEDs may be as low as 1.2 volts. – Peter Bennett Jan 28 at 21:14
You MUST have a current limiting resistor in series with each LED, or each series string of LEDs. Without a current limiting resistor, you will probably destroy the LEDs. – Peter Bennett Jan 28 at 21:53
I have white leds that will be on at 0.1 mA @ 2.54 volts. Very lightly. Typically 20 mA @ 3.3 volts. But the transistors VCE drop will be 0.2 at saturation, not 0.7. 0.7 is the V be drop. – Passerby Jan 28 at 21:59

There are two possibles reasons why is your LEDs not turning on:

  1. The transistor Q1 is never turned on. The AC signal coming out to the headphone must be higher than 0.7V approximately, or else the transistor won't let the current flow through the LEDs. Or the transistor is simply broken, it happens.

  2. The LEDs are burned. Normally, you should put a protection resistor to limit the current flowing through the LEDs as I'm quite surprised there isn't one right now.

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The headphone voltage doesn't matter, they are connecting it after a op amp that's biased to 5V peak to peak. In a direct connection that would be important though – Passerby Jan 28 at 22:05

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