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 [105]
C2 - (not used)
C3 - 0.1uF [104]
C4 - 1nF (1000pF) [102]
C6 - .0047uF [473]
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
Hi, Would you be able to post a picture identifying all the components ? The reason I ask is that the pictures show 2 electrolytics and there's only 1 on the parts list ? Cheers
Both 'lytics (in grey) shown are 1uF caps (C1 & C5). C7 (10uF) is the tantalum which is the silver colored component on the top left of the circuit board.
Very nice! I intend to build one a bit later on, but as a stand-alone unit with a microphone to pick up ambient sound.<br><br>One question: I don't see the potentiometer anywhere on your board! Is it that silver canister that looks like a crystal oscillator?<br><br>Thanks for the great build!
I've omitted the pot for direct mounting on the amp (see Step 4) since I can tailor the input to suit the level on the speakers.
<br> This is a great little project.&nbsp; I don't know why it hasn't sparked more interest.&nbsp;&nbsp; I like the way you've minimised the filtering components.<br> Were the base bias trannies R6 and R7 calculated, or was it a 'suck it and see' job?&nbsp; Also, have you calculated the approximate frequency bands this responds to?<br> I may try a MOSFET variant of this which would (I think) minimise the component count even more.<br>
Thanks for the support. I like this little guy too! :P <br> <br>R6 and R7 are set to the maximum (least drain) that can maintain decent regulation for the 2N4401. <br> <br>I've aimed for 300Hz; 2000Hz and 6000Hz cutoffs. Of course caps are notoriously inaccurate so it's mostly academic. <br> <br>The hard-biasing scheme will not work with MosFETs, and Zero-bias ones are probably too finicky for dependable results. However, I will be very interested to hear about any other approaches you have.
Should have been wearing my glasses - I was misreading a datasheet. It looked like there was a linear region from 0V to 1V as a MOSFET turned on, but I was looking at VDS, not VGS.<br> <sup>(That sounds horribly like a Sheldon Cooper joke ;&not;)</sup><br> I'm fine on the digital side, but my analogue theory's a bit weak.
This looks like a very cool project, and being able to swap the Illumination units easily is a neat trick.
Thanks - I find having the plug-in system lets me 'play' with ideas a lot more.
Great idea. I can envision all types of displays with this. Thanks and Merry Christmas.
Thanks you. And a great XMas and New Year to you as well!

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