This circuit for frequencies of 20 to 2 kHz was obtained in English documentation from 1966. The transistor can be any general purpose and powered by batteries.
This circuit was found in the April 1968 Radio Electronics magazine. The circuit uses two FETs...
Found in an EDN from the 1980s, this circuit uses a double 555. The circuit activates a relay that...
Found in an old Popular Electronics this circuit appears in other variations on this site and in...