This circuit was obtained from an American documentation from 1976. The circuit has a dynamic range of 100 kHz. FETs may be replaced by current equivalents.
This simple metal detector was found in a 1960 Sylvania transistor manual. The phone must be high...
This is one of the many versions of this type of indicator that the reader can find on our...
Found in an English publication from the 1980s, this circuit can be built with equivalent...