Now let's publish a message using mosquitto_pub and receive it in the ESP8266.

For this the ESP8266 needs to subscribe to the same topic on which mosquitto_pub will publish the message. Let's call the topic outdoor/light and it will publish on 0 or 1 values. If the ESP8266 receives the value as 1, it will turn on a LED connected to GPIO 12 and if it will receives a 0, it will turn off that LED:

#include <ESP8266WiFi.h> 
#include <PubSubClient.h> 

Update these with values suitable for your network:

 
const char* wifi_network= "YOUR_WIFI_SSID"; 
const char* password = "YOUR_WIFI_PASSWORD"; 
const char* mqtt_serv_address = "YOUR_MQTT_SERVER_IP"; 
const int mqtt_port_number = 1883; 
#define OUTDOOR_LIGHT  12 
 
WiFiClient  espClient; 
PubsubClient client(espClient); 
long lastMsg; = 0; 

Start the connection to the Wi-Fi network and set the name of the function that will be called when a message is received from the MQTT broker, as follows:

void setup...