Plant Watering Alert System Easy

🔗Goal

Build a system that monitors soil moisture and alerts you when your plant needs watering. When the soil gets too dry, an LED lights up and a buzzer beeps to remind you to water.

Here is how the system works at a high level:

graph LR
    A[Soil Moisture Sensor] -->|Analog signal| B[ESP32]
    B -->|GPIO| C[LED]
    B -->|GPIO| D[Buzzer]
    B -->|Serial| E[Monitor]

The capacitive soil moisture sensor outputs an analog voltage that decreases as the soil dries out. The ESP32 reads this value, compares it to a threshold, and activates the LED and buzzer when watering is needed.

The chart below shows how soil moisture drops over time. Once the reading falls below the threshold, the alert triggers:

🔗Prerequisites

You will need the following components:

Links marked Amazon/AliExpress are affiliate links. We may earn a small commission at no extra cost to you.

The current through the LED is limited by a resistor. For a red LED with a typical forward voltage of $V_f = 2.0\,\text{V}$, the resistor value is calculated using Ohm's law:

$$R = \frac{V_{supply} - V_f}{I} = \frac{3.3 - 2.0}{0.005} = 260\,\Omega$$

A 220 ohm resistor works fine here — the LED will draw about 6 mA, which is bright enough and well within the ESP32 GPIO limit of 12 mA.

🔗Tutorial

🔗Step 1: Wiring

Connect everything to the ESP32 as shown in the table below:

GPIO 34 is an input-only pin with ADC support, which makes it a good choice for reading analog sensors.

🔗Step 2: Determine your threshold

The sensor outputs a value between 0 and 4095 (ESP32 has a 12-bit ADC). The mapping depends on your specific sensor, but as a general guide:

These values vary between sensors. Use the serial monitor (Step 4) to find the right threshold for your sensor and plant.

The decision logic follows this flow:

graph TD
    A[Read sensor] --> B{Reading > threshold?}
    B -->|Yes: soil is dry| C[Turn on LED]
    C --> D[Beep buzzer]
    D --> E[Wait 1 second]
    B -->|No: soil is moist| F[Turn off LED]
    F --> G[Silence buzzer]
    G --> E
    E --> A

🔗Step 3: Upload the code

#define SENSOR_PIN 34
#define LED_PIN    25
#define BUZZER_PIN 26

// Threshold: readings above this mean the soil is dry.
// Adjust based on your sensor calibration.
#define DRY_THRESHOLD 3000

void setup() {
    Serial.begin(115200);
    pinMode(LED_PIN, OUTPUT);
    pinMode(BUZZER_PIN, OUTPUT);
}

void loop() {
    int moisture = analogRead(SENSOR_PIN);

    Serial.print("Soil moisture: ");
    Serial.println(moisture);

    if (moisture > DRY_THRESHOLD) {
        // Soil is dry — alert!
        digitalWrite(LED_PIN, HIGH);
        digitalWrite(BUZZER_PIN, HIGH);
        delay(500);
        digitalWrite(BUZZER_PIN, LOW);
        delay(500);
    } else {
        // Soil is moist — all good
        digitalWrite(LED_PIN, LOW);
        digitalWrite(BUZZER_PIN, LOW);
        delay(1000);
    }
}

🔗Step 4: Calibrate

1. Open the Serial Monitor (115200 baud)
2. Read the value with the sensor in dry air — note this number
3. Read the value with the sensor in a glass of water — note this number
4. Place the sensor in your plant's soil and read the value when the soil feels dry to the touch
5. Set DRY_THRESHOLD to that value

The analog reading maps to a voltage via the ESP32's 12-bit ADC:

$$V_{sensor} = \frac{\text{ADC reading}}{4095} \times 3.3\,\text{V}$$

For example, a reading of 3000 corresponds to $V = \frac{3000}{4095} \times 3.3 \approx 2.42\,\text{V}$.

🔗Common Issues and Solutions