We found this circuit in a Linear Technology documentation from the 80's. It has a demodulator, being suitable for links and remote controls. Note the unusual type of detector configuration.
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...