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.
This amplifier for automotive use is suggested by the Philips Databook of 1990. The integrated circuit...
This circuit is used in transceivers, enabling input selection by diode switching through polarity...
Suitable for operation with high impedance microphones, this circuit was found in American...