This circuit was obtained in a 1967 Raytheon manual. The company's component is an optical coupler with a photo-resistor and a lamp. Transistors support equivalents.
The circuit shown is from an old European publication. In it we have the way to activate a relay...
We found this circuit in a European publication from the 1990s. Transistors are still common types...
This circuit was found in an American documentation and can be added to common amplifiers of any...