The operation of the soil moisture sensor is very simple. The fork-shaped probe with two open conductors acts as a variable resistor (like a potentiometer) whose resistance varies according to the amount of water in the soil.

The resistance value is inversely proportional to the soil moisture:

More water in the soil means better conductivity, which leads to lower resistance.

The less water in the soil means poor conductivity and will result in higher resistance.

Depending on the resistance, the sensor produces an output voltage, which we can measure by measuring the humidity level.

A soil moisture sensor has commonly two components.

The sensor contains a fork-shaped probe that goes into the soil with two conductors exposed or elsewhere to measure the amount of water.

The module has a built-in potentiometer for sensitivity adjustment of the digital output (DO). You can set a limit with a potentiometer; Therefore, when the humidity level exceeds the limit, the module will output a small amount of output.

When the soil moisture exceeds a certain limit, you can activate a relay to start pumping water