This 1/2 hertz high selectivity filter was found in the December 1990 issue of Radio Electronics magazine. The source must be symmetrical. The capacitors can be changed for other frequencies.
We found this circuit in a 1968 Electronics World. The potentiometer should be set to trigger the SCR...
We found this circuit in a documentation from the 90s. It lets frequencies around 1 kHz pass,...
This wired intercom circuit was found in a March 1965 Radio Electronics. The circuit uses two...