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 is a high voltage probe that allows the measurement of high continuous voltages with a common...
The circuit shown in figure is suggested by Texas Instruments (www.ti.com) and makes use of the...
This well-known configuration of the guitar effects pedal was obtained in an English magazine from...