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 was published in an English magazine from 1972, but it can be easily implemented...
This circuit from a Popular Electronics from the 90s allows you to use an old PC as a digital...
With the values of the components of this circuit, tone recognition can be done in the range of...