I'm working on a small-scale audio spectrum analyzer. I take an aux cord L/R inputs into two amplifiers that output to a speaker. Say I pass the output that goes into the speaker into 3 bandpass filters for low,mid, and high ranges.
If I then rectify that output to a DC voltage, how would I configure a "bar" of LEDs to light up based on that voltage? Say I had 6 LEDs per bar. When there's little to no bass I want only the first 1-2 LEDs to light up, and when there's a lot of bass all 6 LEDs to light up.
I'm trying to this project with only analog components, as I realize this is a trivial task with a microcontroller.
I'm not sure if it's important or not in this type of circuit, but other than the signal output I have access to a 5V DC source that I've mainly used to bias the amplifier.
I can't quite figure out how to configure the LEDs in such a way that it matches the behavior I desire. I've searched around online and see some circuits use zener diodes to achieve this sort of effect. I think I want to turn the LEDs on through current and not voltage but am unsure how this works. I breadboarded some LEDs and played around and I couldnt get the right behavior.
As duskwuff pointed out below, the LM3914 is a great and pretty easy solution to this problem. But I hate easy. I've been messing around more and here's what I've been trying to do:
1) Rectify my signal output using a full wave rectifier. There's a problem with this though, my amplifier output signal has a minimum p-p of around 200mV. This means that at lower volumes, no signal will be rectified because the diodes will not allow current through. Could I possibly solve with with an amplifier stage between the output and the rectifier input? I have a 5V DC source available.
2) At this point I'm trying to figure out a way to use NPN transistors as switches that turn on at different rectified output levels. My success with this has been, well, very limited. At first I tried using Zener Diode's taking advantage of their zener voltage, but I looked around and I can't find any with a low enough Zener voltage that would work here. I could put an amplifier between the rectifier stage and the NPN stage, but I'm not sure thats the best solution.
I've included a picture of my entire circuit schematic so anyone can see what I'm working with here.
Starting at the far left I have a balance control, followed by a 3-stage amplifier, followed by a rectifier.
Any help would be appreciated.