Active audio filter (CB21770E)

This circuit was found in a 1977 documentation for radio amateurs. Integrated circuits are common operational amplifiers.

Inverter Amplifier with Arbitrary Reference (CB19844E)
This circuit was obtained in a Motorola manual, but it applies to almost any operational...
Adjustable Zener (CB21532E)
Found in a 1970 EEE this circuit emulates a zener diode whose voltage can be set to R2. Power...
Time Switch (CB12224E)
This circuit different from the timer was found in an English publication from 1978. The circuit uses...