EDIT: Please read the whole post before automatically down voting. I KNOW the diagram is crap. That is what I am asking about. It isn't my diagram.

Original post:

I Web searched a bit and searched this site, but I didn't really see a good answer.

I thought line level voltage was insufficient to switch a transistor and I thought an audio signal coming out of an amp or preamp was a sine wave anyway.

I thought those things made the seemingly simple idea of driving LEDs to pulse with sound or music a little more difficult. Microphone input -> preamp -> rectify it to make a nice fat on/off dc signal -> drive something.

But I see this: leds respond to sound

with a circuit that looks like: enter image description here

I see other circuits (one at Instructables I think) that essentially do the same thing- connect the line level voltage directly to some transistors.

My question: I haven't a clue what is going on here. Why does this work- or does it really? What is happening? Is it that the "audio source" is really headphone/speaker level and not the line level it may be billed at and thus enough to saturate the transistor? Does this look like a recipe for disaster?

EDIT: Trying to focus things per Red Gritty Brick. I know the diagram is crap. How would you do it right?

  • \$\begingroup\$ Link to the source of your schematic is now bad. \$\endgroup\$ Jul 21 '13 at 1:49

That diagram is a rich collection of errors, to long to fit in a comment, so I'll make it an answer.

  • your diagram shows PNP transistors, but wired the wrong way round. But the TIP31C that you show next to a PNP symbol is an NPN type...
  • you have no resistor in the base wire of the transistors to limit the current. Put a 1k resistor there (in each each line)
  • you have the LEDs pointing in the wrong direction, the arrow must point towards the negative. The LED picture next to the symbol that is pointing in the opposite direction does not help for clarity.
  • you have no resistor to limit the current through the LEDs. For standard 20 mA leds put a resistor of ( 12 - ( 4 X 2 )) / 0.02 ~= 220 ohm in series with each LED string.
  • you used the symbol for a foto-transistor, which is a very different component than the power transistor you want (and should) use.

If you want to get this to work, either find a better instructable or dive into the theory yourself.

  • \$\begingroup\$ "I" don't have any of that. I didn't make the diagram- in fact it seems wrong to me as well and that is the root of my question. Frankly, I don't see how it would work as drawn. Edit: I'm hoping somebody will post the real way to do this. Hint. \$\endgroup\$
    – mikeY
    Jun 20 '13 at 19:39
  • \$\begingroup\$ - I'm not sure you read my question. You say "If you want to get this to work, either find a better instructable or dive into the theory yourself." First off, I don't need your open hostility. Second, I thought I displayed a little better knowledge of what is going on than appears to be shown in the diagram. You have done a fine job identifying several of the flaws in the diagram. I was more interested in why this seems to work for the people that posted it and the better/right way to do this. \$\endgroup\$
    – mikeY
    Jun 20 '13 at 19:54
  • \$\begingroup\$ @MikeY your Question contains five separate questions, your comment contains a sixth ("how to make LED brightness respond to a line-level audio signal"). maybe you can edit your Question to focus on a single issue. \$\endgroup\$ Jun 20 '13 at 19:55
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    \$\begingroup\$ My answer to "why does this work" is "this does NOT". There might be something that works, but that is not the diagram you show. \$\endgroup\$ Jun 21 '13 at 5:33
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    \$\begingroup\$ @mikeY - You're missing the critical point that Wouter et.al. are trying to get across. What is in the drawing is not what the people doing the original writeup actually made. They're equally poor in accurately depicting their circuit as they are in drawing a schematic. \$\endgroup\$ Jul 21 '13 at 1:49

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