Domoticz, ESP & MQTT, startup

Domoticz, ESP & MQTT, startup

Of course domotica with Domoticz is fun but it’s more fun if you can combine your own ESPs with Domoticz. That is what I have been fiddling with and what I will describe here.

There are basically 2 ways to connect to Domoticz, through HTTP or MQTT. With HTTP you can only tell Domoticz what to do. With MQTT however, Domoticz shares it’s changes so you can also respond to changes. I choose to go the MQTT route because that way I can create sensors and switches with my ESPs.

To get things started, you need to have an MQTT broker. There have been many people describing this on their blogs, so I will not cover that here. Suffice it to say that I choose to put the MQTT broker on my NAS since that is always up anyway. Once you have the MQTT broker, you need to configure MQTT for Domoticz. This is described nicely on their Wiki. You will also find instructions on how to install Node-RED. I did install that but you don’t need it for these projects. Once you have installed MQTT for Domoticz, you need to tell it to publish everything on the flat version (called out in the drop down list).

We are getting there. Once you have installed and configured MQTT for Domoticz, use an MQTT viewer to view the messages it generates. I personally like mqtt-spy. Remember you have to subscribe to the topic domoticz/out. Look at some of the messages and how they are structured. As you can see it’s JSON formatted. The messages you post to Domoticz on the domoticz/in topic are also JSON formatted.

Ok, let’s configure a dummy switch in Domoticz. I called mine TESTMQTT but you can call it whatever you want. Make sure it is a switch. Once you did that, write down the id number.

Good, now let’s play. Turn it on and back off again. Now go to your viewer and look for the id of your switch. If you are using mqtt-spy, it is the search button on the right of the screen next to where all the messages are shown. Once you find your message, you will see that it looks something like this:

  "Battery" : 255,
  "RSSI" : 12,
  "description" : "",
  "dtype" : "Light/Switch",
  "id" : "00014648",
  "idx" : 1528,
  "name" : "TESTMQTT",
  "nvalue" : 0,
  "stype" : "Switch",
  "svalue1" : "0",
  "switchType" : "On/Off",
  "unit" : 1

I won’t go in detail now, just now that “idx” is your id and in the case of a switch, “nvalue” tells whether it was turned on (1) or off (0).

Next, let’s turn on the switch using MQTT. Make sure the switch is off and than go to your viewer and send this message (remember to use your id number, not mine)

topic: domoticz/in
message: { "command" : "switchlight" , "idx" : 1528 , "switchcmd" : "On" }

And click on publish. Your dummy switch should turn on.

Next up is coding an ESP to be a light. As we need JSON to listen to the switches, we need to install the ArduinoJson library which you install like any library. We also need to install the pubsubclient library which again installs like any library. However in this case we need to make a change after the installation. Go to your libraries directory and go to the pubsubclient-master/src directory. There is a file called PubSubClient.h. Open it to make a change. We need to increase the maximum packet size. Around line 26 there is a line that we need to change. It should read after you’re done:


Next save the file and we are ready to try some code. We will start with something simple, the internal led being turned on and off through Domoticz. I will start by sharing the code (also available on github-mark-32px Github) and discuss it below the code:

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

const char* ssid = "MySSID";
const char* password = "MyPassword";
const char* mqtt_server = "";
#define LED D4
#define MYIDX 1528

WiFiClient espClient;
PubSubClient client(espClient);

long lastMsg = 0;
char msg[50];
int value = 0;
char macAddr[18];
char mqttClient[25];

void setup_wifi() {
  Serial.print("Connecting to ");

  WiFi.begin(ssid, password);

  while (WiFi.status() != WL_CONNECTED) {


  Serial.println("WiFi connected");
  Serial.println("IP address: ");

void actionOn () {

void actionOff () {

void errorOccurred (String err) {

void callback(char* topic, byte* payload, unsigned int length) {
  DynamicJsonBuffer jsonBuffer(512);
  JsonObject& root = jsonBuffer.parseObject(payload);
  if (!root.success()) {
    errorOccurred("Parse json failed");
  } else {
    int idx = root["idx"];
    if (idx==MYIDX) {
      if (root["nvalue"]==0) {
      } else {

void reconnect() {
  while (!client.connected()) {
    Serial.print("Attempting MQTT connection...");
    if (client.connect(mqttClient)) {
      client.publish("outTopic","hello world");
    } else {
      Serial.print("failed, rc=");
      Serial.println(" try again in 5 seconds");

void setup() {
  byte ar[6];

void loop() {
  if (!client.connected()) {

Let’s start at the top. We include the needed libraries and define our constants for wifi and the MQTT broker. I use a Wemos D1 so my internal led is linked to D4 and my test Domoticz dummy switch has id 1528. Initialize the wifi client and the MQTT client. I keep the mac address which I use for creating the client id and the name of the MQTT client.

The function setup_wifi() does what it says. The functions actionOn() and actionOff() are called to turn on and turn off our switch. In this case it is just write to a port but you can put more here. The function errorOccurred gets called when an error happens. It now just outputs to serial but there might be a need to change that.

The function callback() is the interesting one. This one gets called every time we receive a message on a topic that we are subscribed to. We assume the data is in JSON format and decode it using the ArduinoJson library. Next we check if this was a success and if so, if the idx passed is the one we are looking for. If it is, check nvalue and determine what to do based on it’s value.

The function reconnect() is straight from the pubsubclient example and basically reconnects if the connection is dropped and subscribes to our domoticz/out topic.

Finally the function setup() sets everything up, including getting a unique MQTT client name and the function loop() continously checks for messages.

If you upload this to your ESP it should work. Remember that we do not set any initial values so, the light starts on but maybe in Domoticz it is still off which means you will have to click twice for any change.

This is a very simple example, hardly any error checking but for some of you this may be enough to get going. In the next example I will expand a bit and also publish to Domoticz.


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