esp32通过mqtt协议从onenet平台接收到特定的字符串就改变引脚高低电平

作为AI语言模型，我无法直接操作硬件，但我可以提供一些代码示例，在esp32上实现从OneNet平台接收MQTT消息并控制引脚状态。

首先，需要安装ESP32的MQTT库，例如PubSubClient。然后，可以使用以下代码来连接OneNet平台，并订阅特定的MQTT主题：

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

const char* ssid = "your_SSID";
const char* password = "your_PASSWORD";
const char* mqtt_server = "mqtt.heclouds.com";
const char* mqtt_user = "your_MQTT_USER";
const char* mqtt_password = "your_MQTT_PASSWORD";
const char* mqtt_topic = "/mqtt/topic"; // 订阅的MQTT主题

WiFiClient espClient;
PubSubClient client(espClient);

void setup() {
  pinMode(2, OUTPUT); // 控制的引脚
  Serial.begin(9600);
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
    delay(1000);
    Serial.println("Connecting to WiFi...");
  }
  Serial.println("connected to WiFi");

  client.setServer(mqtt_server, 1883);
  client.setCallback(callback);
  while (!client.connected()) {
    Serial.println("Connecting to MQTT server...");
    if (client.connect("ESP32", mqtt_user, mqtt_password)) {
      Serial.println("connected to MQTT server");
      client.subscribe(mqtt_topic);
    } else {
      Serial.print("failed with state ");
      Serial.print(client.state());
      delay(2000);
    }
  }
}

void callback(char* topic, byte* payload, unsigned int length) {
  String message = "";
  for (int i = 0; i < length; i++) {
    message += (char)payload[i];
  }
  Serial.println("MQTT message received: " + message);
  if (message == "ON") {
    digitalWrite(2, HIGH); // 引脚高电平
  } else if (message == "OFF") {
    digitalWrite(2, LOW); // 引脚低电平
  }
}

void loop() {
  if (!client.connected()) {
    reconnect();
  }
  client.loop();
}

void reconnect() {
  while (!client.connected()) {
    Serial.println("Connecting to MQTT server...");
    if (client.connect("ESP32", mqtt_user, mqtt_password)) {
      Serial.println("connected to MQTT server");
      client.subscribe(mqtt_topic);
    } else {
      Serial.print("failed with state ");
      Serial.print(client.state());
      delay(2000);
    }
  }
}

在上面的代码中，我们定义了一个回调函数callback()，当接收到MQTT消息时，会自动调用该函数。在回调函数中，我们检查接收到的字符串是否为"ON"或"OFF"，如果是，则控制引脚的高低电平。

请注意，上面的代码示例仅供参考，并且可能需要根据您的具体需求进行更改和调整。