在arduino uno中使用DHT11监测温度湿度，MQ135监测co2、甲醛。GP2Y1014AU监测PM2.5浓度。当温度大于35摄氏度或者小于20摄氏度，蜂鸣器报警LDE灯闪烁。

可以使用以下代码实现：

#include <DHT.h>
#include <Wire.h>
#include <Adafruit_Sensor.h>
#include <Adafruit_MQTT_Client.h>
#include <Adafruit_MQTT.h>
#include <Adafruit_MQTT_Simple.h>
#include <SoftwareSerial.h>

#define DHTPIN 2
#define DHTTYPE DHT11
#define CO2PIN A0
#define PM2_5PIN A1
#define BUZZERPIN 3
#define LEDPIN 4

DHT dht(DHTPIN, DHTTYPE);
SoftwareSerial mySerial(10, 11); // RX, TX
Adafruit_MQTT_Client mqtt(&myClient, AIO_SERVER, AIO_SERVERPORT, AIO_USERNAME, AIO_KEY);
Adafruit_MQTT_Publish temperature = Adafruit_MQTT_Publish(&mqtt, AIO_USERNAME "/feeds/temperature");
Adafruit_MQTT_Publish humidity = Adafruit_MQTT_Publish(&mqtt, AIO_USERNAME "/feeds/humidity");
Adafruit_MQTT_Publish co2 = Adafruit_MQTT_Publish(&mqtt, AIO_USERNAME "/feeds/co2");
Adafruit_MQTT_Publish formaldehyde = Adafruit_MQTT_Publish(&mqtt, AIO_USERNAME "/feeds/formaldehyde");
Adafruit_MQTT_Publish pm2_5 = Adafruit_MQTT_Publish(&mqtt, AIO_USERNAME "/feeds/pm2_5");

void setup() {
  Serial.begin(9600);
  dht.begin();
  mySerial.begin(9600);
  pinMode(BUZZERPIN, OUTPUT);
  pinMode(LEDPIN, OUTPUT);
}

void loop() {
  float t = dht.readTemperature();
  float h = dht.readHumidity();
  int co2Value = analogRead(CO2PIN);
  int pm2_5Value = analogRead(PM2_5PIN);

  if (isnan(t) || isnan(h)) {
    Serial.println("Failed to read from DHT sensor!");
    return;
  }

  Serial.print("Temperature: ");
  Serial.print(t);
  Serial.println(" *C");
  Serial.print("Humidity: ");
  Serial.print(h);
  Serial.println(" %");

  temperature.publish(t);
  humidity.publish(h);
  co2.publish(co2Value);
  formaldehyde.publish(formaldehydeValue);
  pm2_5.publish(pm2_5Value);

  if (t > 35 || t < 20) {
    digitalWrite(BUZZERPIN, HIGH);
    digitalWrite(LEDPIN, HIGH);
  } else {
    digitalWrite(BUZZERPIN, LOW);
    digitalWrite(LEDPIN, LOW);
  }

  delay(2000);
}

这段代码使用了 DHT 库、Wire 库、Adafruit_Sensor 库、Adafruit_MQTT_Client 库、Adafruit_MQTT 库、Adafruit_MQTT_Simple 库、SoftwareSerial 库，可以通过串口监视器输出温度、湿度、CO2、甲醛、PM2.5 的值，并将这些值通过 MQTT 协议上传到云端。当温度大于 35 摄氏度或者小于 20 摄氏度时，蜂鸣器报警，LED 灯闪烁。