This circuit was found in an American documentation from 1970. We can increase the number of multiplier steps to have higher output voltage. The operating frequency is around 1.35 kHz.
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...