This circuit for an optical audio link using LED or LASER was found in a 90s QST magazine. The circuit can use operational amplifiers like the 741.
This small earphone amplifier with germanium transistors was obtained in a Japanese publication from...
The sensor for this circuit is a phototransistor. We found it in the April 1971 issue of Radio...
Found in a 90s Popular Electronics, this circuit allows to select the operating ranges between...