This circuit was found in a Popular Electronics documentation from the 90s. It is possible to switch the frequency of an oscillator by voltage. The circuit can act as a shield.
The sensor used in this circuit is uncommon today, however equivalent types can be tried. The...
This transistor tester configuration can be found on this website with many variations. The...
This transmitter for the 80-meter range of radio amateurs was found in a 1995 documentation. The...