This filter, which rejects frequencies around 1 kHz, was found in the August 1993 Electronics Now magazine. The fine tuning is done at R3, and the source must be symmetrical.
This version of the circuit is from 2000, using a source of negative voltage that allows to obtain...
This somewhat complex circuit was found in the May 1984 issue of Radio Electronics magazine. The...
This basic circuit is from an old documentation. The relay must be 50 mA maximum and must have a...