This circuit with two transistors was found in ancient European documentation. Transistors support equivalents. The indicator on this circuit is an ordinary 50 to 100 mA lamp.
Obtained from an English publication, this circuit actually needs a current limiting resistor,...
This 2003 circuit serves to digitally show the intensity of a sound captured by a microphone....
Suggested by a Popular Electronics from the 90s, this circuit uses a buzzer to indicate the...