We found this circuit in the July 1969 Radio Electronics magazine. The circuit uses a coupler made with a lamp and an LDR. The power of the lamp determines the types of diodes.
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...