This circuit was found in an ancient American documentation. With it is possible to double the frequency of a transmitter For the range of 6 meters obtaining 8 W. The circuit makes use of varicap.
Old phones with dials can be used in a simple home intercom system, as this circuit from Popular...
This circuit recognizes a tone by its frequency triggering a relay. The LC circuit determines the...
This circuit was obtained in a 1980 publication, but it can be easily assembled today, as all...