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.
This circuit uses a General Electric coupler from the time of the publication in the 1980s....
This circuit with operational amplifier was found in old American documentation. It can be...
This is an integrated recommended for portable amplifiers, recorders, and battery powered radios. It...