This circuit was obtained in an old Elektor and can be adapted to use white power LEDs. The transistor must be equipped with a heat sink. The circuit has a battery charger.
This assembly is from 1957 from a Raytheon manual, but can be modernized using a BC548, 6 V power...
Found in an American publication from the 1980s, this circuit can be found in commercial sound...
This circuit recognizes a tone determined by the tuned input circuit (R3 / R4 / C2). The circuit...