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.
We found this circuit very simple in an old documentation. The relay is high voltage sensitive, but...
This circuit was found in an Elektor from the 90s. The circuit uses common components even today,...
The circuit, shown in figure 2, is a monostable multivibrator with two NAND ports out of the 4...