This circuit protects the speaker from dangerous direct currents that could damage it. The circuit is from an Electronics Experimenter Handbook 1993 edition.
This circuit that draws 1 out of 8 LEDs was found in an 80's American documentation. The circuit...
This protective circuit for power lines was found in a Radio Electronics from November 1957. The...
In figures 1 and 2 we have two current drains that can be made with operational amplifiers and...