Introduction: Musicator Jr - Mk 2

A simplified Sound-to-Light display using a few inexpensive parts.

Step 1: The Circuit - Overview

If you're familiar with the original Musicator-Jr, you'll know it uses a dual op-amp to convert sound from an electret microphone to display on LEDs. This new design uses a much-simplified circuit and is able to support up to 40 LEDs.

Like the USB-Musicator design, this circuit makes use of the LM431 "Programmable Zener" circuit. Although packaged like a transistor in a familiar TO92 form, there is a half-dozen transistors inside which forms a powerful op-amp and driver, which we will make use of here.

We have added a 2N4401 transistor as an output stage, which will allow us to drive LEDs up to 600mA. So it is not advisible to substitute any other device for it.

The basic design is for a 9-volt battery, but 12-volt operation is also possible with a few value changes.

A point-to-point diagram is supplied below, a larger / clearer version is also avilable here.

Step 2: The LED Display Arrays

As you can see, our demo unit consists of 15 LEDs: 10 x Red, 3 x Blue and 2 x White. They are organised as 5 parallel rows of 3 LEDs, each row comprising of 2 Reds and either a White of Blue LED. Since White LEDs are made from Blue LEDs, their specs are essentially the same.

To make the most of available voltage, we'll use the following formulae to work out the optimal LED composition for each chain:

Vs = (V-supply-1) * 0.9, then
Subtract 3 for each White, Blue, Pink or Violet LED
Subtract 2 for each Red, Yellow or Orange LED
 repleat until you are as close to 0 as possible.

 This design requires that at least two of the LEDs be Red, Yellow or Orange, but can be any 2.

In our sample with 2 reds and a blue and Vs = 7.2; so 7.2 - 2 - 2 - 3 = 0.2, which is very close to optimal.

Additional chains composed of similar LEDs (same number of 2 and 3 -type LEDs) can be attached to points (A) and (B) on the scematic, up to a limit of 400mA, or about 20 chains of 20mA LEDs.

 Here's a video of it in action:

Step 3: Parts List

Q1 LM431 Programmable Zener
Q2 2N4401 NPN Driver Transistor
C-in 1uF to 5uF capacitor, + to Electret Mic side

R-bias 10K
R-drive 1.5K
R-feedback 150k to 560k. I used 2 x 470k in parallel to get 235k-ohm. Use 150k for the lowest sensitivity, ie LOUD environments. This is connected to the top of the 2 x RED LEDs.

 An additional 0.1uF capacitor can be inserted from the top end of R-feedback to ground to extend the time LEDs stay on, but is entirely optional.

A series of images are provided to guide you in the wiring.

Step 4: Variations

The circuit is versatile enough to support almost any LED combinations. Here is another version with Amber and Pink LEDs in an eye-watering arrangement. A 500k-ohm trimmer is placed in series with a 47K resistor as an adjustable R-feedback.

For other voltages, use the formulae presented in step (2) and adjust R-bias to V*1000-ohms, R-drive to V*150-ohms.

Have fun! Visit my website for more ideas on lights and LEDs.

Here's a video of it working:
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