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Qs wrote up an instructable for his “Musicator Jr.”, an LED light organ about the size of a 9V battery –

The ‘brains’ of this project is a LM358 general-purpose op-amp which costs under 30-cents. The first half of the circuit is an amplifier which boosts the 500-micro-volts from an electret mic to about 1-volt. This level is generally called ‘Line-level’ and can be used to drive our LEDs, an audio amp, or even the input pins of an Arduino processor.

The second half of the op-amp is used as a voltage-to-current converter, which limits the brightness of the LEDs to 10mA or less.

Nice and simple build with plenty of ‘wow-factor’ for those who see you wearing it! Check out the full step-by-step here.