Found in American documentation from the 1980s, this circuit has a diode feedback. At the exit we have an indicator LED. The LM3900 is a transconductance operational amplifier.
The oscillator shown in the figure keeps its frequency constant in an active cycle adjustment...
For bench use, this circuit functions as a bistable safety switch. The circuit is from a Radio...
In fact, this is the circuit also known as “vox”, that is, a relay triggered by sound. This...