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 source circuit was obtained in a 1976 publication for radio amateurs. The transistors can be...
This low-pass filter with 741 op and symmetrical source was found in the August 1993 issue of...
This circuit was found in a 1993 Electronics Hobbyst Handbook. The circuit operates in the FM...