This 555-based circuit was found in the August 1976 issue of Radio Electronics magazine. The tone frequencies are provided by the capacitors connected to the 555 IC.
This well-known configuration that appears elsewhere on the site is from an English documentation. In it,...
We found this simple operating tester 741 in a European publication in 1991. The circuit...
Found in Radio Electronics, this circuit doubles the voltage of the 117V power grid to activate...