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 using an operational amplifier was found in old American documentation. It can be...
If the circuit oscillates, emitting sound, it is because the transistor is good. This simple...
Like the previous circuit, this is used in the analog part of the telephone lines. The same...