This circuit was found in a British documentation from 1969. It can be implemented with modern transistors. The pass band is 6 MHz.
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...