Using LM3909 integrated circuits, this circuit was found in an American documentation from the 90's. The frequencies can be changed by changing the capacitors.
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...