MQTT 是轻量级的、灵活的物联网消息交换和数据传递协议，致力于为 IoT 开发人员实现灵活性与硬件/网络资源的平衡。
NodeMCU 是一个开源的物联网平台。它使用 Lua 语言编程。该平台基于eLua开源项目，底层使用ESP8266 sdk 0.9.5版本。
在此项目中我们将实现 NodeMCU(ESP8266) 与 EMQX Cloud 运营和维护的免费公共 MQTT 服务器远程控制 LED 灯，并使用 Arduino IDE 来对 NodeMCU ESP8266 进行编程。 EMQX Cloud 是由 EMQ 推出的安全的 MQTT 物联网云服务平台，它提供一站式运维代管、独有隔离环境的 MQTT 5.0 接入服务。
所需组件NodeMCUArduino IDELED * 1，330 Ω 电阻MQTTX: 优雅的跨平台 MQTT 5.0 客户端工具免费的公共 MQTT 服务器Broker: broker.emqx.ioTCP Port: 1883Websocket Port: 8083NodeMCU ESP8266 和 LED 连接图代码编写
首先我们将导入 ESP8266WiFi 和 PubSubClient 库，ESP8266WiFi 库能够将 ESP8266 连接到 WiFi 网络，PubSubClient 库，使我们能够连接到 MQTT 代理并发布/订阅主题消息。
#include #include我们将使用 NodeMCU ESP8266 的 D1 引脚来连接到 LED，实际上该引脚内部连接到 ESP8266 模块的 GPIO5。
// GPIO 5 D1#define LED 5设置 WIFI 名称和密码，以及 MQTT Broker 连接地址和端口
// WiFiconst char *ssid = "mousse"; // Enter your WiFi nameconst char *password = "qweqweqwe"; // Enter WiFi password// MQTT Brokerconst char *mqtt_broker = "broker.emqx.io";const char *topic = "esp8266/led";const char *mqtt_username = "emqx";const char *mqtt_password = "public";const int mqtt_port = 1883;我们打开了一个串行连接，以便于输出程序的结果并且连接到WiFi网络
// Set software serial baud to 115200;Serial.begin(115200);// connecting to a WiFi networkWiFi.begin(ssid, password);while (WiFi.status() != WL_CONNECTED) { delay(500); Serial.println("Connecting to WiFi..");}我们将设置 MQTT Broker，同时将连接信息打印到串口监视器上
//connecting to a mqtt brokerclient.setServer(mqtt_broker, mqtt_port);client.setCallback(callback);while (!client.connected()) { String client_id = "esp8266-client-"; client_id += String(WiFi.macAddress()); Serial.printf("The client %s connects to the public mqtt broker ", client_id.c_str()); if (client.connect(client_id.c_str(), mqtt_username, mqtt_password)) { Serial.println("Public emqx mqtt broker connected"); } else { Serial.print("failed with state "); Serial.print(client.state()); delay(2000); }}MQTT Broker 连接成功后，ESP8266 将向 MQTT Broker 发布和订阅消息
// publish and subscribeclient.publish(topic, "hello emqx");client.subscribe(topic);编写回调函数，从串行监视器读取下发指令并且控制 LED 的开和关
void callback(char *topic, byte *payload, unsigned int length) { Serial.print("Message arrived in topic: "); Serial.println(topic); Serial.print("Message: "); String message; for (int i = 0; i < length; i++) { message += (char) payload[i]; // Convert *byte to string } Serial.print(message); if (message == "on" && !ledState) { digitalWrite(LED, HIGH); // Turn on the LED ledState = true; } if (message == "off" && ledState) { digitalWrite(LED, LOW); // Turn off the LED ledState = false; } Serial.println(); Serial.println("-----------------------");}完整代码
#include #include // GPIO 5 D1#define LED 5// WiFiconst char *ssid = "mousse"; // Enter your WiFi nameconst char *password = "qweqweqwe"; // Enter WiFi password// MQTT Brokerconst char *mqtt_broker = "broker.emqx.io";const char *topic = "esp8266/led";const char *mqtt_username = "emqx";const char *mqtt_password = "public";const int mqtt_port = 1883;bool ledState = false;WiFiClient espClient;PubSubClient client(espClient);void setup() { // Set software serial baud to 115200; Serial.begin(115200); delay(1000); // Delay for stability // Connecting to a WiFi network WiFi.begin(ssid, password); while (WiFi.status() != WL_CONNECTED) { delay(500); Serial.println("Connecting to WiFi..."); } Serial.println("Connected to the WiFi network"); // Setting LED pin as output pinMode(LED, OUTPUT); digitalWrite(LED, LOW); // Turn off the LED initially // Connecting to an MQTT broker client.setServer(mqtt_broker, mqtt_port); client.setCallback(callback); while (!client.connected()) { String client_id = "esp8266-client-"; client_id += String(WiFi.macAddress()); Serial.printf("The client %s connects to the public MQTT broker ", client_id.c_str()); if (client.connect(client_id.c_str(), mqtt_username, mqtt_password)) { Serial.println("Public EMQX MQTT broker connected"); } else { Serial.print("Failed with state "); Serial.print(client.state()); delay(2000); } } // Publish and subscribe client.publish(topic, "hello emqx"); client.subscribe(topic);}void callback(char *topic, byte *payload, unsigned int length) { Serial.print("Message arrived in topic: "); Serial.println(topic); Serial.print("Message: "); String message; for (int i = 0; i < length; i++) { message += (char) payload[i]; // Convert *byte to string } Serial.print(message); if (message == "on" && !ledState) { digitalWrite(LED, HIGH); // Turn on the LED ledState = true; } if (message == "off" && ledState) { digitalWrite(LED, LOW); // Turn off the LED ledState = false; } Serial.println(); Serial.println("-----------------------");}void loop() { client.loop(); delay(100); // Delay for a short period in each loop iteration}连接和测试请使用 Arduino IDE 将完整代码上传 ESP8266，并打开串口监视器
建立 MQTTX 客户端 与 MQTT Broker 连接, 并向 ESP8266 发送指令
总结
至此，我们成功实现 NodeMCU ESP8266 与免费公共 MQTT 服务器远程控制 LED 灯，该例子只是描述了一个简单的场景，在实际的项目中，需要更加安全的连接方式，以及对物联网数据进行持久化等功能。