"AC source is an electret microphone"
Bias voltages in this circuit are just enough to collect little glow in the 9 LEDs, then the circuit is free to be "bothered" (LEDs filcker) by the smallest noises, via its 2N3904 realizing an impressive gain for its weight. Simplicity is key; this circuit consumes little current, and lights up many of these LED triplets in parallel, in this very configuration.
***IMPORTANT***
Source resistance is actually 90-300 Ohm; when there's no noise, the LEDs are a bit glowy/flickery, and the voltage across that resistor is only < 20 mV...
Read the sentence again because I don't think you got it.
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Enjoy The Ballerina!
