Practical case: IoT Mailbox Monitor with ESP32

#include <Arduino.h>
#include <WiFi.h>
#include <HTTPClient.h>

// ==========================================
// Configuration
// ==========================================
const char* WIFI_SSID     = "IoT_Network";
const char* WIFI_PASSWORD = "SecurePassword123";
const char* WEBHOOK_URL   = "http://192.168.1.100/mailbox-alert";

// Pin Definitions
const gpio_num_t REED_SWITCH_PIN = GPIO_NUM_33; // Must be an RTC GPIO for ext0 wakeup

// Timeouts
const int WIFI_TIMEOUT_MS = 10000; // 10 seconds max to connect

// ==========================================
// Helper Functions
// ==========================================

void printWakeupReason() {
    esp_sleep_wakeup_cause_t wakeup_reason = esp_sleep_get_wakeup_cause();
    switch(wakeup_reason) {
        case ESP_SLEEP_WAKEUP_EXT0:     
            Serial.println("Wakeup caused by external signal using RTC_IO (Door Opened!)"); 
            break;
        case ESP_SLEEP_WAKEUP_TIMER:    
            Serial.println("Wakeup caused by timer"); 
            break;
        default:                        
            Serial.printf("Wakeup was not caused by deep sleep: %d\n", wakeup_reason); 
            break;
    }
}

void sendAlert() {
    Serial.print("Connecting to Wi-Fi: ");
    Serial.println(WIFI_SSID);

    WiFi.begin(WIFI_SSID, WIFI_PASSWORD);

    unsigned long startAttemptTime = millis();

    // Wait for connection with a timeout to prevent draining battery
    while (WiFi.status() != WL_CONNECTED && millis() - startAttemptTime < WIFI_TIMEOUT_MS) {
        Serial.print(".");
        delay(500);
    }

    Serial.println();

    if (WiFi.status() != WL_CONNECTED) {
        Serial.println("Failed to connect to Wi-Fi. Going back to sleep.");
        return; // Abort and let the main loop put the device to sleep
    }

    Serial.println("Wi-Fi connected!");
    Serial.print("IP address: ");
    Serial.println(WiFi.localIP());

    // Send HTTP GET request
    if (WiFi.status() == WL_CONNECTED) {
        HTTPClient http;
        Serial.print("Sending alert to: ");
        Serial.println(WEBHOOK_URL);

        http.begin(WEBHOOK_URL);
        int httpResponseCode = http.GET();

        if (httpResponseCode > 0) {
            Serial.print("HTTP Response code: ");
            Serial.println(httpResponseCode);
        } else {
            Serial.print("Error code: ");
            Serial.println(httpResponseCode);
        }
        http.end();
    }

    // Disconnect Wi-Fi to save power before sleeping
    WiFi.disconnect(true);
    WiFi.mode(WIFI_OFF);
}

// ==========================================
// Main Setup and Loop
// ==========================================

void setup() {
    Serial.begin(115200);
    delay(1000); // Allow serial monitor to catch up

    Serial.println("\n--- Mailbox Monitor Booting ---");

    // Determine why the ESP32 woke up
    esp_sleep_wakeup_cause_t wakeup_reason = esp_sleep_get_wakeup_cause();
    printWakeupReason();

    // If woke up because the door opened (EXT0)
    if (wakeup_reason == ESP_SLEEP_WAKEUP_EXT0) {
        sendAlert();
    } else {
        // Normal boot (e.g., first power on or reset)
        Serial.println("Initial boot. System is armed and ready.");
    }

    // Configure Deep Sleep Wakeup
    // Door open = magnet away = switch open = pull-down resistor pulls to LOW (0).
    // Wake up when GPIO 33 goes LOW (0).
    esp_sleep_enable_ext0_wakeup(REED_SWITCH_PIN, 0); 

    Serial.println("Entering deep sleep now. Waiting for door to open...");
    Serial.flush(); 

    // Enter Deep Sleep
    esp_deep_sleep_start();
}

void loop() {
    // The loop is intentionally empty.
    // The ESP32 goes to sleep in setup() and resets entirely upon waking.
}