This circuit from a Popular Electronics from the 90s uses a common optocoupler and a general-purpose transistor to control a sensitive relay that can be from 6 to 12 V.
We found this circuit in an Electronics Handbook from the 1980s. It filters out noise from a radio...
This circuit can be implemented with discrete transistors. We found it in General Electric's 1964...
We found this powerful amplifier that uses common transistors in a 1998 documentation. The power...