How to use humidity sensor with Arduino

Arduino, Electronics, Tutorials

In this tutorial, you will learn how to use a basic analog soil moisture sensor with an Arduino Uno to measure soil humidity. Instead of using a digital humidity sensor, we will power the analog sensor selectively to reduce corrosion, read the moisture level, and visually represent it using 5 LEDs ranging from dry to wet soil conditions.

📝 Required components

Arduino Uno
Analog Soil Moisture Sensor
5 LEDs (any color)
1 × 220Ω resistors (for LEDs)
Breadboard
Jumper wires

Step 1: wiring the electronics

Connect the sensor’s VCC to Arduino pin A0.
Connect the sensor’s GND to Arduino GND.
Connect the sensor’s analog OUT to Arduino pin A1.
Connect each LED (via 220Ω resistor) to Arduino pins 8 to 12:

- Pin 8 → Very Dry
- Pin 9 → Dry
- Pin 10 → Moderate
- Pin 11 → Moist
- Pin 12 → Very Wet

All LED cathodes connect to GND.

🖥️Step 2: Writing the Code

Below you will find the complete code to use the humidity sensor:

				
					// C++ code
//
int moisture = 0;

void setup()
{
  pinMode(A0, OUTPUT);
  pinMode(A1, INPUT);
  Serial.begin(9600);
  pinMode(8, OUTPUT);
  pinMode(9, OUTPUT);
  pinMode(10, OUTPUT);
  pinMode(11, OUTPUT);
  pinMode(12, OUTPUT);
}

void loop()
{
  // Apply power to the soil moisture sensor
  digitalWrite(A0, HIGH);
  delay(10); // Wait for 10 millisecond(s)
  moisture = analogRead(A1);
  // Turn off the sensor to reduce metal corrosion
  // over time
  digitalWrite(A0, LOW);
  Serial.println(moisture);
  digitalWrite(8, LOW);
  digitalWrite(9, LOW);
  digitalWrite(10, LOW);
  digitalWrite(11, LOW);
  digitalWrite(12, LOW);
  if (moisture < 200) {
    digitalWrite(12, HIGH);
  } else {
    if (moisture < 400) {
      digitalWrite(11, HIGH);
    } else {
      if (moisture < 600) {
        digitalWrite(10, HIGH);
      } else {
        if (moisture < 800) {
          digitalWrite(9, HIGH);
        } else {
          digitalWrite(8, HIGH);
        }
      }
    }
  }
  delay(100); // Wait for 100 millisecond(s)
}

🔎 Breaking down the Code

Let’s review the code section by section:

				
					int moisture = 0;

Declares a variable to store the soil moisture value read from the sensor.

				
					void setup() {
  pinMode(A0, OUTPUT);     // Power pin for the sensor
  pinMode(A1, INPUT);      // Analog signal input from the sensor
  Serial.begin(9600);      // Start Serial Monitor
  pinMode(8, OUTPUT);      // LED for very dry
  pinMode(9, OUTPUT);      // LED for dry
  pinMode(10, OUTPUT);     // LED for medium
  pinMode(11, OUTPUT);     // LED for moist
  pinMode(12, OUTPUT);     // LED for very wet
}

Sets pin A0 as output to control power to the sensor.
Reads the sensor signal from A1.
Initializes 5 digital pins for controlling the moisture indicator LEDs.
Starts serial communication for debugging or monitoring.

				
					  digitalWrite(A0, HIGH);       // Turn on the sensor
  delay(10);                    // Wait 10 ms to stabilize
  moisture = analogRead(A1);    // Read analog value from the sensor
  digitalWrite(A0, LOW);        // Turn off sensor to prevent corrosion

The sensor is powered briefly just before taking a reading.
Analog values range from:0 (wet) to 1023 (dry).

				
					  Serial.println(moisture);

Sends the moisture value to the Serial Monitor so you can see live readings.

				
					  digitalWrite(8, LOW);
  digitalWrite(9, LOW);
  digitalWrite(10, LOW);
  digitalWrite(11, LOW);
  digitalWrite(12, LOW);

All LEDs are turned off before updating the current state.

				
					  if (moisture < 200) {
    digitalWrite(12, HIGH);     // Very wet
  } else if (moisture < 400) {
    digitalWrite(11, HIGH);     // Moist
  } else if (moisture < 600) {
    digitalWrite(10, HIGH);     // Medium
  } else if (moisture < 800) {
    digitalWrite(9, HIGH);      // Dry
  } else {
    digitalWrite(8, HIGH);      // Very dry
  }

Depending on the sensor value, only one LED turns on to indicate the moisture level.

				
					  delay(100);

Adds a short pause between readings to stabilize behavior and avoid flooding the Serial Monitor.

🛠️Step 3: Troubleshooting

Sensor always reads the same value:

Check that A0 powers the sensor briefly and that A1 is connected correctly.

LEDs never light up:

Make sure the cathodes are connected to GND and you use current-limiting resistors.

Moisture readings seem inverted:

Try watering the soil and compare changes in the serial monitor.