Its pretty simple. I took the flashing circuit for an led box and built it into a ready made 2.1 system, then made a new housing for it.
Have a look here for a video of the finished product:http://youtu.be/0kvhXYVkGGU
Credit to motadacruz for his instructable as it was his awesome guide that got me doing this one, anyway have a look at this for the circuit but forget about the stereo cable as we are going to hard wire this into our audio circuit
The Circuit to control the LEDs consists of a NPN transistor (everyone recommends tip31 but i used tip 121 as thats what we had in the electronics cupboard). The base (left leg) connects to the signal source i.e the positive wire on the speaker. The collector (middle leg) connects to the first negative leg in the chain of LEDs (the amount of LEDs you can run depends on your voltage) the emitter connects to the 0v rail on your power supply. Thats it. there is no need to ground the negative audio signal like everyone else states.
Step 1: Getting the circuit and testing
I chose a cheap 2.1 system, stripped it down and played with the circuit. Although this system was AC (i live in the UK) it had transformer and bridge rectifier to lower the voltage and produce DC current. A fiddle with the multimeter showed it produced 17V
Sand down the rails to expose the copper before drilling holes to run your power leads from (i drilled 3 holes so i could have lights on the left, right and bass)
once you have run a power supply off the amplifier board and created your circuit you should get something like this:
(i have these LEDs in series and paralel in order to test but basically they are superbright Blue LEDs, they have a safe working voltagge of upto 5.5v 17/5.5 = 3.09 so i can have 3 LEDs in series and a protector resistor.
Now plan out how you want the lights and it will determine the design of your housing.
I went for 3 lights around each speaker and flashing bass ports. The spacing of the volume and bass dials could bot be changed so I placed them in the middle of the housing.
To difuse the lights around the speakers i used acrylic rod, drilled at one end at 5mm to accept the LEDs and sanded at the other with 400 grit wet and dry to defuse the light. The bass tubes are acrylic tube drilled 5mm again to accept the LEDs but this time the lenses of the LEDs have been sanded away as i want the light to diffuse through the side of the led and not protrude into the tube.
My design looked like the last picture on this step: