This circuit was found in Radio Electronics from the 90s. The sensor is an oscillator whose frequency depends on humidity, an uncommon component nowadays.
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...