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 efficient filter against EMI has several coils, being L1 and L2 formed by 100 turns of wire...
This optical sensor circuit for a robot was found in the March 1986 issue of Radio Electronics magazine....
This circuit can provide both positive and negative voltage outputs from 2 to 12 V with current up to...