This circuit from the 60's uses germanium transistors of the time. Listening is done on high impedance headphones.
This circuit, found in a 1986 documentation, uses an optical sensor to count pulses and, in its...
Found in General Electric's 1969 documentation, this circuit uses two Foto Darlingtons to control...
The output voltage of this inverter depends on the employed transformer. The circuit is from a 90s...