This circuit was found in a 1964 Electronic Design. The components are of the time, and modern equivalents can be used. Note the multi-emitter transistor.
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...