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 transmitter emits signals in the FM range, reaching a few meters away. The modulation...
This audio amplifier has an internal DC volume control with logarithmic characteristics and a...
This configuration found in GE documentation from the 90s, can also be seen elsewhere in this...