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 from a Popular Electronics shows how to take the audio signal from a telephone line...
The coil of this transmitter has 4 coils of wire 26 or 28 in the form of 1 cm without core and on it...
In the figure we have another configuration of filter that uses the four Norton amplifiers of the...