We found this circuit in an Electronic Design. The main integrated circuit is the 4017. The frequencies are determined by the RC circuits as well as the duration of the generated pulse train.
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...