We found this circuit in an old Mullard (Philips) documentation. The sensor consists of a reed-switch that is coupled to a magnet on one of the wheels of the vehicle.
This circuit is from a 1980 publication. The circuit has high gain with weak signals and lower gain...
This circuit is for old telephones with analog lines. It was found in the May 1988 Radio...
By LED flashes you can get an idea of the degree of moisture in the soil. The sensors are wires...