This speaker overload protection circuit was found in Radio Electronics from the 1980s. The circuit uses a 9 V relay but can be powered by 12 V for a relay of this voltage.
We found this circuit in an Electronic Design from the 80's. The generated tone can be changed as well...
This basic tone control circuit employs a Philips NE5538 operational amplifier, but can be adapted to work...
This two-phase stepper motor circuit was found in an old documentation using an operational amplifier...