A seasonal display can be seen here:
Step 1: Quick-change Light System
For portability and ease of use, we've mounted the circuit on a mini rechargeable speaker system (obtained from DealExtreme) and we've wired it directly to its speakers and power supply. Therefore this design will be using a supply of 3.7 to 5-volt.
Here are a couple of videos of it in action.
Step 2: The Circuit and Parts List
R1, R2 - 100R
R3 - 4.7K
R4 - 560K
R5 - 10K
R6, R7 - 22K
R8, R9 - (not used)
R10 - 3.3K
R11 - 6.8K (Try 4.7k or 2.2k to enhance Bass response)
TR1 - 1K Potentiometer
C1, C5 - 1uF 
C2 - (not used)
C3 - 0.1uF 
C4 - 1nF (1000pF) 
C6 - .0047uF 
C7 - 10uF, 10v or more Electrolytic
D1 - 1N914 or 1N4148 (Low-voltage switching diodes only)
Q1-Q4 - 2N4401 (Do not substitute)
For each light display assembly:
Limiting R - 22R to 150R (Lower for brighter)
Wires and mounting hardware.
Step 3: The Quick-change System
The following photos show a closeup of the connections. Of the 6-pins, only 4 are used - the spacing is to allow more room for mounting parts and to protect reverse connecting the strip from damaging anything.
The last image shows the easiest connect to make: a 100-ohm resistor joined to a (common-anode) RGB 5mm LED whose legs had been spread to fit. A translucent sphere (courtesy of the last Beer-Pong competition) acts as an integrator to show almost endlessly changing color.
Step 4: Circuit Description
For simplicity, TR1 can be left out if the output is from earphones. Connect the -ve side of C1 directly to the resistors.
For a small amplifier like the mini-speakers I used, change R1 and R2 to 10K and TR1 can be skipped as well. Some closeups are included to show where I connected the signal and power leads.
Care must be taken with higher powered speakers: voltages can be quite high and for bridged amps, there could be a large DC voltage between the (+) of the speakers.
Step 5: The Pre-amplifier and Drivers
Each of the transistor Q2 and Q3 is hard-biased by the network consisting of R5, D1 and C3. The voltage on the anode of D1 is within milli-volts of the B-E bias of the transistors, which ensures that it has maximum sensitivity to the audio input. This also forms a half-wave rectifier to drive our circuit.
I've used Copper-clad boards to ease construction and the photos below show fairly simple layout. The only critical area is the 6-pin header interface. The last photo is the 'X-Ray' view to show component and wiring layout.
Visit youtube for more videos. More will be added as time permits.
To all a Happy and Safe Holiday! qs