The circuit shown is from the November 1996 Electronics Now magazine. The basic circuit is in the previous 3 sections of this section. The circuit measures resistances up to 2M ohms.
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...