Sample Arduino code for publishing devices using Home Assistant MQTT Discovery. See the following for use case: https://youtu.be/VHiCtZqllU8 or https://resinchemtech.blogspot.com/2023/12/mqtt-auto-discovery.html

mqtt_discovery.cpp

/*  ===================================================================
    SAMPLE CODE Segments for Home Assistant MQTT Discovery
    This is NOT complete code but shows an example of generating a unique ID
    for topic/entity publishing based on the device MAC address.
    It also includes sample code for creating a Home Assistant device with four
    entities via MQTT Discovery, as shown here: 
      Video: https://youtu.be/VHiCtZqllU8   
      Blog: https://resinchemtech.blogspot.com/2023/12/mqtt-auto-discovery.html
    
    It is highly recommended that you include some sort of end-user accessible setting to 
    enable/disable Home Assistant discovery as some users may not want your device auto-discovered
    and Home Assistant currently doesn't provide any prompting... devices/entities are immediately
    added as soon as a valid topic/payload are received.
    
    THIS IS AN INCOMPLETE EXAMPLE ONLY!!!!
    ===================================================================== */
    
//Partial list of libaries - also need Wifi, ESP8266/ESP32 libraries, etc.
#include <PubSubClient.h>
#include <ArduinoJson.h>
#include <ESP8266WebServer.h>  //Used for HTTP callback to enable/disable discovery

//Auto-discover enable/disable option
bool auto_discovery = false;      //default to false and provide end-user interface to allow toggling 

//Variables for creating unique entity IDs and topics
byte macAddr[6];                  //Device MAC address
char uidPrefix[] = "rctdev";      //Prefix for unique ID generation (limit to 20 chars)
char devUniqueID[30];             //Generated Unique ID for this device (uidPrefix + last 6 MAC characters) 

// =====================================
//  Create Unique ID for topics/entities
// =====================================
void createDiscoveryUniqueID() {
  //Generate UniqueID from uidPrefix + last 6 chars of device MAC address
  //This should insure that even multiple devices installed in same HA instance are unique
  
  strcpy(devUniqueID, uidPrefix);
  int preSizeBytes = sizeof(uidPrefix);
  int preSizeElements = (sizeof(uidPrefix) / sizeof(uidPrefix[0]));
  //Now add last 6 chars from MAC address (these are 'reversed' in the array)
  int j = 0;
  for (int i = 2; i >= 0; i--) {
    sprintf(&devUniqueID[(preSizeBytes - 1) + (j)], "%02X", macAddr[i]);   //preSizeBytes indicates num of bytes in prefix - null terminator, plus 2 (j) bytes for each hex segment of MAC
    j = j + 2;
  }
  // End result is a unique ID for this device (e.g. rctdevE350CA) 
  Serial.print("Unique ID: ");
  Serial.println(devUniqueID);  
}
// ===============================
//  Main HA MQTT Discover Function
//    This creates a single fictional device with four entities:
//      - A dimmer switch with light level, temperature and IP address
// ===============================
void haDiscovery() {
  char topic[128];
  if (auto_discovery) {
    char buffer1[512];
    char buffer2[512];
    char buffer3[512];
    char buffer4[512];
    char uid[128];
    DynamicJsonDocument doc(512);
    doc.clear();
    Serial.println("Discovering new devices...");


    Serial.println("Adding light switch...");
    //Create unique topic based on devUniqueID
    strcpy(topic, "homeassistant/light/");
    strcat(topic, devUniqueID);
    strcat(topic, "S/config");
    //Create unique_id based on devUniqueID
    strcpy(uid, devUniqueID);
    strcat(uid, "S");
    //Create JSON payload per HA documentation
    doc["name"] = "My MQTT Light";
    doc["obj_id"] = "mqtt_light";
    doc["uniq_id"] = uid;
    doc["stat_t"] = "stat/mydevice/switch";
    doc["cmd_t"] = "cmnd/mydevice/switch";
    doc["brightness"] = "true";
    doc["bri_scl"] = "100";
    doc["bri_stat_t"] = "stat/mydevice/brightness";
    doc["bri_cmd_t"] = "cmnd/mydevice/brightness";
    JsonObject device = doc.createNestedObject("device");
    device["ids"] = "mymqttdevice01";
    device["name"] = "My MQTT Device";
    device["mf"] = "Resinchem Tech";
    device["mdl"] = "ESP8266";
    device["sw"] = "1.24";
    device["hw"] = "0.45";
    device["cu"] = "http://192.168.1.226/config";  //web interface for device, with discovery toggle
    serializeJson(doc, buffer1);
    //Publish discovery topic and payload (with retained flag)
    client.publish(topic, buffer1, true);    

    //Lux Sensor
    Serial.println("Adding light sensor...");
    //Create unique Topic based on devUniqueID
    strcpy(topic, "homeassistant/sensor/");
    strcat(topic, devUniqueID);
    strcat(topic, "L/config");
    //Create unique_id based on decUniqueID
    strcpy(uid, devUniqueID);
    strcat(uid, "L");
    //Create JSON payload per HA documentation
    doc.clear();
    doc["name"] = "Light Level";
    doc["obj_id"] = "mqtt_light_level";
    doc["dev_cla"] = "illuminance";
    doc["uniq_id"] = uid;
    doc["stat_t"] = "stat/mydevice/lightlevel";
    doc["unit_of_meas"] = "lx";
    JsonObject deviceS = doc.createNestedObject("device");
    deviceS["ids"] = "mymqttdevice01";
    deviceS["name"] = "My MQTT Device";
    serializeJson(doc, buffer2);
    //Publish discovery topic and payload (with retained flag)
    client.publish(topic, buffer2, true);
    
    //Temperature Sensor
    Serial.println("Adding Temp Sensor...");
    //Create unique Topic based on devUniqueID
    strcpy(topic, "homeassistant/sensor/");
    strcat(topic, devUniqueID);
    strcat(topic, "T/config");
    //Create unique_id based on decUniqueID
    strcpy(uid, devUniqueID);
    strcat(uid, "T");
    //Create JSON payload per HA documentation
    doc.clear();
    doc["name"] = "Temperature";
    doc["obj_id"] = "mqtt_temperature";
    doc["deve_cla"] = "temperature";
    doc["uniq_id"] = uid;
    doc["stat_t"] = "stat/mydevice/temperature";
    doc["unit_of_meas"] = "°F";
    JsonObject deviceT = doc.createNestedObject("device");
    deviceT["ids"] = "mymqttdevice01";
    deviceT["name"] = "My MQTT Device";
    serializeJson(doc, buffer3);
    //Publish discovery topic and payload (with retained flag)
    client.publish(topic, buffer3, true);

    //IP Address Diagnostic
    Serial.println("Adding IP Diagnostic Sensor...");
    //Create unique Topic based on devUniqueID
    strcpy(topic, "homeassistant/sensor/");
    strcat(topic, devUniqueID);
    strcat(topic, "I/config");
    //Create unique_id based on decUniqueID
    strcpy(uid, devUniqueID);
    strcat(uid, "I");
    //Create JSON payload per HA documentation
    doc.clear();
    doc["name"] = "IP Address";
    doc["uniq_id"] = uid;
    doc["ent_cat"] = "diagnostic";
    doc["stat_t"] = "stat/mydevice/ipaddress";
    JsonObject deviceI = doc.createNestedObject("device");
    deviceI = doc.createNestedObject("device");
    deviceI["ids"] = "mymqttdevice01";
    deviceI["name"] = "My MQTT Device";
    serializeJson(doc, buffer4);
    //Publish discovery topic and payload (with retained flag)
    client.publish(topic, buffer4, true);
   
    Serial.println("All devices added!");

  } else {

    //Remove all entities by publishing empty payloads
    //Must use original topic, so recreate from original Unique ID
    //This will immediately remove/delete the device/entities from HA
    Serial.println("Removing discovered devices...");

    //Lux Sensor
    strcpy(topic, "homeassistant/sensor/");
    strcat(topic, devUniqueID);
    strcat(topic, "L/config");
    client.publish(topic, "");

    //Temperature Sensor
    strcpy(topic, "homeassistant/sensor/");
    strcat(topic, devUniqueID);
    strcat(topic, "T/config");
    client.publish(topic, "");

    //IP Address Sensor
    strcpy(topic, "homeassistant/sensor/");
    strcat(topic, devUniqueID);
    strcat(topic, "I/config");
    client.publish(topic, "");
    
    //Light (switch)
    strcpy(topic, "homeassistant/light/");
    strcat(topic, devUniqueID);
    strcat(topic, "S/config");
    client.publish(topic, "");

    Serial.println("Devices Removed...");
  }
}

// =====================
//   MAIN SETUP
// =====================
void Setup() {
  // The following should be included in (or called from) the main setup routine

  //Get MAC address when joining wifi and place into char array
  WiFi.macAddress(macAddr);  
  
  //Call routing (or embed here) to create initial Unique ID
  createDiscoveryUniqueID();
 
  //Handle web callbacks for enabling or disabling discovery (using this method is just one of many ways to do this)
  server.on("/discovery_on",[]() {
    server.send(200, "text/html", "<h1>Discovery ON...<h1><h3>Home Assistant MQTT Discovery enabled</h3>");
    delay(200);
    auto_discovery = true;
    haDiscovery();
  });
  server.on("/discovery_off",[]() {
    server.send(200, "text/html", "<h1>Discovery OFF...<h1><h3>Home Assistant MQTT Discovery disabled. Previous entities removed.</h3>");
    delay(200);
    auto_discovery = false;
    haDiscovery();
  });
  server.begin();  
    
}

Just remember there are really two parts here... the discovery topic and the discovery payload. The discovery topic is what MUST contain that unique object ID. Technically that's the only place you need to have a unique ID.

The payload are the specifics for creating the entity or device in Home Assistant. While it is good practice to include a unique ID so that the entity can be edited in the Home Assistant UI, it technically isn't required. You can use the same unique ID as you used in the discovery topic, but even that isn't necessary. It just need to be unique across all other entities in Home Assistant.

As far as the state and command topics within the payload, these are the specific MQTT commands that the device or entity will use after it is discovered and integrated into Home Assistant. They have absolutely nothing to do with the discovery topic and can be anything you want them to be. Your state topic could be something as simple as /abc and the command topic of /xyz. This would be the same as if you defined it in YAML. In fact, for something like a sensor, there won't even be a command topic, as it only sends data and doesn't receive commands. Assure you understand how MQTT works and how Home Assistant entities and automations utilize MQTT. Just remember that the state and command topics used within the payload for the device are just the topics that the device and Home Assistant are going to subscribe and publish to for exchanging data. They have nothing to do with the discovery topic and payload... or the unique ID as used for discovery.

:-) Thank you

Can add and remove entities after:

• I added my mosquito user and password to the client.connect("arduinoClient", "me" , "mypass")
• I studied the sprintf(&devUniqueID[(preSizeBytes - 1) + (j)], "%02X", macAddr[i]); line and realized it won't work. (write outside buffer)

I know its just Sample code. :-)

Thank you. I think I got it now.

Cleaned up some code, trying to implement the light only.

I cannot get the brightness to work. Checked and double checked the state_topic and brightness topic with my code and MQTT-Explorer.
The broker gets all commands and states correctly.
But then I found this in the homeassistant log

2024-08-10 13:49:22.686 WARNING (MainThread) [homeassistant.components.light] light.office_light_my_mqtt_light (<class 'homeassistant.components.mqtt.light.schema_basic.MqttLight'>) set to unsupported color mode onoff, expected one of {<ColorMode.BRIGHTNESS: 'brightness'>}, this will stop working in Home Assistant Core 2025.3, please create a bug report at https://github.com/home-assistant/core/issues?q=is%3Aopen+is%3Aissue+label%3A%22integration%3A+mqtt%22

Home assistant seems to get the on/off and brightness states correctly from my code too.

Then checking with the Developer tools in HA:
The brightness attribute always gets set to null.
(when I touch the control for either the switch or the brightness slider)
And there is a color_mode attribute that I did not set in my arduino code.

Are you using any mqtt discovered lights yourself? And do they still work, inlcuding brightness setting?

I have already reported to home-assistant too. But perhaps you have some workaround?

Thanks in advance.

Well, every month a new version of Home Assistant is released, with breaking changes. And it seems that every month, MQTT is listed in the breaking changes. In this case, I believe the "brightness: true" is no longer valid. You can always check the latest valid parameters and format by searching for "Home Assistant MQTT entitytype". For lights, see: MQTT Light. Also note that if you are using abbreviations, you should also check the main Home Assistant MQTT Documentation page for valid abbreviations, as these may change with other parameter changes.

As mentioned before, the code I include here is really just a guideline and, as you found, may not always be 100% correct with later versions of Home Assistant. I actually do not even have an MQTT light (although I do have many switches, sensors. etc.) so the code above was used to create a discovered light, but wasn't actually installed on or connected to a 'real' light. Due to the rapid changing nature of Home Assistant, it is almost always necessary to take an sample code and then compare that to the latest official documentation (although sometimes even the official documentation lags behind the changes as well). But it looks like you are very close with your device. Probably just a small tweak related to the brightness attribute changes in MQTT.

LoL.. Yeah, I got that. The MQTT-integration is frequently part of the breaking changes. It's frustrating.
I read the Home assistant documentation for mqtt discovery. I tried using the full name instead of the abbreviations.. etc etc.

I read some topic named "Do not allow mqtt lights to set brightness to zero #91296"
But thats on the core... thats above my paygrade. (if it is relevant)

The example for an mqtt device with light on the HA website, is 6 years old. Its almost criminal to have it on the website. :-)

There is a library: https://github.com/dawidchyrzynski/arduino-home-assistant/tree/main
I may try that out (I am scanning it to see if it does things significantly different from the simple code I have so far.)
But it adds an extra level of abstraction. Once the HA implementation changes, I would depend on that library to do its maintenance.
It's a little similar to the reason I hessitate using ESP-Home. Very frequent updates and it's basically an abstraction layer on top of arduino code.
I love how mqtt-discovery creates device on HA, but if it breaks every two months..

So far I create quite some HomeAssistant entities in node-red. And then sending regular text commands to TCP or UDP node in node-red.
It works, but it is a task to manage them.
Anyway, we got options.

Hey, thank you man for at least introducing me to mqtt discovery and getting me this far.

Dear Resinchem,

I thought it fair (and possibly helpfull to others that read your blog or watch your video) to report back that I figured it out.
And its simple but deceptive trick in the MQTT-Callback function:

I previously responded to receiving a switch command, by publishing the new switch_state.
And responded to receiving a brightness command, by publishing the new brightness_state.
(I had tried to simply respond by publishing all states upon receiving any command, but that did not work)

I now have included code that when receiving a brightness command: when it is >0 , ALSO publish switch_state = on
And when the brightness command has a value == 0, then ALSO publish the switch_state = off

And it magically works.
When checking the brightness value in the Developer_tools, it also updates.
When the switch is off, the brightness value is null. when I toggle the switch, it gets the (original) value through mqtt.
When I drag the slider, the light goes brighter/less bright, and goes off when slider set to lowest position.

Thanks again.

Edit.... OR..... it got solved in the core_2024.8.1 , because it now behaves quite a bit diffferent
Anyway.. will figure it out.

P.S. It works better if you set the [bri_scl] = "255" .. because then the attribite "color_mode" will switch between "brightnes" (switch is on) and "null" (switch is off)
And the attribute "brightness" will get a value depending on the mqtt state value .. or null when the switch is off.
OK, I will stop bothering you with updates.
Thanks for the last time.