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 basic tone control circuit employs a Philips NE5538 operational amplifier, but can be adapted to work...
The light effect obtained with the circuit shown in figure 6 is different from the others: it...
Found in an old Radio Electronics, this circuit is intended for the modulation of composite video...