This circuit was found in a 1967 US documentation. It shows how to build a gain control by digital signals. The circuit can be used as a shield. FET can be a modern equivalent.
The circuit with a common operational amplifier uses a zener as a reference. The circuit is from...
This circuit was found in a 90’s Popular Electronics. The circuit is used to analyze noise in...
This 1989 circuit activates a protection relay if one or two phases are missing from a three-phase...