This 1968 Mallory circuit uses an uncommon integrated circuit today. The circuit has a 30 to 20,000 Hz response with lower power and is powered by 12 V.
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...