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 high Q “rejector” or notch filter rejects signals whose frequencies are in the range...
This circuit was found in documentation from the 90's, having an LC compensation network. The...
This two-transistor circuit was found in the January 1994 issue of Electronics Now magazine....