MQTT and IoT Protocols

MQTT 브로커에 센서 데이터를 발행하고 명령 토픽을 구독하여 원격 제어.

What is MQTT?

MQTT (Message Queuing Telemetry Transport) is a lightweight publish–subscribe protocol designed for constrained IoT devices. A broker (server) routes messages between clients. Clients publish messages to topics (like channels) and subscribe to receive them.

ESP32 (publisher) ──→ Broker ──→ Dashboard (subscriber)
                              ←── Mobile app (publisher)

Topics are hierarchical strings: pailab/sensor/temperature, pailab/control/led.

Setting Up a Broker

Local (Mosquitto on PC):

# Install: https://mosquitto.org
mosquitto -v           # start broker on port 1883
mosquitto_sub -t "#"   # subscribe to ALL topics (debug)
mosquitto_pub -t "test" -m "hello"

Cloud (HiveMQ public):
Host: broker.hivemq.com, Port: 1883, no credentials needed (public, unencrypted).

Installing PubSubClient

In the Arduino IDE Library Manager, install PubSubClient by Nick O’Leary.

Publishing Sensor Data

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

const char* SSID     = "YourSSID";
const char* PASSWORD = "YourPass";
const char* BROKER   = "broker.hivemq.com"; // or your local IP
const int   PORT     = 1883;
const char* CLIENT_ID = "esp32-pailab-001"; // must be unique per device

WiFiClient   wifi;
PubSubClient mqtt(wifi);

void reconnect() {
  while (!mqtt.connected()) {
    Serial.print("Connecting to MQTT...");
    if (mqtt.connect(CLIENT_ID)) {
      Serial.println("connected");
      mqtt.subscribe("pailab/control/led"); // subscribe after connect
    } else {
      Serial.print("failed, rc="); Serial.print(mqtt.state());
      delay(2000);
    }
  }
}

void onMessage(char* topic, byte* payload, unsigned int length) {
  String msg;
  for (unsigned int i = 0; i < length; i++) msg += (char)payload[i];
  Serial.printf("Topic: %s  Message: %s\n", topic, msg.c_str());

  if (String(topic) == "pailab/control/led") {
    digitalWrite(13, msg == "1" ? HIGH : LOW);
  }
}

void setup() {
  Serial.begin(115200);
  pinMode(13, OUTPUT);
  WiFi.begin(SSID, PASSWORD);
  while (WiFi.status() != WL_CONNECTED) delay(500);

  mqtt.setServer(BROKER, PORT);
  mqtt.setCallback(onMessage);
}

unsigned long lastPublish = 0;

void loop() {
  if (!mqtt.connected()) reconnect();
  mqtt.loop(); // process incoming messages

  if (millis() - lastPublish > 5000) {
    lastPublish = millis();
    float temp = 25.4; // replace with DHT reading
    char payload[16];
    snprintf(payload, sizeof(payload), "%.1f", temp);
    mqtt.publish("pailab/sensor/temperature", payload);
    Serial.printf("Published: %s\n", payload);
  }
}

Monitoring in Node-RED (optional)

1. Install Node-RED: npm install -g node-red
2. Add MQTT-in node → Chart node → Wire → Deploy
3. Set broker to match your Mosquitto host, topic to pailab/sensor/#

QoS Levels

mqtt.publish("pailab/sensor/temp", payload, false, 1); // QoS 1

Exercises

1. Publish temperature AND humidity as separate topics every 10 seconds
2. Subscribe to pailab/control/brightness (value 0–255) and set LED PWM accordingly
3. Implement a “last will” message: if the device disconnects, publish “offline” to pailab/status