This transistorized circuit appeared in a Radio Electronics magazine in March 1958. The frequency range depends on the tuned circuit, being within a few megahertz.
The operational amplifier is old. The circuit is from an 80's documentation but can be designed...
This configuration, explored in other applications of this section and website, was found in a 90s...
The sensors can be screens having porous tissue between them. When the water moistens them, the...