Arduino to Arduino MQTT Communication

In this tutorial, we are going to learn:

communication between two arduino via MQTT

Hardware Required

2×Arduino UNO R4 WiFi
2×USB Cable Type-C
1×(Recommended) Screw Terminal Block Shield for Arduino Uno
1×(Optional) Transparent Acrylic Enclosure For Arduino Uno

Or you can buy the following sensor kit:

1×DIYables Sensor Kit 30 types, 69 units
Please note: These are Amazon affiliate links. If you buy the components through these links, We will get a commission at no extra cost to you. We appreciate it.

About Arduino and MQTT

We have a detailed tutorial on how to use Arduino with MQTT here:

Communication between two Arduino via MQTT

Two Arduino boards can communicate with each other through an MQTT server. If you wish for two Arduinos to communicate directly without using an MQTT server, please refer to the tutorial on Arduino to Arduino TCP Client/Server communication.

When Arduino #1 and Arduino #2 exchange data via an MQTT broker:

  • Both Arduinos connect to the MQTT broker.
  • To have Arduino #2 send data to Arduino #1:
    • Arduino #1 subscribes to a topic, for instance: arduino-1/data.
    • Arduino #2 can send data to Arduino #1 by publishing the data to the topic that Arduino #1 is subscribed to.
  • Similarly, to have Arduino #1 send data to Arduino #2:
    • Arduino #2 subscribes to a topic, for instance: arduino-2/data.
    • Arduino #1 can send data to Arduino #2 by publishing the data to the topic that Arduino #2 is subscribed to.

    By following this method, two Arduinos can exchange data bidirectionally.

Example Use Case

Let's realize the following application: A button/switch connected to Arduino #1 controls an LED connected to Arduino #2 via MQTT.

communication between two arduino

As mentioned above, there are some application protocols we can use. In this example, to make it simple, we will define a protocol by ourself (a self-defined protocol)

Self-defined Protocol

Let's define a simple protocol:

  • Arduino #1 and Arduino #2 connects to MQTT Broker (MQTT server)
  • Arduino #1:
    • Arduino #1 publish a MQTT message to a topic when the state of switch is changed.
    • When the button/switch is switched to on, the MQTT message payload is 1.
    • When the button/switch is switched to off, the MQTT message payload is 0.
  • Arduino #2:
    • Arduino #2 subscribes to the topic
    • If Arduino #2 receives an MQTT message with payload is 1, it turns on LED
    • If Arduino #2 receives an MQTT message with payload is 0, it turns off LED

    Wiring Diagram

    • Arduino #1 - Wiring diagram between Arduino and button
    Arduino Button Wiring Diagram

    This image is created using Fritzing. Click to enlarge image

    • Arduino #2 - Wiring diagram between Arduino and LED
    Arduino LED Wiring Diagram

    This image is created using Fritzing. Click to enlarge image

Communication between two Arduino via MQTT

Arduino Code #1

/* * Created by ArduinoGetStarted.com * * This example code is in the public domain * * Tutorial page: https://arduinogetstarted.com/tutorials/arduino-to-arduino-mqtt-communication */ // ARDUINO #1: CONNECTED TO A BUTTON/SWITCH, ACTED AS A MQTT PUBLISHER #include <WiFiS3.h> #include <MQTTClient.h> #include <ezButton.h> const int BUTTON_PIN = 7; const char WIFI_SSID[] = "YOUR_WIFI_SSID"; // CHANGE TO YOUR WIFI SSID const char WIFI_PASSWORD[] = "YOUR_WIFI_PASSWORD"; // CHANGE TO YOUR WIFI PASSWORD const char MQTT_BROKER_ADRRESS[] = "test.mosquitto.org"; // CHANGE TO MQTT BROKER'S ADDRESS //const char MQTT_BROKER_ADRRESS[] = "192.168.0.26"; // CHANGE TO MQTT BROKER'S IP ADDRESS const int MQTT_PORT = 1883; const char MQTT_CLIENT_ID[] = "arduino-001"; // CHANGE IT AS YOU DESIRE const char MQTT_USERNAME[] = ""; // CHANGE IT IF REQUIRED, empty if not required const char MQTT_PASSWORD[] = ""; // CHANGE IT IF REQUIRED, empty if not required // The MQTT topics that Arduino should publish/subscribe const char MQTT_TOPIC[] = "arduino/command"; // CHANGE IT AS YOU DESIRE WiFiClient network; MQTTClient mqtt = MQTTClient(256); ezButton button(BUTTON_PIN); // create ezButton that attach to pin 7 void setup() { Serial.begin(9600); button.setDebounceTime(100); // set debounce time to 100 milliseconds Serial.println("ARDUINO #1: CONNECTED TO A BUTTON/SWITCH, ACTED AS A MQTT PUBLISHER"); int status = WL_IDLE_STATUS; while (status != WL_CONNECTED) { Serial.print("Arduino - Attempting to connect to SSID: "); Serial.println(WIFI_SSID); // Connect to WPA/WPA2 network. Change this line if using open or WEP network: status = WiFi.begin(WIFI_SSID, WIFI_PASSWORD); // wait 10 seconds for connection: delay(10000); } // print your board's IP address: Serial.print("IP Address: "); Serial.println(WiFi.localIP()); connectToMQTT(); } void loop() { mqtt.loop(); button.loop(); // MUST call the loop() function first if (button.isPressed()) { Serial.println("- The button is pressed, send command: 1"); sendToMQTT('1'); } if (button.isReleased()) { Serial.println("- The button is released, send command: 0"); sendToMQTT('0'); } } void connectToMQTT() { // Connect to the MQTT broker mqtt.begin(MQTT_BROKER_ADRRESS, MQTT_PORT, network); Serial.print("Arduino - Connecting to MQTT broker"); while (!mqtt.connect(MQTT_CLIENT_ID, MQTT_USERNAME, MQTT_PASSWORD)) { Serial.print("."); delay(100); } Serial.println(); if (!mqtt.connected()) { Serial.println("Arduino - MQTT broker Timeout!"); return; } Serial.println("Arduino - MQTT broker Connected!"); } void sendToMQTT(char command) { char messageBuffer[1]; messageBuffer[0] = command; mqtt.publish(MQTT_TOPIC, messageBuffer); Serial.println("Arduino - sent to MQTT:"); Serial.print("- topic: "); Serial.println(MQTT_TOPIC); Serial.print("- payload:"); Serial.println(messageBuffer); }

Arduino Code #2

/* * Created by ArduinoGetStarted.com * * This example code is in the public domain * * Tutorial page: https://arduinogetstarted.com/tutorials/arduino-to-arduino-mqtt-communication */ // ARDUINO #2: CONNECTED TO A LED, ACTED AS A MQTT SUBSCRIBER #include <WiFiS3.h> #include <MQTTClient.h> const int LED_PIN = 9; const char WIFI_SSID[] = "YOUR_WIFI_SSID"; // CHANGE TO YOUR WIFI SSID const char WIFI_PASSWORD[] = "YOUR_WIFI_PASSWORD"; // CHANGE TO YOUR WIFI PASSWORD const char MQTT_BROKER_ADRRESS[] = "test.mosquitto.org"; // CHANGE TO MQTT BROKER'S ADDRESS //const char MQTT_BROKER_ADRRESS[] = "192.168.0.26"; // CHANGE TO MQTT BROKER'S IP ADDRESS const int MQTT_PORT = 1883; const char MQTT_CLIENT_ID[] = "arduino-002"; // CHANGE IT AS YOU DESIRE const char MQTT_USERNAME[] = ""; // CHANGE IT IF REQUIRED, empty if not required const char MQTT_PASSWORD[] = ""; // CHANGE IT IF REQUIRED, empty if not required // The MQTT topics that Arduino should publish/subscribe const char MQTT_TOPIC[] = "arduino/command"; // CHANGE IT AS YOU DESIRE WiFiClient network; MQTTClient mqtt = MQTTClient(256); void setup() { Serial.begin(9600); pinMode(LED_PIN, OUTPUT); Serial.println("ARDUINO #2: CONNECTED TO A LED, ACTED AS A MQTT SUBSCRIBER"); int status = WL_IDLE_STATUS; while (status != WL_CONNECTED) { Serial.print("Arduino - Attempting to connect to SSID: "); Serial.println(WIFI_SSID); // Connect to WPA/WPA2 network. Change this line if using open or WEP network: status = WiFi.begin(WIFI_SSID, WIFI_PASSWORD); // wait 10 seconds for connection: delay(10000); } // print your board's IP address: Serial.print("IP Address: "); Serial.println(WiFi.localIP()); connectToMQTT(); } void loop() { mqtt.loop(); } void connectToMQTT() { // Connect to the MQTT broker mqtt.begin(MQTT_BROKER_ADRRESS, MQTT_PORT, network); // Create a handler for incoming messages mqtt.onMessage(messageReceived); Serial.print("Arduino - Connecting to MQTT broker"); while (!mqtt.connect(MQTT_CLIENT_ID, MQTT_USERNAME, MQTT_PASSWORD)) { Serial.print("."); delay(100); } Serial.println(); if (!mqtt.connected()) { Serial.println("Arduino - MQTT broker Timeout!"); return; } // Subscribe to a topic, the incoming messages are processed by messageReceived() function if (mqtt.subscribe(MQTT_TOPIC)) Serial.print("Arduino - Subscribed to the topic: "); else Serial.print("Arduino - Failed to subscribe to the topic: "); Serial.println(MQTT_TOPIC); Serial.println("Arduino - MQTT broker Connected!"); } void messageReceived(String &topic, String &payload) { Serial.println("Arduino - received from MQTT:"); Serial.println("- topic: " + topic); Serial.print("- payload: "); Serial.println(payload); char command = payload[0]; if (command == '1') { Serial.print("- Received command: "); Serial.print(command); Serial.println(" => Turned LED on"); digitalWrite(LED_PIN, HIGH); // Turn LED on } else if (command == '0') { Serial.print("- Received command: "); Serial.print(command); Serial.println(" => Turned LED off"); digitalWrite(LED_PIN, LOW); // Turn LED off } else { Serial.print("- Received an unsupported command: "); Serial.println(command); } }

Quick Steps

  • Wire a button/switch to Arduino #1
  • Wire an LED to Arduino #2
  • Open Arduino IDE (called Arduino IDE #1)
  • Open another Arduino IDE window (called Arduino IDE #2) by clicking on Arduino IDE icon on your PC (important!(**))
  • Open the Library Manager by clicking on the Library Manager icon on the left navigation bar of Arduino IDE
  • Type MQTT on the search box, then look for the MQTT library by Joel Gaehwiler.
  • Click Install button to install MQTT library.
Arduino MQTT library
  • Type ezButton on the search box, then find the button library by ArduinoGetStarted
  • Click Install button to install ezButton library.
Arduino button library
  • Connect Arduino #1 to PC via USB cable and select COM port of Arduino #1 on Arduino IDE #1
  • Connect Arduino #2 to PC via USB cable and select COM port of Arduino #2 on Arduino IDE #2
  • Copy Arduino #1 code, paste to Arduino IDE #1 and save it (named Arduino-1)
  • Copy Arduino #2 code, paste to Arduino IDE #2 and save it (named Arduino-2)
  • Replace the WiFi information (SSID and password) in both codes with your own.
  • Replace the MQTT broker address in both codes (domain name or IP address).
  • Upload Arduino #1 code to Arduino #1
  • Upload Arduino #2 code to Arduino #2
  • Open Serial Monitor on Arduino IDE #1
  • Open Serial Monitor on Arduino IDE #2
  • Press and hold the button on Arduino #1 → see LED's state on Arduino #2 (ON)
  • Release button on Arduino #1 → see LED's state on Arduino #2 (OFF)
  • Press, hold, and release the button several times.
  • See output on both Serial Monitors
    • Serial Monitor of Arduino #1
    COM6
    Send
    Arduino #1: CONNECTED TO A BUTTON/SWITCH, ACTED AS A MQTT PUBLISHER Arduino - Attempting to connect to SSID: YOUR_WIFI_SSID IP Address: 192.168.0.2 Arduino - Connecting to MQTT broker Arduino - MQTT broker Connected! - The button is pressed, send command: 1 Arduino - sent to MQTT: - topic: arduino/command - payload:1 - The button is released, send command: 0 Arduino - sent to MQTT: - topic: arduino/command - payload:0
    Autoscroll Show timestamp
    Clear output
    9600 baud  
    Newline  
    • Serial Monitor of Arduino #2
    COM6
    Send
    Arduino #2: CONNECTED TO A LED, ACTED AS A MQTT SUBSCRIBER Arduino - Attempting to connect to SSID: YOUR_WIFI_SSID IP Address: 192.168.0.2 Arduino - Connecting to MQTT broker Arduino - Subscribed to the topic: arduino/command Arduino - MQTT broker Connected! Arduino - received from MQTT: - topic: arduino/command - payload: 1 - Received command: 1 => Turned LED on Arduino - received from MQTT: - topic: arduino/command - payload: 0 - Received command: 0 => Turned LED off
    Autoscroll Show timestamp
    Clear output
    9600 baud  
    Newline  

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