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.
Using a capacitor as a sensor, this circuit was found in the 1989 Radio Electronics Experimenter magazine....
Using bipolar, FETs and unijunction this long-lasting timer can reach 10 hours with good quality...
This circuit is found in an American documentation from the 80's. The circuit measures the power of...