Here i will show you how to create a RGB music controller, which lights RGB leds to music. I also added an AGC(Automatic gain control), which keeps the voltage at the same level. This is very useful, because you often play music at different volumes, and without the AGC, the leds will be very bright at high volumes, and be really dim at low volumes. Since the AGC automatically keeps the voltage at the same level, the leds will stay at the same brightness no matter what volume you're playing at. I created a circuit board using ISIS/ARES, but you could create one on a breadboard.

To use this, you need to have a power supply with +12V, ground and -12V

You also might need an amplifier, depending on your audio set-up. The AGC starts amplifying at around 20mV, so if you're playing on low volumes you might need and amplifier. My internal laptop sound card outputs around 5-10mV on low volumes, and 1V on high volumes.

Step 1: How It Works

When you send a sound signal into the input of the AGC, it either amplifies it, or dampens it, so that the output voltage(controlled by potentiometer RV4) is always the same. It then goes through 3 audio filters, to separate low, mid and high frequencies. The leds are controlled by transistors which leads current from collector to emitter when there's 0.6V difference between base and emitter. The output of each of the audio filters are connected to their own transistor, so you get different led light, when there's different audio.

Instructables lowers the resolution of the image, so here's a higher res of the schematic:

http://i.imgur.com/FqjSHoR.jpg

Step 2: The AGC

Here I will show you how to make the AGC part.

You need an OP-amp(U2:A), I used an Tl074, which needs +12V, GND and -12V, but you can use any OP-amp.

You just connect everything as the image, where the input of R20 is the audio signal (needs to be atleast 20mV)

I used a 10K pot to regulate the voltage out the AGC.

Step 3: Filter

I start with having an op-amp (U2:B) with a negative feedback loop from the output of the AGC to a potentiometer for the each of the three filters. The voltage then goes through each filter, and then to a transistor at the end driving the voltage to ground when it receives voltage. The potentiometers regulate the amplitude of the signal, which is useful if you for example want bass frequencies to light more than others.

In my schematic I have the cut off frequency for low tones at around 200hz, the start for middle at around 250hz.
Cut off frequency for mid tones is at around 2,3Khz and start for high tones at around 2,5Khz.

You can change all this to your liking by configuring the capacitor and the resistors in each of the filters. I used these calculators, which worked excellent:

low pass: http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm
high pass: http://sim.okawa-denshi.jp/en/OPseikiHikeisan.htm

Step 4: Finalization

I took my RGB music controller, and added it in a small box together with a power supply and an amplifier. I also added a speaker output, so that I can listen to music too. I connected red to low tones, green to mid and blue to high tones.

I've added the ISIS/ARES files here, so you can download and print it for yourself if you have the things you need to print a circuit board.