audio reactive lights question (slightly ot)

[snufkin]

I'm currently putting together an led lighting thing to be placed in loud venues and I want it to be audio reactive

now I don't need anything fancy but what I was thinking was to use the audio going in to a small mic to control the amount of power going to the leds

what approach would you guys use (link me some circuits) to do this ? i'm probably talking a 9v psu here  LDR or something as a variable resistor ?

[valdiorn]

Me, I love op-amps. Would amplify and half-wave rectify the input signal (from the mic or line input or whatever), then put that through a low-pass filter with a VERY low cutoff like ~ 3-5 hz). Now you have a voltage that is proportional to the volume of the input signal. You can now use that voltage for whatever you want, for example, use it to drive the gate of a FET that supplies current to the diodes, that would be one way to control the LEDs, another is to do some kind of PWM with it, but that's more complex...

[head_spaz]

Take a good look at National Semiconductor's LM3915 chip.
It is used mainly for driving LED (or LCD) VU meter displays.
One chip can drive as many as 10 LEDs, and you can cascade chips if you need better resolution or more dynamic range.
You can configure it for DOT or BAR mode display with a single jumper.
It incorporates built-in current limiting for the LEDs too.
Plus you can mix and match colored LEDs.
It's a pretty cool chip.