This sensor circuit from a 1967 Ferranti documentation uses components from the era. The circuit can be used as a shield, although the integrated one used is unusual today.
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...