The Soil Moisture Sensor module is made up of two conductive plates that act as a probe and together act as a variable resistor. When the sensor is inserted into the water, the resistance will decrease, and you will get better conductivity between the plates.
Working principle of soil moisture sensor module
The above figure shows the functional principle of the soil moisture sensor module. The soil moisture sensor has two conductive plates. The first board is connected to the +5 volt supply via a 10K Ohm series resistor. And the second board is connected directly to the ground. It simply acts as a bias network of the voltage divider. And the output is taken now from the first terminal of the sensor pin as shown in the picture above, the output changes in the range of 0-5 volts, proportional to the change in the water content in the soil. Ideally, when there is no soil moisture, the sensor will act as an open circuit, that is, an infinite resistance. For this condition, we get 5V at the output.
The sensor contains a fork-shaped probe with two exposed conductors that go deep into the ground or wherever the water content is to be measured. As mentioned above, it acts as a variable resistor, the resistance of which changes according to the moisture of the soil.
The sensor also contains an electronic module that connects the probe to the Arduino. The module generates an output voltage according to the resistance of the probe. And is available at an analogue output pin (AO). The same signal is fed to an LM393 High Precision Comparator for digitisation and is available at a digital output pin (DO). The module has a built-in potentiometer for setting the sensitivity of the digital output (DO). You can set a threshold with a potentiometer. So if the humidity exceeds the threshold value, the module returns LOW, otherwise HIGH. This setting is very useful when you want to take action when a certain threshold is reached. For example, when the moisture content in the soil exceeds a threshold. You can activate a relay to start pumping water. You have the idea!