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 optical counter was found in a 1964 Electronics Design. It makes use of an uncommon...
Found in an English publication in 1969, this circuit has a 600 kHz pass band. Ferranti's...
The transistor may be any general purpose, but the integrated transistor is not common today....