This circuit was taken from Maxim documentation from the 1990s. It can be implemented on the basis of modern operational amplifiers. Power supply doesn't have to be symmetrical.
Old phones with dials can be used in a simple home intercom system, as this circuit from Popular...
This circuit was found in the July 1992 issue of Radio Electronics magazine. The circuit has a...
This simple circuit indicates if the polarity of a load is reversed when it is connected to a...