This circuit was found in Raytheon's 1967 documentation. The circuit uses an operating of the era, but the configuration can be applied to modern operational amplifiers.
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...