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.
The circuit shown is from an old European publication. In it we have the way to activate a relay...
We found this circuit in a European publication from the 1990s. Transistors are still common types...
This circuit was found in an American documentation and can be added to common amplifiers of any...