This optically coupled circuit was found in the February 1968 Radio Electronics magazine. The circuit uses an LDR in modulation. Transistors are types of the time.
This circuit was found in a 1965 Radio Shack documentation. The transistors are from that time....
This is a typical source of the time (1989) using the well-known regulator integrated circuit 723....
The integrated circuits used in this circuit are from the 70s and are therefore no longer common in...