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In figure 1, we can see the iPod audio signals are brought into the circuit via J1. J2 provides a headphone output. U2A buffers the audio signal which is then passed to each of the filter circuits.

• Figure 1 - Left Filter Bank

In figure 2, all of the filtered audio signals are fed into voltage to current converters which drive high intensity LEDS. Figure 2 also contains the bias voltage generator with creates a Vbat/2 voltage for the op-amps.

• Figure 2 - Left LED Drivers

Figures 3 and 4 contain the circuitry necessary for the right channel.

• Figure 3 - Right Filter Bank
• Figure 4 - Right LED Drivers

Notice that the filter audio signals are AC coupled to the inputs of the V to I converters. This means that the inputs to these op-amps will see signals that swing below ground. Fortunately, the OPA422 can handle this as long as the current is limited.

The circuit assumes a 1 volt RMS input signal. This translates to peak voltages of about 1.41 volts on the voltage to current converter inputs. The gain of the V to I converter is I=V/27 so the LEDs will see peak currents of 52mA or an average of 26mA since the duty cycle is 50%. If you want to extend battery life and/or reduce the brightmess, you can increase the value of the feedback resistors R18, R19, R25, R43, R44, and R48.

This design can be built into a small enclosure such as an Altoids Tin. The LEDs can be mounted on the left and right sides with the audio jacks and switch on the bottom.

So, that's all for now. I'm going to plug in my iPod, jam in my earbuds and chill out on some Alison Krauss.