This circuit found in a 90s Popular Electronics, triggers a huge array of LEDs from the sounds captured by a microphone, thus generating visual effects.
Some old multimeters have a high resistance scale that has an internal circuit powered by a 22.5 V...
This circuit is indicated to operation with piezoeletric transdicers or other with high impedance...
Found in Radio Electronics from the 90s, this circuit generates signals from 100 Hz to 1.5 kHz....