This Capacitive Soil Moisture Sensor measures soil moisture levels by capacitive sensing, rather than resistive sensing like other types of moisture sensor. Capacitive Soil moisture sensor is made of a corrosion-resistant material giving it long service life. Insert capacitive soil moisture sensor into the soil and impress your friends with the real-time soil moisture data!. The product includes an on-board voltage regulator which gives it an operating voltage range of 3.3 ~ 5.5V. It is compatible with low-voltage MCUs (both 3.3V and 5V logic). To make it compatible with a Raspberry Pi, an ADC converter is required.

Features Of Capacitive Soil Moisture Sensor:

1. Operating Voltage: 3.3 ~ 5.5 VDC
2. Output Voltage: 0 ~ 3.0VDC
3. Interface: PH2.0-3P
4. Dimension: 98mm * 23mm (3.86in x 0.905in)
5. Supports Gravity 3-Pin interface
6. Analog output

Applications:

1. Garden plants
2. Moisture detection
3. Intelligent agriculture

Product Wiki

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This example reads Capacitive Soil Moisture Sensor.
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const int AirValue = 520;   //you need to replace this value with Value_1
const int WaterValue = 260;  //you need to replace this value with Value_2
int intervals = (AirValue - WaterValue)/3;   
int soilMoistureValue = 0;
void setup() {
Serial.begin(9600); // open serial port, set the baud rate to 9600 bps
}
void loop() {
soilMoistureValue = analogRead(A0);  //put Sensor insert into soil
if(soilMoistureValue > WaterValue && soilMoistureValue < (WaterValue + intervals))
{
Serial.println("Very Wet");
}
else if(soilMoistureValue > (WaterValue + intervals) && soilMoistureValue < (AirValue - intervals))
{
Serial.println("Wet");
}
else if(soilMoistureValue (AirValue - intervals))
{
Serial.println("Dry");
}
delay(100);
}