Practical case: ESP32 Soil Moisture Monitor

#include <Arduino.h>

// --------------------------------------------------------
// Hardware Pin Definitions
// --------------------------------------------------------
const int MOISTURE_PIN = 34; // ADC1 Channel 6
const int LED_PIN      = 25; // Digital Output for Status LED
const int BUZZER_PIN   = 26; // Digital Output for Piezo Buzzer

// --------------------------------------------------------
// PWM Configuration for Piezo Buzzer
// --------------------------------------------------------
const int PWM_FREQ       = 2000; // 2 kHz audio frequency
const int PWM_CHANNEL    = 0;    // ESP32 Hardware Timer Channel 0
const int PWM_RESOLUTION = 8;    // 8-bit resolution (0-255)

// --------------------------------------------------------
// Calibration Constants
// --------------------------------------------------------
// IMPORTANT: Capacitive sensors output a HIGHER voltage when dry 
// and a LOWER voltage when wet. These values represent the 12-bit 
// ADC readings (0-4095). You must calibrate these for your specific sensor.
const int DRY_VALUE = 3200;  // Expected ADC reading in completely dry air
const int WET_VALUE = 1400;  // Expected ADC reading submerged in water

// --------------------------------------------------------
// Application Logic Constants
// --------------------------------------------------------
const int ALARM_THRESHOLD_PERCENT = 30; // Trigger alert at or below 30% moisture

void setup() {
    // Initialize Serial Communication for debugging
    Serial.begin(115200);

    // Configure Digital Output Pins
    pinMode(LED_PIN, OUTPUT);
    digitalWrite(LED_PIN, LOW); // Ensure LED is off at boot

    // Configure LEDC PWM peripheral for the Buzzer
    // This allows us to generate a clean square wave for a passive piezo
    ledcSetup(PWM_CHANNEL, PWM_FREQ, PWM_RESOLUTION);
    ledcAttachPin(BUZZER_PIN, PWM_CHANNEL);
    ledcWrite(PWM_CHANNEL, 0); // Ensure buzzer is silent at boot

    Serial.println("=========================================");
    Serial.println(" ESP32 Soil Moisture Monitor Initialized ");
    Serial.println("=========================================");

    // Allow hardware to stabilize
    delay(1000); 
}

void loop() {
    // 1. Read the raw analog voltage from the sensor
    int rawAdcValue = analogRead(MOISTURE_PIN);

    // 2. Map the raw ADC value to a 0-100 percentage scale.
    // We invert the mapping parameters because DRY is high, WET is low.
    int moisturePercent = map(rawAdcValue, DRY_VALUE, WET_VALUE, 0, 100);

    // 3. Constrain the percentage to strictly 0-100.
    // This prevents negative numbers or values > 100% if the sensor 
    // reads slightly outside the hardcoded calibration range.
    moisturePercent = constrain(moisturePercent, 0, 100);

    // 4. Output telemetry to the Serial Monitor
    Serial.print("Raw ADC: ");
    Serial.print(rawAdcValue);
    Serial.print("\t | Moisture: ");
    Serial.print(moisturePercent);
    Serial.println("%");

    // 5. Evaluate the alert threshold
    if (moisturePercent <= ALARM_THRESHOLD_PERCENT) {
        // Condition: Soil is dangerously dry.
        // Action: Flash the LED and pulse the buzzer.

        digitalWrite(LED_PIN, HIGH);
        // Set PWM duty cycle to 50% (128 out of 255) to generate sound
        ledcWrite(PWM_CHANNEL, 128); 
        delay(500); // Wait 500ms

        digitalWrite(LED_PIN, LOW);
        // Set PWM duty cycle to 0% to silence buzzer
        ledcWrite(PWM_CHANNEL, 0);   
        delay(500); // Wait 500ms

    } else {
        // Condition: Soil moisture is adequate.
        // Action: Keep alerts turned off and delay before next reading.

        digitalWrite(LED_PIN, LOW);
        ledcWrite(PWM_CHANNEL, 0);
        delay(1000); // Poll sensor once per second
    }
}