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 step-down converter with 10 to 60 V input produces a 5 V output with a maximum current of 5...
Found in an old American documentation, this step has as input and output transformers coupling. Two...
This circuit was found in a 1963 magazine, but can still be easily assembled with some current...