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 circuit is from a 1980s documentation, but the configuration can be applied to most modern...
This 0.5 Hz filter was found in the December 1990 issue of Radio Electronics magazine. The power...
This circuit was found in a 1977 Radio Shack documentation. The circuit is TTL, which must be...