This circuit was found in an Electronic Design from 1964. The components are from that time and can be replaced by equivalents. The modulation ranges from 0.25 to 0.75 of the 20 kHz pulses.
This circuit is from a 1980s documentation, but the configuration can be applied to most modern...
This 0.5 Hz filter was found in the December 1990 issue of Radio Electronics magazine. The power...
This circuit was found in a 1977 Radio Shack documentation. The circuit is TTL, which must be...