This circuit was obtained from an Electronics Design of 1964. The circuit makes use of tunnel diodes that are uncommon today. The circuit works with very weak signals.
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...