In the previous instructions you read about MQTT. If you are not familiar with this, go and check our instructions about setting up IoTaaP MQTT instance and testing it.

In this instructions you will learn how to connect to MQTT broker using IoTaaP and we will create a simple program that will publish accelerometer coordinates and listen for LED status commands.


We must define our features before #include

In setup() function we wil configure WiFi and connect IoTaaP to WiFi AP (with internet access)

Now it’s time to initialize MQTT object and connect IoTaaP to our IoTaaP MQTT instance.

iotaap.mqtt.connect("iotaap_client", "", 8883, callback, false, "USERNAME", "PASSWORD");
This line of code will connect IoTaaP to the IoTaaP MQTT broker. You can get connection credentials in your instance details in IoTaaP Console.

callback() is the function that will be called every time when something arrives to our subscribed topic(s). We will define this function later.

This line of code will subscribe IoTaaP to “ledstatus/led1” topic. We will define our callback in the next step.

Callback function

Callback function signature is already defined so it cannot be changed and it looks like this:

oid callback(char* topic, byte* message, unsigned int length) 
Now we will define the function body:

void callback(char* topic, byte* message, unsigned int length) {
  String messageTemp;

  for (int i = 0; i < length; i++) {
    messageTemp += (char)message[i];

  if (String(topic) == "ledstatus/led1") {
    if(messageTemp == "on"){
    else if(messageTemp == "off"){
This function will convert our message into String type and it will check if our trigger topic was “ledstatus/led1“, if this is true, our function will check message content. Content must be “on” in order to turn the LED1 on, or “off” to turn it off.


In our main loop we will publish our accelerometer data in jSON format, and we will call mandatory function keepAlive() that will keep our MQTT connection active.

IoTaaP library comes with ArduinoJson (C++ JSON library for Arduino and IoT) for easier handling of the jSON format. We will use only serializeJson() function in this example, but feel free to check other features of ArduinoJson library.

void loop()

  accelerometer acc = iotaap.accelerometer.getRaw();

  doc["x"] = acc.x;
  doc["y"] = acc.y;
  doc["z"] = acc.z;

  char accJson[255];

  serializeJson(doc, accJson);

  iotaap.mqtt.publish("iotaap/accelerometer", accJson);

This code will keep our MQTT connection active, but it will also read accelerometer data (in accelerometer structure). Axis values will be saved to “doc” with appropriate axis names. Function serializeJson(doc,accJson); will convert doc to jSON string, and finally iotaap.mqtt.publish(“iotaap/accelerometer”, accJson); will publish values to “iotaap/accelerometer” topic.

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We will use MQTT.fx for testing. MQTT.fx installation process and usage are described in MQTT Setup. Simply connect to your IoTaaP MQTT instance and publish on or off to “ledstatus/led1” topic. Be sure to subscribe to the “iotaap/accelerometer” topic and you will se accelerometer coordinates in jSON format

  "x" : 1921,
  "y" : 1936,
  "z" : 1109

Now, create a code that will control 2 LED’s and publish button status, share your solutions with the community.