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 is the receiver for the previous circuit, also found in 90s Radio Electronics. The circuit...
This circuit from a 1978 American publication is used on vintage vehicles that still have contact...
This circuit was suggested by Popular Elecronics from the 90's. The circuit has high input...