This simple circuit can work with either 4.5 V or 6 or 9 V if a 1k resistor is added in series with the set of LEDs. The circuit is from an old French publication.
Found in an Electronic Design from 1965, this circuit makes use of transistors of the time. The...
Found in an EDN from the 90s, this circuit uses an electret microphone. The output can be used to...
This circuit compares the input voltages by providing an output according to the curve next to the...