Suggested by a Popular Electronics from the 90s, this circuit triggers an external load, relay or recorder, when the telephone line receives a call, thus enabling its recording.
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...