I'm at the beginning stages of making an LED visualizer for live music. My ideal end product will most likely be a wearable device that has the arduino switching transistors to connect different led configurations that will get their supply voltage from an anolog signal (ultimately for vocals, but I have a guitar and not a nice microphone, so right now with a guitar)
Where I am right now is a simple circuit with the guitar input to a LM386 OP amp IC and an LED out. It works fine when I have a 8 Ohm speaker (from a Hallmark card) attatched as an output also. When I have no speaker, and just the LED is when I run into trouble. The led starts off working but fades out in like a second. I'm not sure why this is, but hopefully someone can point me in the right direction.
Also, what can I do to make the LED brighter? It works at a reasonable brightness, but nothing close to what I would get if I fed 3V straight into the LED. The difference appears greater when instead of a standard LED I use one of the 8000mcd jumbo 10mm LEDs from RadioShack. This is the LED that I ultimately want to use (it's super bright and big).
Here is the circuit I have set up as of right now (forgive the sloppy mspaint):
You're feeding the LED AC voltage (which the speaker likes).
What you need is an envelop follower / rectifier (same thing basically) that converts AC->DC. Easiest way is diode + resistor + capacitor. The LED has to be above a certain voltage (its usually around 2VDC for red LEDs) but can be higher for other colors (there's also a current requirement but you shouldn't run into any trouble with that).
Feeding the LED AC voltage means the LED will only be "on" for part of the time. It turns on an off as the voltage rises and falls, which is why its much less bring than supplying it with DC voltage. Also watch out because you can burn out LEDs by putting too much current/voltage through them. Usually you put a 'current limiting resistor' in series with the LED to keep this from happening. 220 or 1k are common value for that. I'll try to find some schematics of this stuff
The LED doesn't have the same impedance as a speaker and I wouldn't be surprised if your op amp was shutting down due to excessive current through the diode when you remove the speaker, but I could be wrong.
I vaguely remember an old sound-to-light controller I had from Edmund Scientific decades ago. When I opened it up it was just a couple simple R-C filters (i.e. tone control circuits: bass, mid, treble) hooked up to some triacs to make AC lights flash when an appropriate frequency was input. You don't need triacs, but you do need a transistor so that your LED can be driven at the proper voltage/current.
Try looking up some passive low/high pass filter schematics, then send your op amp output to your filter and send the filter output to a transistor to drive the LED (with proper resistor to limit the LED current, R = V/I).
The real question is how does the guitar sound with your setup?
I've been toying with the idea for sometime of making an Arduino based LED controller that would put on a little light show that could be used by street musicians. I've got some of the 10,000 mcd LEDs from SparkFun that are practically like little spot lights. I can imagine a little accelerometer that would sense the vibration from change being thrown in the guitar case and flash a special light sequence, or maybe a sonic rangefinder to up the light show when someone walks near.
Technically I don't think the op amp can output AC since it doesn't have a negative power supply relative to ground, rather it's a DC wave form. Current always goes the same direction, kind of what you would get with an AC wave if you added a DC offset equal to the amplitude.
pardon the kluge. But you could replace the speaker with an inductor like the torroidal coil in a "Joule thief". 30 turns around the torroid out of a mini flourescent light for starters. I suspect that the speaker is acting like a voltage boost/buck power supply. Doesnt require AC, just for voltage to go to zero.
Also this guitar effects page has a lot of good audio filter stuff.