Soil Moisture Sensor Soil Hygrometer Humidity Detection Module
Soil Moisture Sensor Module is used to accurately measure the moisture in the substrate (Moisture Sensor), which gives you the opportunity to give your plants the best care. It uses 2 soil probes to detect the moisture of the soil. When the surface of the plates touches the moist soil, a signal is generated at the digital output of the module. The operating voltage is 5V, which allows it to be used with common microcontrollers.

Digital output D0 can be connected directly to the microcontroller to detect soil moisture. If the analog output A0 and the AD module are connected via an AD converter, you can determine more accurate values of soil moisture. The sensor has a switching contact adjustable with potentiometer.

Soil Moisture Sensor Hygrometer Soil Moisture Modules Compatible with Arduino and Raspberry Pi.

Pin Out:
The soil Moisture sensor FC-28 has four pins:

VCC: For power
A0: Analog output
D0: Digital output
GND: Ground
This Module also contains a potentiometer which will set the threshold value and then this threshold value will be compared by the LM393 comparator. The output LED will light up and down according to this threshold value