This circuit rejects frequencies around 1 kHz and can be adjusted using C1. Equivalent operational amplifiers can be used, and the power supply must be symmetrical.
This 2003 circuit serves to digitally show the intensity of a sound captured by a microphone....
The circuit shown in the figure makes it possible to control a stepper motor via a PC's serial port...
This circuit was found in an old American documentation. Its purpose is a proportional scanning signal...