This circuit uses a resonant reed relay to recognize the frequency of an input signal. Adaptations can be made to obtain functioning as a shield. The circuit is from 1965 documentation.
This 1-transistor Hartley oscillator generates an audio signal that is dependent on capacitor C1....
We found this circuit in an old French documentation from the 70s, but it can be assembled using...
This crystal-controlled oscillator was obtained from an American 1980s documentation. The circuit...