Soil drying alarm circuit

In order to monitor soil moisture and save water,
electronic devices that reflect changes in soil resistance are sometimes used (the drier the soil, the greater the resistance). When a certain resistance range is reached, the alarm sends a signal that water must be watered. The circuit
is shown in Figure 13-133 (a). Structural diagram
    of the alarm sensor , which consists of the following parts:
a pillar 1 made of organic glass strips with a tip; a contact piece 2f made of a thin brass sheet or copper sheet with a thickness of 0.1 to 0.2mm The electrode
3 is made of a section of graphite pencil rod with a length of 2.2mm and 20mm .     Multi-core insulated wires 4 are welded on the contact piece 2, and the contact piece is attached to both sides of the pillar and fixed with a section of polyvinyl chloride pipe 5 , and then a section of polyvinyl chloride pipe 6 is used to set it up as shown in Figure 13-133.     When using, insert the tip pillar of the sensor into the soil so that the depth of the electrode inserted into the soil is 5mm, and then use your fingers to carefully compact the soil around the electrode .     Adjust the potentiometer RP to change the humidity set value.