This is a very simple Musicator that runs off the USB port. In its basic form, it only uses 4 components plus a single white LED. The 'star' of the show is the LM431 'Programmable Zener Diode' - which costs under 20c, so our project can be made for about a dollar in parts!

This allows us to double it up and have separate Left and Right channels, as shown here and below.

Step 1: Some Sample Videos

As you can see, the circuit is very sensitive and is able to display even soft passages on low listening volumes.

Step 2: What's Needed

The parts:

LM431 Programmable Zener
15K resistor
120-ohm resistor
1uF capacitor

You will also need a means to plug into the earphone socket of your music player and a male-"A" USB plug.

You can perhaps see an earlier prototype in the rear of the image below. There I've modified a perf board as a USB plug. It works, but definitely is not recommended unless you know what is involved and you have an inexpensive USB port to test on.

Step 3: How Does It Work?

How do we get a Zener diode to play music you may ask. The answer is the unique circuit inside the LM431. Within its transistor-like package, it packs a comparator and a precision voltage reference of2.5v, which is useful here, because it is exactly mid-way of our USB supply.

We are using this precise voltage to bias the LED just below its Vf, in fact you will see a faint glow even with no signal.

The input pin is monitored by the 'reference' pin and any changes in the signal shows up as fluctuations in our LED. The 15K resistor controls the sensitivity and can be as low as 3.3K if you like your music loud, or if your phones are less efficient. The LED brightness is controlled by the 120-ohm resistor.

Step 4: Assembly

With a handful of parts, the circuit is simple to assemble.

The images below should help you arrange the parts.

Note: You should not space the LEDs more than 3/4" (18mm) apart, otherwise your eyes will not detect out-of-phase signals used for surround sound effects.

See more of my circuits here.

Step 5: Variations

The LM431 is truly quite impressive. Here in a variation of our circuit, we have it driving a 1/2-W LED, not only that, it is also powering a little speaker at the same time!

Step 6: Variations 2

WIth some minor changes, our circuit can be adapted to other voltages. Because all the LEDs are in series, they can all be controlled by the LM431 by monitoring the last LED in the chain.

Basic calculations:
E = 0.9 * (V-1)
e = 3 for White, Blue, Pink, Violet or Green
e = 2 for Red, Orange, Yellow or Yellow-Green
Subtract e of each LED from E until it is close to 0. This is the most efficient combination for a given supply voltage.

R1 = 50 - 100 ohms (Brightness)
R2 = 10k - 47k-ohm (Sensitivity)
C = 0.5 - 1uF. Positive side towards TL431. Connect the Negative side to either the tip (Left Channel) or the first Ring (Right) of a Stereo plug.

Step 7: Variations 3

With a small sacrifice in sensitivity, we can substitute an NPN transistor for the LM431. Use the same process as above to calcuate LEDs supported.

Choose an audio-frequency transistor with a high beta (gain) of 150 or more. 2N4401, BC337 are good choices. Also acceptable are 2N2222 and 2N3906. Otherwise, check the datasheet for a device that:

is NPN
has a gain (beta) of 150-500
can handle 500mW or more
can handle 150mA or more
Voltage 50% higher than the Supply

R1 = 50-100ohm (Bright)
R2 = 100k - 390k (Sensitivity)
C = 0.1 to 1 uF. Negative side to music output.