This 80's Exar circuit has a carrier detector for frequencies up to 500 kHz. The circuit uses a company 567 which is still a common component.
This circuit uses a contactor having been found in a 1987 publication. More modern equivalent...
This circuit was found in a 1977 Radio Shack documentation. Its purpose is to generate 1 Hz...
This circuit was found in a 1980s documentation. It decodes by identifying a certain frequency tone...