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.
Two inverters, of the 6 available in a Low-power Schottky 74LS04, are used in this anti-ring...
Found in Radio Electronics from the 90's, this circuit can be implemented with other operational...
Taken from a 1956 Raytheon transistor manual, this circuit uses varistors to obtain modulation of a signal...