This variable filter that lets frequencies from 500 Hz to 1500 Hz pass through was found in a Popular Electronics from the 90's. The operational amplifiers are the 741.
We find this circuit in a 2005 Linear Technology manual. The circuit applies to small DC motors...
This circuit, which can also be adapted to function as a microcontroller shield, is from 2003...
Using a CA3130, operational amplifier with FET, we found this circuit in a 1977 publication. The...