This very low current circuit was found in the April 1995 Electronics Now magazine. The frequency should be chosen for maximum efficiency according to the small transformer used.
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...