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.
Found in a 1967 Electronics World, this circuit can use any modern equivalent FET. Observe the...
Found in American documentation from the 90's, this circuit applies to other operational...
This circuit was obtained from the February 1992 Radio Electronics magazine. In it, a bar of 4...