Example MQTT implementation scketch for an ESP8266 board and a DS18B20 temperature probe sensor

mqtt_example.ino

#include <ESP8266WiFi.h>              // Library for WiFi
#include <OneWire.h>                  // Library for DS18B20 sensor
#include <DallasTemperature.h>        // Library for DS18B20 sensor
#include <Adafruit_MQTT.h>            // MQTT client
#include <Adafruit_MQTT_Client.h>     // MQTT client
#include "credentials.h"
    // Content - credentials.h  ##########################################################
    // ###################################################################################
       // WiFi Credentials 
       // #define WLAN_SSID       "YOUR SSID"
       // #define WLAN_PASSWORD   "YOUR PASSWORD"
       // MQTT Credentials
       // #define MQTT_SERVER     "YOUR MQTT BROKER IP"
       // #define MQTT_USER       "MQTT USER"
       // #define MQTT_PASSWORD   "MQTT PASSWORD"
    // ###################################################################################
    // ###################################################################################

// Network Configuration
IPAddress ip(192, 168, 0, 102);   // Your device IP
IPAddress gateway(192, 168, 0, 1);
IPAddress subnet(255, 255, 255, 0);

// MQTT client configuration
WiFiClient esp_client;
Adafruit_MQTT_Client mqtt(&esp_client, MQTT_SERVER, 1883, MQTT_USER, MQTT_PASSWORD);
Adafruit_MQTT_Publish current_temp = Adafruit_MQTT_Publish(&mqtt, "current_temp", MQTT_QOS_1); // Define MQTT topic

// DS18B20 Sensor Libraries Configuration
OneWire ourWire(4);                  // Set D2(GPIO-4) pin as OneWire bus
DallasTemperature sensors(&ourWire); // Declare sensor object

// Global variables definition
float temp;                          //Define variable to store temp readings
unsigned long previousMillis = 0;    //Temporal variable
const long interval = 2000;          //Set minimal interval between readings

void MQTT_connect();

void setup() {
  Serial.begin(115200);
  delay(10);

  // Connect to WiFi network
  Serial.println(); Serial.println();
  Serial.print("Connecting to ");
  Serial.println(WLAN_SSID);

  WiFi.config(ip, gateway, subnet, dns1, dns2);
  WiFi.mode(WIFI_STA);
  WiFi.begin(WLAN_SSID, WLAN_PASSWORD);
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }
  
  // Initialize sensor
  sensors.begin();                // Initialize DS18B20 sensor
}

void loop() {
  
  // Send reading via MQTT message
  MQTT_connect();

  read_sensor();
  
  // ping the server to keep the mqtt connection alive
  if (! mqtt.ping()) {
    mqtt.disconnect();
  }
}

// FUNCTIONS #########################################################################
// ###################################################################################

// Get sensor readings
void read_sensor() {
  unsigned long currentMillis = millis();
  if (currentMillis - previousMillis >= interval) {
    previousMillis = currentMillis;

    sensors.requestTemperatures();         //Request temp readings
    temp = sensors.getTempCByIndex(0);    //Request temp in C by index

    Serial.print(F("\nSending reading: "));
    Serial.print(temp);
    Serial.print("...");
    if (! current_temp.publish(temp)) {
      Serial.println(F("Failed"));
    } else {
      Serial.println(F("OK!"));
    }
  }
}

// MQTT functions
void MQTT_connect() {
  int8_t ret;

  // Stop if already connected.
  if (mqtt.connected()) {
    return;
  }

  Serial.print("Connecting to MQTT... ");

  uint8_t retries = 3;
  while ((ret = mqtt.connect()) != 0) { // connect will return 0 for connected
    Serial.println(mqtt.connectErrorString(ret));
    Serial.println("Retrying MQTT connection in 5 seconds...");
    mqtt.disconnect();
    delay(5000);  // wait 5 seconds
    retries--;
    if (retries == 0) {
      // basically die and wait for WDT to reset me
      while (1);
    }
  }
  Serial.println("MQTT Connected!");
}