This circuit with 24 dB per octave attenuation was found in the November 1973 Radio Electronics magazine. The components are for 1 kHz but can be recalculated for other frequencies.
This optically coupled circuit was found in the February 1968 Radio Electronics magazine. The...
This circuit uses a special 1950's RCA strobe tube. It can be built with equivalent components for the rest...
This circuit, already explored on this site, can be applied to other comparators in the same...