ESP32 MQTT Home Automation (Hands-on Project)

#include <WiFi.h>
#include <PubSubClient.h>

// --- WiFi Configuration ---
const char* ssid = "YOUR_WIFI_SSID";
const char* password = "YOUR_WIFI_PASSWORD";

// --- MQTT Broker Configuration ---
// We are using HiveMQ's free public sandbox broker
const char* mqtt_broker = "broker.hivemq.com"; 
const int mqtt_port = 1883;

// --- MQTT Topics ---
const char* publish_topic = "sparkovation/esp32/temp";
const char* subscribe_topic = "sparkovation/esp32/led";

// ESP32 Client Instances
WiFiClient espClient;
PubSubClient client(espClient);

const int ledPin = 2; // Onboard LED Pin
unsigned long lastMsg = 0;
float temperature = 24.0;

void setup_wifi() {
  delay(10);
  Serial.println();
  Serial.print("Connecting to ");
  Serial.println(ssid);

  WiFi.begin(ssid, password);

  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }

  Serial.println("");
  Serial.println("WiFi connected!");
  Serial.print("IP address: ");
  Serial.println(WiFi.localIP());
}

// Callback Function: Triggered when a new message arrives on a subscribed topic
void callback(char* topic, byte* payload, unsigned int length) {
  Serial.print("Message arrived [");
  Serial.print(topic);
  Serial.print("]: ");
  
  String message = "";
  for (int i = 0; i < length; i++) {
    message += (char)payload[i];
  }
  Serial.println(message);

  // Switch LED control logic
  if (String(topic) == subscribe_topic) {
    if (message == "ON") {
      digitalWrite(ledPin, HIGH);
      Serial.println("LED turned ON!");
    } else if (message == "OFF") {
      digitalWrite(ledPin, LOW);
      Serial.println("LED turned OFF!");
    }
  }
}

// Reconnect Function: Reconnects to the broker if connection drops
void reconnect() {
  while (!client.connected()) {
    Serial.print("Attempting MQTT connection...");
    
    // Create a unique client ID to prevent connection collisions
    String clientId = "ESP32Client-";
    clientId += String(random(0xffff), HEX);
    
    if (client.connect(clientId.c_str())) {
      Serial.println("connected!");
      // Re-subscribe to control topic
      client.subscribe(subscribe_topic);
      Serial.print("Subscribed to: ");
      Serial.println(subscribe_topic);
    } else {
      Serial.print("failed, rc=");
      Serial.print(client.state());
      Serial.println(" trying again in 5 seconds");
      delay(5000);
    }
  }
}

void setup() {
  pinMode(ledPin, OUTPUT);
  digitalWrite(ledPin, LOW);
  
  Serial.begin(115200);
  setup_wifi();
  
  client.setServer(mqtt_broker, mqtt_port);
  client.setCallback(callback);
}

void loop() {
  if (!client.connected()) {
    reconnect();
  }
  client.loop(); // Keeps the MQTT client running and handles keep-alives

  unsigned long now = millis();
  // Publish simulated temperature reading every 5 seconds
  if (now - lastMsg > 5000) {
    lastMsg = now;
    
    // Simulate slight temperature changes (+/- 0.5 degrees)
    temperature += random(-5, 6) / 10.0;
    
    // Convert float value to a string
    char tempString[8];
    dtostrf(temperature, 1, 2, tempString);
    
    Serial.print("Publishing temperature: ");
    Serial.println(tempString);
    client.publish(publish_topic, tempString);
  }
}