Electronic – Line level voltage to drive LED display

ledpreampsoundvoltage

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?

Best Answer

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.

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