I will be using GUI-O IoT (MQTT) connection, but the example can be easily be ported to other connection types.

Software prerequisites:

• Arduino IDE (https://www.arduino.cc/en/software)

• ESP32 Arduino board support, if using ESP32 based board (see https://docs.espressif.com/projects/arduino-esp32/en/latest/installing.html)

• Arduino Client for MQTT(https://github.com/knolleary/pubsubclient)
  You can follow the setup instructions or directly download and include the PubSubClient.zip library within the Arduino IDE (Sketch -> Include Library -> Add .ZIP library)

• GUI-O design tool (https://www.gui-o.com/design-tool)

• GUI-O application (https://play.google.com/store/apps/details?id=com.guio.guioapp)

• For additional information about the GUI-O application, download the developer manual from https://www.gui-o.com/

Components needed:

• ESP32-WROOM-32 (or any other Arduino supported WiFi capable board; see https://github.com/knolleary/pubsubclient#compatible-hardware for compatible hardware)
• 10k resistor
• thermistor

The entire tutorial is split into various steps. All necessary information is given in each step.

0. DESIGN THE GUI (optional)

The best way to create a GUI layout is to use GUI-O live designer tool.

Note that the Arduino source code already includes the necessary commands, so this step is not needed, unless you want to make some visual adjustments. If you make adjustments, please include the generated ASCII code in the Arduino source code (see section 3. MODIFY AND UPLOAD THE SOURCE CODE).

First, you need to establish a TCP/IP connection between the designer tool and GUI-O application:

1. Determine the local IP address of your PC's network interface (WiFi or Ethernet)

• Under Windows, open the command prompt, enter ipconfig and press Enter
• Under Linux, open the terminal, enter ifconfig and press Enter

2. Open GUI-O application and open settings menu. Select "Connections -> Ethernet" and create a new device with IP address (determined from 1.) and any port between 49152 - 65535

3. Open GUI-O designer and select "TCP/IP connection" tab. Set the IP address and port. Both values must match the device settings created within the GUI-O application. Click "Start server" button.

4. Within the GUI-O application, tap the created device and wait for successful connection.

5. In the GUI-O designer, select "File -> Load designer file" and load the TemperatureReader.gdf design file. Make the desired adjustments, if necessary. Copy / replace the GUI-O commands into the Arduino source code (see section 3. MODIFY AND UPLOAD THE SOURCE CODE).

1. CONNECT THE COMPONENTS

Connecting the components is straightforward. The "D35" is one of the ADC1 pins on ESP32 (note that ADC2 pins are not available when WiFi is enabled).

2. CREATE A UNIQUE MQTT CHANNEL

Open GUI-O application and navigate to settings menu. Select "Connections -> IoT" and add a new device. After adding the device, note the In and Out token (you can share this tokens e.g., to your e-mail by pressing the "share" button).

Finally, press "Connect" menu entry to establish the connection with the MQTT server.

3. MODIFY AND UPLOAD THE SOURCE CODE

The source code has inline comments, describing the important parts of the code. You can copy the source code from the snippet below, or download it here.

The only thing that needs to be done is to set the ssid and password of your router and the unique In and Out channels that were generated by the GUI-O application (see section 2. CREATE A UNIQUE MQTT CHANNEL).

After setting these values, upload the code to your board (make sure that the correct board and upload port are selected). Reset the board after upload.

/*
 * GUI-O Thermistor MQTT example (using ESP32-WROOM-32)
 *
 * Copyright (C) 2022, kl3m3n
 * last updated on 30.10.2022
 *
 * SPDX-License-Identifier: BSD-3-Clause
 */

#include <WiFi.h>
#include <PubSubClient.h>

static const char *ssid = "<ssid>"; // router name
static const char *pass = "<pass>"; // router password

static const char *mqttServer = "mqtt.gui-o.com";   // host name
static const char *mqttUser = "gui-o-mqtt-generic"; // user name
static const char *mqttPass = "lqXeZpv5VJyv0XBT";   // password

// IMPORTANT NOTE: if optional user name was specified when adding a new IoT device,
// the user name should also be included when setting the topics (e.g., "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx/<user_name>")
static const char *Out = "<Out>"; // GUI-O app publish topic
static const char *In = "<In>";  // GUI-O app subscribe topic

// mqtt client
WiFiClient wiFiClient;
PubSubClient mqttClient(wiFiClient);

// forward declare functions for mqtt messages handling
void mqttCallback(char* topic, byte* message, unsigned int length);
void parseGuioMsg(const String &msg);

// setup thermistor read settings
char buf[100];
const int thermistorPin = 35;
unsigned long startTimestampMsec, currentTimestampMsec;
const unsigned long readIntervalMsec = 500; // milliseconds

void setup() {
  // debug output
  Serial.begin(115200);
  
  // connect WiFi (keep trying...)
  Serial.print("Connecting to ");
  Serial.println(ssid);
  
  WiFi.begin(ssid, pass);
  while(WiFi.status() != WL_CONNECTED) { 
    Serial.print(".");
    delay(500); 
  }
  
  Serial.println("WiFi connected!"); 
  
  // setup mqtt
  mqttClient.setServer(mqttServer, 1883);
  mqttClient.setCallback(mqttCallback);

  startTimestampMsec = millis();
}

void loop() {
  while(!mqttClient.connected()) {
    Serial.println("MQTT connecting...");

    // mqtt client id is the mac address (AABBCCDDEEFF)
    char mqttClientId[15];    
    uint8_t mac[6];
    WiFi.macAddress(mac);

    snprintf(mqttClientId, sizeof(mqttClientId), "%02x%02x%02x%02x%02x%02x", mac[0], mac[1], mac[2], mac[3], mac[4], mac[5]);

    if(mqttClient.connect(mqttClientId, mqttUser, mqttPass)) {
      Serial.println("MQTT connected!");
      mqttClient.subscribe(&Out[0]);
    }
    else {
      Serial.print("MQTT connection failed (");
      Serial.print(mqttClient.state());
      Serial.println(")! Retrying...");      
      delay(2500);
    }
  }
  mqttClient.loop();

  // get and display temperature 
  currentTimestampMsec = millis();
  
  if(currentTimestampMsec - startTimestampMsec > readIntervalMsec) {
    // read adc
    const int adcValue = analogRead(thermistorPin);
    // calculate voltage
    const float voltage = adcValue * 3.3 / 4096.0;
    // calculate resistance (voltage divider equation)
    const float resistance = 10.0 * voltage / (3.3 - voltage);
    // calculate temperature using thermistor equation
    // using thermistor with thermal index = 3950, nominal resistance = 10k
    float temp = 1.0 / (log(resistance / 10.0) / 3950 + 1.0 / (273.15 + 25.0));
    temp -= 273.15;

    // label
    snprintf(buf, sizeof(buf), "@temp TXT:\"%.2f °C\"\r\n", temp);
    mqttClient.publish(&In[0], &buf[0]);
    // chart
    snprintf(buf, sizeof(buf), "@tempCh PLI:\"id0\" PLC:\"#4C4C4C\" YP:\"%.1f\"\r\n", temp);
    mqttClient.publish(&In[0], &buf[0]);
    
    Serial.print("T = ");
    Serial.print(temp);
    Serial.println(" °C");

    // reset read interval
    startTimestampMsec = millis();
  }
}

// mqtt callback function
void mqttCallback(char* topic, byte* message, unsigned int length) {
  // build message string
  String msg;
  for(int i = 0; i < length; i++) {
    msg += (char) message[i];
  }
  // parse message string
  parseGuioMsg(msg);
}

void parseGuioMsg(const String &msg) {
  if(msg.startsWith("@init")) {
    Serial.println("GUI-O app is requesting INITIALIZATION!");

    // clear screen and set background
    mqttClient.publish(&In[0], "@cls\r\n");
    mqttClient.publish(&In[0], "@guis BGC:#FFFFFF\r\n");
    delay(100);

    // initialize GUI
    mqttClient.publish(&In[0], "|LB UID:title X:50 Y:20 FSZ:4 FFA:\"font8\" TXT:\"Temperature<br>reader\"\r\n");
    mqttClient.publish(&In[0], "|BSR UID:container X:50 Y:40 W:30 H:5 RAD:1 BTH:0\r\n");
    mqttClient.publish(&In[0], "|LB UID:temp X:50 Y:40 FGC:#FFFFFF TXT:\"n/a\"\r\n");
    mqttClient.publish(&In[0], "|CH UID:tempCh X:50 Y:65 YLO:15 YHI:35\r\n");
    mqttClient.publish(&In[0], "|LB UID:details X:50 Y:90 FSZ:2 TXT:\"GUI-O temperature reader<br>demonstration by kl3m3n\"\r\n");
  }  
}

4. ESTABLISH CONNECTION

Make sure that the GUI-O application is connected to the MQTT server.
Also make sure that the ESP32 board (or other Arduino supported board) is connected to the MQTT server (you can check this by observing the serial debug messages using the Arduino serial monitor).

Press the Initialize button (see image below) from the GUI-O application home screen.

5. THE RESULT

Image below shows the result (screen capture) on my Android device after pressing the "Initialize" button. The label and chart are updated every time the ESP32 board calculates the temperature and publishes new data to the MQTT server.

If you have any questions or run into any problems, please let me know!

Best regards,
kl3m3n