This allows us to double it up and have separate Left and Right channels, as shown here and below.
Step 1: Some Sample Videos
Step 2: What's Needed
LM431 Programmable Zener
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?
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
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
Step 6: Variations 2
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
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:
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.