This code key uses a PLL and an oscillator that generates two frequencies that must be recognized for the relay to be activated. The circuit is from an 80's Popular Electronics.
This circuit called “surf man” was published in the August 1992 Radio Electronics magazine. It...
This Hartley oscillator uses an audio output transformer that can be obtained from small, unused...
The oscillator shown in the figure can generate signals between 3 and 15 MHz approximately, depending...