This circuit uses discrete devices to filter and drive 3 LEDs each based on Low, Medium and High frequency signals.
A seasonal display can be seen here:
Step 1: Quick-change Light System
A quick-connect system allows us to swap displays in seconds.
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
The complete 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
To allow quick display changes, a 6-pin computer terminal strip is used on the main board, while a matching one is used to mount the display and the limiting resistor.
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
Resistors R1 and R2 add the Left and Right channel signals from our audio output.
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
Transistor Q1 forms a simple amplifier and its output is used to drive a filter chain consisting of C4 (Highs), R10, C5 (Mids) and R11, C6 (Lows).
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