This circuit was found as part of another project in a January 1988 Radio Electronics magazine. The frequencies are given by the timing capacitors.
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...