ESP32 Web Bluetooth (BLE): Getting Started Guide

This guide provides a beginner-friendly introduction to using Web Bluetooth with the ESP32. We’ll explain what Web Bluetooth is and walk you through creating a web application for interacting with an ESP32 Bluetooth Low Energy (BLE) device. Within the web app, you’ll be able to control the ESP32 GPIOs and retrieve values sent by the ESP32 through writing to and reading from its BLE characteristics.

Table of Contents

Throughout this tutorial, we’ll cover the following:

• Web Bluetooth Introduction
• Bluetooth Low Energy Introduction – Basic Concepts
  • BLE Peripheral and Controller (Central Device)
  • BLE Server and Client
  • GATT
  • UUID
• Project Overview
• Part 1: ESP32 BLE Device – Arduino Code
• Part 2: Creating the Web BLE App
• Part 3: Hosting your Web BLE App

Introducing Web Bluetooth

Web Bluetooth (also sometimes referred to as Web BLE) is a technology that allows you to connect and control BLE-enabled devices, like the ESP32, directly from your web browser using JavaScript.

With Web BLE, you can create web applications that interact with your ESP32 devices via Bluetooth, enabling you to control GPIO pins, exchange data, and manage your devices remotely through a web interface (this means any device that supports a web browser like your computer or smartphone).

One of the key advantages of Web BLE is its cross-platform compatibility. Unlike traditional mobile apps developed for Android or iOS, Web BLE applications are web-based and can run on any device with a modern web browser that supports Web BLE.

This cross-platform compatibility removes the need for users to download and install dedicated mobile apps, simplifying the user experience and reducing development efforts.

This means you can use a smartphone, tablet, or desktop computer to connect and control ESP32 devices using a Web BLE application.

The Web Bluetooth API is still under development, but it is generally considered to be stable and usable. It has been implemented in Chrome, Edge, Opera, and Firefox, and it is supported on Android and Windows. However, it is not yet supported on iOS.

Note: At the time of writing this tutorial, Web BLE is not supported on iOS.

Bluetooth Low Energy Introduction – Basic Concepts

Before proceeding, it’s important to get familiar with some basic BLE concepts. We also recommend that you take a quick look at our BLE getting started guides and tutorials:

• Getting Started with ESP32 Bluetooth Low Energy (BLE) on Arduino IDE
• ESP32 BLE Server and Client (Bluetooth Low Energy)

BLE Peripheral and Controller (Central Device)

When using Bluetooth Low Energy (BLE), it’s important to understand the roles of BLE Peripheral and BLE Controller (also referred to as the Central Device).

In our particular example, the ESP32 takes the role of the BLE Peripheral, serving as the device that provides data or services. Your smartphone or computer acts as the BLE Controller, managing the connection and communication with the ESP32.

BLE Server and Client

With Bluetooth Low Energy, there are two types of devices: the server and the client. The ESP32 can act either as a client or as a server. But, in our particular example, it will act as a server, exposing its GATT structure containing data. The BLE Server acts as a provider of data or services, while the BLE Client consumes or uses these services.

The server advertises its existence, so it can be found by other devices and contains data that the client can read or interact with. The client scans the nearby devices, and when it finds the server it is looking for, it establishes a connection and can interact with that device by reading or writing on its characteristics.

GATT

GATT, which stands for Generic Attribute Profile, is a fundamental concept in Bluetooth Low Energy (BLE) technology. Essentially, it serves as a blueprint for how BLE devices communicate with each other. Think of it as a structured language that two BLE devices use to exchange information seamlessly.

• Profile: standard collection of services for a specific use case;
• Service: collection of related information, like sensor readings, battery level, heart rate, etc. ;
• Characteristic: it is where the actual data is saved on the hierarchy (value);
• Descriptor: metadata about the data;
• Properties: describe how the characteristic value can be interacted with. For example: read, write, notify, broadcast, indicate, etc.

For a more detailed introduction to these BLE concepts, read: Getting Started with ESP32 Bluetooth Low Energy (BLE) on Arduino IDE.

UUID

A UUID is a unique digital identifier used in BLE and GATT to distinguish and locate services, characteristics, and descriptors. It’s like a distinct label that ensures every component in a Bluetooth device has a unique name.

Each service, characteristic, and descriptor has a UUID (Universally Unique Identifier). A UUID is a unique 128-bit (16 bytes) number. For example:

55072829-bc9e-4c53-938a-74a6d4c78776

There are shortened and default UUIDs for services, and characteristics specified in the SIG (Bluetooth Special Interest Group). This means, that if you have a BLE device that uses the default UUIDs for its services and characteristics, you’ll know exactly how to interact with that device to get or interact with the information you’re looking for.

Most of the time, with the ESP32, you’ll use your custom UUIDs. You can generate them using this UUID generator website.

Project Overview

Now that you’re more familiar with BLE concepts, let’s go through a quick overview of the project we’ll build.

The ESP32 will act as a BLE Peripheral/BLE Server that advertises its existence. Your computer, smartphone, or tablet will act as a BLE Controller/Client that interacts with the ESP32 device.

The ESP32 GATT structure will have one service with two characteristics. One characteristic (let’s call it sensor characteristic) will be the place to save a value that changes over time (like sensor readings). The other characteristic (let’s call it LED characteristic) will be the place to save the state of a GPIO. By changing the value of that characteristic, we’ll be able to control an LED connected to that GPIO.

The ESP32 will write a new value to the sensor characteristic periodically. Your browser will connect to the ESP32 Bluetooth device and will receive notifications whenever the value changes and will display the new value on the web page (see Fetched Value in the picture below).

Additionally, your browser will also connect to the LED characteristic and will change its value (see the ON and OFF buttons above). The ESP32 checks that the value of that characteristic changed, and it will change the state of the GPIO accordingly, turning the LED either on or off.

ESP32 BLE Device – Arduino Code

The following code turns the ESP32 into a BLE device with one service and two characteristics as we’ve mentioned previously.

Upload the following code to your board, and it will work straight away.

/*
  Rui Santos & Sara Santos - Random Nerd Tutorials
  Complete project details at https://RandomNerdTutorials.com/esp32-web-bluetooth/
  Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files.
  The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
*/
#include <BLEDevice.h>
#include <BLEServer.h>
#include <BLEUtils.h>
#include <BLE2902.h>

BLEServer* pServer = NULL;
BLECharacteristic* pSensorCharacteristic = NULL;
BLECharacteristic* pLedCharacteristic = NULL;
bool deviceConnected = false;
bool oldDeviceConnected = false;
uint32_t value = 0;

const int ledPin = 2; // Use the appropriate GPIO pin for your setup

// See the following for generating UUIDs:
// https://www.uuidgenerator.net/
#define SERVICE_UUID        "19b10000-e8f2-537e-4f6c-d104768a1214"
#define SENSOR_CHARACTERISTIC_UUID "19b10001-e8f2-537e-4f6c-d104768a1214"
#define LED_CHARACTERISTIC_UUID "19b10002-e8f2-537e-4f6c-d104768a1214"

class MyServerCallbacks: public BLEServerCallbacks {
  void onConnect(BLEServer* pServer) {
    deviceConnected = true;
  };

  void onDisconnect(BLEServer* pServer) {
    deviceConnected = false;
  }
};

class MyCharacteristicCallbacks : public BLECharacteristicCallbacks {
  void onWrite(BLECharacteristic* pLedCharacteristic) {
    String value = pLedCharacteristic->getValue();
    if (value.length() > 0) {
      Serial.print("Characteristic event, written: ");
      Serial.println(static_cast<int>(value[0])); // Print the integer value

      int receivedValue = static_cast<int>(value[0]);
      if (receivedValue == 1) {
        digitalWrite(ledPin, HIGH);
      } else {
        digitalWrite(ledPin, LOW);
      }
    }
  }
};

void setup() {
  Serial.begin(115200);
  pinMode(ledPin, OUTPUT);

  // Create the BLE Device
  BLEDevice::init("ESP32");

  // Create the BLE Server
  pServer = BLEDevice::createServer();
  pServer->setCallbacks(new MyServerCallbacks());

  // Create the BLE Service
  BLEService *pService = pServer->createService(SERVICE_UUID);

  // Create a BLE Characteristic
  pSensorCharacteristic = pService->createCharacteristic(
                      SENSOR_CHARACTERISTIC_UUID,
                      BLECharacteristic::PROPERTY_READ   |
                      BLECharacteristic::PROPERTY_WRITE  |
                      BLECharacteristic::PROPERTY_NOTIFY |
                      BLECharacteristic::PROPERTY_INDICATE
                    );

  // Create the ON button Characteristic
  pLedCharacteristic = pService->createCharacteristic(
                      LED_CHARACTERISTIC_UUID,
                      BLECharacteristic::PROPERTY_WRITE
                    );

  // Register the callback for the ON button characteristic
  pLedCharacteristic->setCallbacks(new MyCharacteristicCallbacks());

  // https://www.bluetooth.com/specifications/gatt/viewer?attributeXmlFile=org.bluetooth.descriptor.gatt.client_characteristic_configuration.xml
  // Create a BLE Descriptor
  pSensorCharacteristic->addDescriptor(new BLE2902());
  pLedCharacteristic->addDescriptor(new BLE2902());

  // Start the service
  pService->start();

  // Start advertising
  BLEAdvertising *pAdvertising = BLEDevice::getAdvertising();
  pAdvertising->addServiceUUID(SERVICE_UUID);
  pAdvertising->setScanResponse(false);
  pAdvertising->setMinPreferred(0x0);  // set value to 0x00 to not advertise this parameter
  BLEDevice::startAdvertising();
  Serial.println("Waiting a client connection to notify...");
}

void loop() {
  // notify changed value
  if (deviceConnected) {
    pSensorCharacteristic->setValue(String(value).c_str());
    pSensorCharacteristic->notify();
    value++;
    Serial.print("New value notified: ");
    Serial.println(value);
    delay(3000); // bluetooth stack will go into congestion, if too many packets are sent, in 6 hours test i was able to go as low as 3ms
  }
  // disconnecting
  if (!deviceConnected && oldDeviceConnected) {
    Serial.println("Device disconnected.");
    delay(500); // give the bluetooth stack the chance to get things ready
    pServer->startAdvertising(); // restart advertising
    Serial.println("Start advertising");
    oldDeviceConnected = deviceConnected;
  }
  // connecting
  if (deviceConnected && !oldDeviceConnected) {
    // do stuff here on connecting
    oldDeviceConnected = deviceConnected;
    Serial.println("Device Connected");
  }
}

View raw code

This is one of the simplest BLE code examples that turn the ESP32 into a BLE device that writes and listens to changes on characteristics. If you understand how this code works, you can easily modify it to serve more complex projects with more characteristics and services.

We recommend continuing reading to learn how the code works.

How the Code Works

In summary, this code sets up a BLE server with two characteristics—one for reading sensor data and another for controlling an LED. When a BLE client connects, it can read sensor data and send commands (write on the characteristic) to turn the LED on or off. The code also handles device connection and disconnection events.

Including Libraries

First, you need to import the following libraries to deal with Bluetooth.

#include <BLEDevice.h>
#include <BLEServer.h>
#include <BLEUtils.h>
#include <BLE2902.h>

Creating Global Variables

Then you create some global variables to use later in your code.

BLEServer* pServer = NULL;
BLECharacteristic* pSensorCharacteristic = NULL;
BLECharacteristic* pLedCharacteristic = NULL;
bool deviceConnected = false;
bool oldDeviceConnected = false;
uint32_t value = 0;

const int ledPin = 2; // Use the appropriate GPIO pin for your setup

• pServer: Pointer to the BLEServer object.
• pSensorCharacteristic: Pointer to the BLECharacteristic for reading the sensor value.
• pLedCharacteristic: Pointer to the BLECharacteristic for controlling an LED.
• deviceConnected: A boolean variable to track whether a BLE device is connected.
• oldDeviceConnected: A boolean variable to track the previous connection status.
• value: An integer variable used to store and send sensor values.
• ledPin: The GPIO pin number to which an LED is connected.

UUID Definitions

Then, you set the UUIDs for the Service and Characteristics. Those UUIDs were created using the uuidgenerator website. You can generate your own UUIDs for your application, but for this example, we recommend using the same UUIDs we’re using.

#define SERVICE_UUID        "19b10000-e8f2-537e-4f6c-d104768a1214"
#define SENSOR_CHARACTERISTIC_UUID "19b10001-e8f2-537e-4f6c-d104768a1214"
#define LED_CHARACTERISTIC_UUID "19b10002-e8f2-537e-4f6c-d104768a1214"

BLE Server and Callbacks

Then, you create several callback functions. The MyServerCallbacks defines a callback function for device connection and disconnection events. In this case, we change the value of the deviceConnected variable to true or false depending on the connection state.

class MyServerCallbacks: public BLEServerCallbacks {
    void onConnect(BLEServer* pServer) {
      deviceConnected = true;
    };

    void onDisconnect(BLEServer* pServer) {
      deviceConnected = false;
    }
};

The MyCharacteristicCallbacks defines a callback function to read the value of a characteristic when it changes. In our code, we’ll set this callback function for the LED_CHARACTERISTIC to detect when the value of the characteristic has changed. We’ll turn the LED on and off accordingly to the value of that characteristic.

class MyCharacteristicCallbacks : public BLECharacteristicCallbacks {
    void onWrite(BLECharacteristic* onCharacteristic) {
        String value = onCharacteristic->getValue();
        if (value.length() > 0) {
            Serial.print("Characteristic event, written: ");
            Serial.println(static_cast<int>(value[0])); // Print the integer value

            int receivedValue = static_cast<int>(value[0]);
            if (receivedValue == 1) {
                digitalWrite(ledPin, HIGH);
            } else {
                digitalWrite(ledPin, LOW);
            }
        }
    }
};

setup()

In the setup(), initialize serial communication for debugging.

Serial.begin(115200);

Set the ledPin as an output.

pinMode(ledPin, OUTPUT);

Initialize the ESP32 as BLE device called ESP32. You can call it any other name, but for this project we recommend using this name to be compatible with the app we’ll build later on.

Then, create a BLE server and set its callbacks for connection and disconnection.

// Create the BLE Server
pServer = BLEDevice::createServer();
pServer->setCallbacks(new MyServerCallbacks());

Create a BLE service with the UUID we’ve defined earlier.

// Create the BLE Service
BLEService *pService = pServer->createService(SERVICE_UUID);

Create a BLE characteristic (inside the service we just created) for sensor values and set its properties (read, write, notify, indicate).

// Create a BLE Characteristic
pSensorCharacteristic = pService->createCharacteristic(
                    SENSOR_CHARACTERISTIC_UUID,
                    BLECharacteristic::PROPERTY_READ   |
                    BLECharacteristic::PROPERTY_WRITE  |
                    BLECharacteristic::PROPERTY_NOTIFY |
                    BLECharacteristic::PROPERTY_INDICATE
                  );

Create a BLE characteristic for the LED characteristic and set its property (write).

// Create the ON button Characteristic
pLedCharacteristic = pService->createCharacteristic(
                    LED_CHARACTERISTIC_UUID,
                    BLECharacteristic::PROPERTY_WRITE
                  );

Register the callback for the LED characteristic, so that we detect when a new value was written on that characteristic.

pLedCharacteristic->setCallbacks(new MyCharacteristicCallbacks());

Add BLE descriptors (BLE2902) to both characteristics.

Start the BLE service.

// Start the service
pService->start();

And finally, configure advertising settings and start advertising.

// Start advertising
BLEAdvertising *pAdvertising = BLEDevice::getAdvertising();
pAdvertising->addServiceUUID(SERVICE_UUID);
pAdvertising->setScanResponse(false);
pAdvertising->setMinPreferred(0x0);  // set value to 0x00 to not advertise this parameter
BLEDevice::startAdvertising();
Serial.println("Waiting a client connection to notify...");

loop()

In the loop(), if a BLE device is connected (deviceConnected is true), it updates the sensor value, notifies the client, and increments the value.

if (deviceConnected) {
    pSensorCharacteristic->setValue(String(value).c_str());
    pSensorCharacteristic->notify();
    value++;
    Serial.print("New value notified: ");
    Serial.println(value);
    delay(3000); // bluetooth stack will go into congestion, if too many packets are sent, in 6 hours test i was able to go as low as 3ms
}

If a device disconnects and oldDeviceConnected is true, it restarts advertising and logs a message.
If a device connects and oldDeviceConnected is false, it logs a message (you can add your own logic here for actions to be taken on connection).

// disconnecting
if (!deviceConnected && oldDeviceConnected) {
    Serial.println("Device disconnected.");
    delay(500); // give the bluetooth stack the chance to get things ready
    pServer->startAdvertising(); // restart advertising
    Serial.println("Start advertising");
    oldDeviceConnected = deviceConnected;
}
// connecting
if (deviceConnected && !oldDeviceConnected) {
    // do stuff here on connecting
    oldDeviceConnected = deviceConnected;
    Serial.println("Device Connected");
}

Uploading the Code

Upload the code to your ESP32 board. After uploading, open the Serial Monitor and restart your board.

You’ll see that it initialized the BLE service and is waiting for a connection from a client.

Now that you’ve set the ESP32 as a BLE Client, we’ll create the web app so that we can interact with the ESP32 via Bluetooth using our web browser.

Alternatively, you can use our web app by going to this URL:

•  https://ruisantosdotme.github.io/esp32-web-ble

Connect to your ESP32 BLE device and see the values sent by the ESP32 being displayed on the interface and control the ESP32 on-board LED with the ON and OFF buttons. If you want to learn how to build the web app, continue reading.

Creating the Web BLE App

Create an HTML file called index.html with the following code (it contains both the HTML to build the web page and Javascript to handle Web Bluetooth).

<!--
  Rui Santos
  Complete project details at https://RandomNerdTutorials.com/esp32-web-bluetooth/

  Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files.
  The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
-->

<!DOCTYPE html>
<html>
<head>
    <title>ESP32 Web BLE App</title>
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <link rel="icon" type="image/png" href="">
</head>
<body>
  <h1>ESP32 Web BLE Application</h1>
  <button id="connectBleButton">Connect to BLE Device</button>
  <button id="disconnectBleButton">Disconnect BLE Device</button>
  <p>BLE state: <strong><span id="bleState" style="color:#d13a30;">Disconnected</span></strong></p>
  <h2>Fetched Value</h2>
  <p><span id="valueContainer">NaN</span></p>
  <p>Last reading: <span id="timestamp"></span></p>
  <h2>Control GPIO 2</h2>
  <button id="onButton">ON</button>
  <button id="offButton">OFF</button>
  <p>Last value sent: <span id="valueSent"></span></p>
  <p><a href="https://randomnerdtutorials.com/">Created by RandomNerdTutorials.com</a></p>
  <p><a href="https://RandomNerdTutorials.com/esp32-web-bluetooth/">Read the full project here.</a></p>
</body>
<script>
    // DOM Elements
    const connectButton = document.getElementById('connectBleButton');
    const disconnectButton = document.getElementById('disconnectBleButton');
    const onButton = document.getElementById('onButton');
    const offButton = document.getElementById('offButton');
    const retrievedValue = document.getElementById('valueContainer');
    const latestValueSent = document.getElementById('valueSent');
    const bleStateContainer = document.getElementById('bleState');
    const timestampContainer = document.getElementById('timestamp');

    //Define BLE Device Specs
    var deviceName ='ESP32';
    var bleService = '19b10000-e8f2-537e-4f6c-d104768a1214';
    var ledCharacteristic = '19b10002-e8f2-537e-4f6c-d104768a1214';
    var sensorCharacteristic= '19b10001-e8f2-537e-4f6c-d104768a1214';

    //Global Variables to Handle Bluetooth
    var bleServer;
    var bleServiceFound;
    var sensorCharacteristicFound;

    // Connect Button (search for BLE Devices only if BLE is available)
    connectButton.addEventListener('click', (event) => {
        if (isWebBluetoothEnabled()){
            connectToDevice();
        }
    });

    // Disconnect Button
    disconnectButton.addEventListener('click', disconnectDevice);

    // Write to the ESP32 LED Characteristic
    onButton.addEventListener('click', () => writeOnCharacteristic(1));
    offButton.addEventListener('click', () => writeOnCharacteristic(0));

    // Check if BLE is available in your Browser
    function isWebBluetoothEnabled() {
        if (!navigator.bluetooth) {
            console.log("Web Bluetooth API is not available in this browser!");
            bleStateContainer.innerHTML = "Web Bluetooth API is not available in this browser!";
            return false
        }
        console.log('Web Bluetooth API supported in this browser.');
        return true
    }

    // Connect to BLE Device and Enable Notifications
    function connectToDevice(){
        console.log('Initializing Bluetooth...');
        navigator.bluetooth.requestDevice({
            filters: [{name: deviceName}],
            optionalServices: [bleService]
        })
        .then(device => {
            console.log('Device Selected:', device.name);
            bleStateContainer.innerHTML = 'Connected to device ' + device.name;
            bleStateContainer.style.color = "#24af37";
            device.addEventListener('gattservicedisconnected', onDisconnected);
            return device.gatt.connect();
        })
        .then(gattServer =>{
            bleServer = gattServer;
            console.log("Connected to GATT Server");
            return bleServer.getPrimaryService(bleService);
        })
        .then(service => {
            bleServiceFound = service;
            console.log("Service discovered:", service.uuid);
            return service.getCharacteristic(sensorCharacteristic);
        })
        .then(characteristic => {
            console.log("Characteristic discovered:", characteristic.uuid);
            sensorCharacteristicFound = characteristic;
            characteristic.addEventListener('characteristicvaluechanged', handleCharacteristicChange);
            characteristic.startNotifications();
            console.log("Notifications Started.");
            return characteristic.readValue();
        })
        .then(value => {
            console.log("Read value: ", value);
            const decodedValue = new TextDecoder().decode(value);
            console.log("Decoded value: ", decodedValue);
            retrievedValue.innerHTML = decodedValue;
        })
        .catch(error => {
            console.log('Error: ', error);
        })
    }

    function onDisconnected(event){
        console.log('Device Disconnected:', event.target.device.name);
        bleStateContainer.innerHTML = "Device disconnected";
        bleStateContainer.style.color = "#d13a30";

        connectToDevice();
    }

    function handleCharacteristicChange(event){
        const newValueReceived = new TextDecoder().decode(event.target.value);
        console.log("Characteristic value changed: ", newValueReceived);
        retrievedValue.innerHTML = newValueReceived;
        timestampContainer.innerHTML = getDateTime();
    }

    function writeOnCharacteristic(value){
        if (bleServer && bleServer.connected) {
            bleServiceFound.getCharacteristic(ledCharacteristic)
            .then(characteristic => {
                console.log("Found the LED characteristic: ", characteristic.uuid);
                const data = new Uint8Array([value]);
                return characteristic.writeValue(data);
            })
            .then(() => {
                latestValueSent.innerHTML = value;
                console.log("Value written to LEDcharacteristic:", value);
            })
            .catch(error => {
                console.error("Error writing to the LED characteristic: ", error);
            });
        } else {
            console.error ("Bluetooth is not connected. Cannot write to characteristic.")
            window.alert("Bluetooth is not connected. Cannot write to characteristic. \n Connect to BLE first!")
        }
    }

    function disconnectDevice() {
        console.log("Disconnect Device.");
        if (bleServer && bleServer.connected) {
            if (sensorCharacteristicFound) {
                sensorCharacteristicFound.stopNotifications()
                    .then(() => {
                        console.log("Notifications Stopped");
                        return bleServer.disconnect();
                    })
                    .then(() => {
                        console.log("Device Disconnected");
                        bleStateContainer.innerHTML = "Device Disconnected";
                        bleStateContainer.style.color = "#d13a30";

                    })
                    .catch(error => {
                        console.log("An error occurred:", error);
                    });
            } else {
                console.log("No characteristic found to disconnect.");
            }
        } else {
            // Throw an error if Bluetooth is not connected
            console.error("Bluetooth is not connected.");
            window.alert("Bluetooth is not connected.")
        }
    }

    function getDateTime() {
        var currentdate = new Date();
        var day = ("00" + currentdate.getDate()).slice(-2); // Convert day to string and slice
        var month = ("00" + (currentdate.getMonth() + 1)).slice(-2);
        var year = currentdate.getFullYear();
        var hours = ("00" + currentdate.getHours()).slice(-2);
        var minutes = ("00" + currentdate.getMinutes()).slice(-2);
        var seconds = ("00" + currentdate.getSeconds()).slice(-2);

        var datetime = day + "/" + month + "/" + year + " at " + hours + ":" + minutes + ":" + seconds;
        return datetime;
    }


</script>

</html>

View raw code

How does it Work?

This HTML and JavaScript code represents a web application that allows you to connect to an ESP32 device over Bluetooth Low Energy (BLE). The application provides a user interface to interact with the ESP32, including reading values and controlling an LED. Let’s break down the code and understand how it works.

Below you can see the application we’ll build (on the right with CSS, and on the left without CSS).

HTML Page

First, we create a simple HTML page. We first create two buttons: one to connect the browser to BLE devices and another to disconnect.

<button id="connectBleButton"> Connect to BLE Device</button>
<button id="disconnectBleButton"> Disconnect BLE Device</button>

Then, we have a paragraph that will then display the BLE connection state.

<p>BLE state: <strong><span id="bleState" style="color:#d13a30;">Disconnected</span></strong></p>

Then, we have a section to display the value written by the ESP32 on the sensor value characteristic.

<h2>Fetched Value</h2>
<p><span id="valueContainer">NaN</span></p>

We’ll also display the timestamp of when the value was received in the following paragraph.

<p>Last reading: <span id="timestamp"></span></p>

Finally, we have a section with two buttons. One to write 1 on the LED characteristic (to turn it on) and another to write 0 on the LED characteristic (to turn it off).

<button id="onButton">ON</button>
<button id="offButton">OFF</button>

We also have a paragraph to display the last value that was sent to control the LED.

<p>Last value sent: <span id="valueSent"></span></p>

JavaScript – Web BLE

Next, inside the <script></script> tags, we have the Javascript code responsible for handling the buttons and connecting to the ESP32 via Bluetooth and interacting with its characteristics.

DOM Elements

First, we select the HTML elements and assign them to a variable name for easier manipulation along the code.

const connectButton = document.getElementById('connectBleButton');
const disconnectButton = document.getElementById('disconnectBleButton');
const onButton = document.getElementById('onButton');
const offButton = document.getElementById('offButton');
const retrievedValue = document.getElementById('valueContainer');
const latestValueSent = document.getElementById('valueSent');
const bleStateContainer = document.getElementById('bleState');
const timestampContainer = document.getElementById('timestamp');

BLE Device Specifications

Then, we add the BLE Device specifications we want to connect to. We already set up the ESP32 as a BLE server. So, we can set its specs here, the name, the UUID of the service, and the UUIDs of the characteristics we want to interact with—these should be the same you’ve set on your Arduino code.

//Define BLE Device Specs
var deviceName ='ESP32';
var bleService = '19b10000-e8f2-537e-4f6c-d104768a1214';
var ledCharacteristic = '19b10002-e8f2-537e-4f6c-d104768a1214';
var sensorCharacteristic= '19b10001-e8f2-537e-4f6c-d104768a1214';

Defining Global Variables

Then, we create some global variables to handle Bluetooth communication and device discovery later on in our code.

var bleServer;
var bleServiceFound;
var sensorCharacteristicFound;

Assigning Events to Buttons

Then, we add event listeners to the buttons to trigger actions when they are clicked.

Connect To BLE Device Button

The Connect To BLE Device button will trigger the connectToDevice() function. But first, we check if the Web BLE Javascript API is available in your browser before proceeding and we display a message on the bleStateContainer in case Web BLE is not supported.

// Connect Button (search for BLE Devices only if BLE is available)
connectButton.addEventListener('click', (event) => {
    if (isWebBluetoothEnabled()){
        connectToDevice();
    }
});

To check if the Web Bluetooth is enabled in your browser, we created a function called isWebBluetoothEnabled(). The method that checks if Web BLE is enabled is nagivator.bluetooth.

// Check if BLE is available in your Browser
function isWebBluetoothEnabled() {
    if (!navigator.bluetooth) {
        console.log("Web Bluetooth API is not available in this browser!");
         bleStateContainer.innerHTML = "Web Bluetooth API is not available in this browser/device!";
         return false
    }
    console.log('Web Bluetooth API supported in this browser.');
    return true
}

Disconnect BLE Device Button

The Disconnect BLE Device Button will call the disconnectDevice function.

// Disconnect Button
disconnectButton.addEventListener('click', disconnectDevice);

ON and OFF Buttons

The on and off buttons will trigger the writeOnCharacteristic function. In case of the ON button, we’ll pass a 1 as a parameter, and in case of the OFF button, we’ll pass 0 as a parameter.

// Write to the ESP32 LED Characteristic
onButton.addEventListener('click', () => writeOnCharacteristic(1));
offButton.addEventListener('click', () => writeOnCharacteristic(0));

Connecting to BLE Device and search Services and Characteristics

The connectToDevice() function is triggered when you click on the Connect to BLE Device button. This function searches for our specific BLE Device, its Service and Characteristics.

function connectToDevice(){

The following lines of code search for BLE Devices with the name and Service that we’ve defined. There are other filters you can define to search for BLE devices, or you can opt to not add any filters and return all the BLE devices found.

navigator.bluetooth.requestDevice({
    filters: [{name: deviceName}],
    optionalServices: [bleService]
})

Once we’ve connected to your device, we display on the HTML interface that we are now connected to our device. We also add an event listener to our device, in case it disconnects (it will call the onDisconnected function).

.then(device => {
    console.log('Device Selected:', device.name);
    bleStateContainer.innerHTML = 'Connected to device ' + device.name;
    bleStateContainer.style.color = "#24af37";
    device.addEventListener('gattservicedisconnected', onDisconnected);
    return device.gatt.connect();
})

From the device, we can get our GATT server (the hierarchical structure that stores data in BLE protocol). We save our GATT server on our bleServer global variable. From the GATTT server, we can get the service with the UUID we’ve defined at the beginning of the code, bleService.

.then(gattServer =>{
    bleServer = gattServer;
    console.log("Connected to GATT Server");
    return bleServer.getPrimaryService(bleService);
 })

Once we’ve found our service, we save it on the global variable bleServiceFound, and get the sensor characteristic from the service.

.then(service => {
    bleServiceFound = service;
    console.log("Service discovered:", service.uuid);
    return service.getCharacteristic(sensorCharacteristic);
})

Now that we’ve found our sensor characteristic, we assigned it to the global sensorCharacteristicFound variable. We add an event listener to our characteristic to handle what happens when the characteristic value changes—we call the handleCharacteristicChange function. We also start notifications on that characteristic. Finally, we return the current value written on the characteristic.

.then(characteristic => {
    console.log("Characteristic discovered:", characteristic.uuid);
    sensorCharacteristicFound = characteristic;
    characteristic.addEventListener('characteristicvaluechanged', handleCharacteristicChange);
    characteristic.startNotifications();
    console.log("Notifications Started.");
    return characteristic.readValue();
})

Next, we display the value read from the characteristic on the corresponding HTML element.

.then(value => {
    console.log("Read value: ", value);
    const decodedValue = new TextDecoder().decode(value);
    console.log("Decoded value: ", decodedValue);
    retrievedValue.innerHTML = decodedValue;
})

Handle Characteristic Change

The handleCharacteristicChange() function will be called when the sensor characteristic value changes. That function gets the new value written on the characteristic and places it on the corresponding HTML element. Additionally, we also get the date and time to display when was the last time that the characteristic value has changed.

function handleCharacteristicChange(event){
    const newValueReceived = new TextDecoder().decode(event.target.value);
    console.log("Characteristic value changed: ", newValueReceived);
    retrievedValue.innerHTML = newValueReceived;
    timestampContainer.innerHTML = getDateTime();
}

Write on Characteristics

When you click on the ON or OFF buttons, the writeOnCharacteristicfunction will be called. That function first checks if we are connected to the BLE server. If we are, it gets the LED characteristic from the BLE Service we’ve found previously, and it writes the value we’ve passed as an argument on the characteristic.

if (bleServer && bleServer.connected) {
    bleServiceFound.getCharacteristic(ledCharacteristic)
     .then(characteristic => {
        console.log("Found the LED characteristic: ", characteristic.uuid);
        const data = new Uint8Array([value]);
        return characteristic.writeValue(data);
})

If we’re successful in writing to the characteristic, we update the last value written on the HTML interface.

.then(() => {
    latestValueSent.innerHTML = value;
    console.log("Value written to LEDcharacteristic:", value);
})

In case we try to write to the characteristic without being connected to BLE first, we’ll create a pop-up window informing that we need to connect to BLE first.

} else {
    console.error ("Bluetooth is not connected. Cannot write to characteristic.")
    window.alert("Bluetooth is not connected. Cannot write to characteristic. \n Connect to BLE first!")
}

Disconnect BLE Device

When you click on the Disconnect BLE Device button, the disconnectDevice() function is called. This function first checks if we’re connected to the server. Then, we stop the notifications on the sensorCharacteristic and we disconnect from the GATT server. Additionally, we also display some messages on the HTML interface and a pop-up window in case Bluetooth is not connected.

function disconnectDevice() {
    console.log("Disconnect Device.");
    if (bleServer && bleServer.connected) {
        if (sensorCharacteristicFound) {
            sensorCharacteristicFound.stopNotifications()
                .then(() => {
                    console.log("Notifications Stopped");
                    return bleServer.disconnect();
                })
                .then(() => {
                    console.log("Device Disconnected");
                    bleStateContainer.innerHTML = "Device Disconnected";
                    bleStateContainer.style.color = "#d13a30";

                })
                .catch(error => {
                    console.log("An error occurred:", error);
                });
        } else {
            console.log("No characteristic found to disconnect.");
        }
    } else {
        // Throw an error if Bluetooth is not connected
        console.error("Bluetooth is not connected.");
        window.alert("Bluetooth is not connected.")
    }
}

Testing the Web BLE App

Save your index.html file and drag it to your browser. The following page will open.

With the ESP32 running the code we’ve provided previously, let’s test the web app.

Start by clicking on the Connect to BLE Device button. A window will pop up and you should see the ESP32 BLE Device. Connect to that device.

You’ll see that the BLE Status will change to connected and you’ll start receiving the values written by the ESP32 on the sensor characteristic.

Simultaneously, you should get the following messages on the Arduino Serial Monitor showing that the connection was successful and the values being written on the sensor characteristic.

Getting back to the app, if you click the ON and OFF buttons you’ll be able to control the ESP32 LED on and off.

On the Serial Monitor, you’ll see that it detects the changes in the LED characteristic value.

Consequently, it will control the ESP32 onboard LED according to the value written on that characteristic.

Styling the Web BLE App

To make your web interface look better, we’ll add some CSS.

index.html

Copy the following to your index.html file. This is the same HTML we provided previously, but we added some CSS classes to style the web page.

<!--
  Rui Santos
  Complete project details at https://RandomNerdTutorials.com/esp32-web-bluetooth/

  Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files.
  The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
-->

<!DOCTYPE html>
<html>
<head>
    <title>ESP32 Web BLE App</title>
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <link rel="icon" type="image/png" href="favicon.ico">
    <link rel="stylesheet" type="text/css" href="style.css">
    <meta charset="UTF-8">
</head>
<body>
    <div class="topnav">
        <h1>ESP32 Web BLE Application</h1>
    </div>
    <div class="content">
        <div class="card-grid">
            <div class="card">
                <p>
                    <button id="connectBleButton" class="connectButton"> Connect to BLE Device</button>
                    <button id="disconnectBleButton" class="disconnectButton"> Disconnect BLE Device</button>
                </p>
                <p class="gray-label">BLE state: <strong><span id="bleState" style="color:#d13a30;">Disconnected</span></strong></p>
            </div>
        </div>
        <div class="card-grid">
            <div class="card">
                <h2>Fetched Value</h2>
                <p class="reading"><span id="valueContainer">NaN</span></p>
                <p class="gray-label">Last reading: <span id="timestamp"></span></p>
            </div>

            <div class="card">
                <h2>Control GPIO 2</h2>
                <button id="onButton" class="onButton">ON</button>
                <button id="offButton" class="offButton">OFF</button>
                <p class="gray-label">Last value sent: <span id="valueSent"></span></p>
            </div>
        </div>
    </div>
    <div class="footer">
        <p><a href="https://randomnerdtutorials.com/">Created by RandomNerdTutorials.com</a></p>
        <p><a href="https://RandomNerdTutorials.com/esp32-web-bluetooth/">Read the full project here.</a></p>
    </div>
</body>
<script>
    // DOM Elements
    const connectButton = document.getElementById('connectBleButton');
    const disconnectButton = document.getElementById('disconnectBleButton');
    const onButton = document.getElementById('onButton');
    const offButton = document.getElementById('offButton');
    const retrievedValue = document.getElementById('valueContainer');
    const latestValueSent = document.getElementById('valueSent');
    const bleStateContainer = document.getElementById('bleState');
    const timestampContainer = document.getElementById('timestamp');

    //Define BLE Device Specs
    var deviceName ='ESP32';
    var bleService = '19b10000-e8f2-537e-4f6c-d104768a1214';
    var ledCharacteristic = '19b10002-e8f2-537e-4f6c-d104768a1214';
    var sensorCharacteristic= '19b10001-e8f2-537e-4f6c-d104768a1214';

    //Global Variables to Handle Bluetooth
    var bleServer;
    var bleServiceFound;
    var sensorCharacteristicFound;

    // Connect Button (search for BLE Devices only if BLE is available)
    connectButton.addEventListener('click', (event) => {
        if (isWebBluetoothEnabled()){
            connectToDevice();
        }
    });

    // Disconnect Button
    disconnectButton.addEventListener('click', disconnectDevice);

    // Write to the ESP32 LED Characteristic
    onButton.addEventListener('click', () => writeOnCharacteristic(1));
    offButton.addEventListener('click', () => writeOnCharacteristic(0));

    // Check if BLE is available in your Browser
    function isWebBluetoothEnabled() {
        if (!navigator.bluetooth) {
            console.log('Web Bluetooth API is not available in this browser!');
            bleStateContainer.innerHTML = "Web Bluetooth API is not available in this browser/device!";
            return false
        }
        console.log('Web Bluetooth API supported in this browser.');
        return true
    }

    // Connect to BLE Device and Enable Notifications
    function connectToDevice(){
        console.log('Initializing Bluetooth...');
        navigator.bluetooth.requestDevice({
            filters: [{name: deviceName}],
            optionalServices: [bleService]
        })
        .then(device => {
            console.log('Device Selected:', device.name);
            bleStateContainer.innerHTML = 'Connected to device ' + device.name;
            bleStateContainer.style.color = "#24af37";
            device.addEventListener('gattservicedisconnected', onDisconnected);
            return device.gatt.connect();
        })
        .then(gattServer =>{
            bleServer = gattServer;
            console.log("Connected to GATT Server");
            return bleServer.getPrimaryService(bleService);
        })
        .then(service => {
            bleServiceFound = service;
            console.log("Service discovered:", service.uuid);
            return service.getCharacteristic(sensorCharacteristic);
        })
        .then(characteristic => {
            console.log("Characteristic discovered:", characteristic.uuid);
            sensorCharacteristicFound = characteristic;
            characteristic.addEventListener('characteristicvaluechanged', handleCharacteristicChange);
            characteristic.startNotifications();
            console.log("Notifications Started.");
            return characteristic.readValue();
        })
        .then(value => {
            console.log("Read value: ", value);
            const decodedValue = new TextDecoder().decode(value);
            console.log("Decoded value: ", decodedValue);
            retrievedValue.innerHTML = decodedValue;
        })
        .catch(error => {
            console.log('Error: ', error);
        })
    }

    function onDisconnected(event){
        console.log('Device Disconnected:', event.target.device.name);
        bleStateContainer.innerHTML = "Device disconnected";
        bleStateContainer.style.color = "#d13a30";

        connectToDevice();
    }

    function handleCharacteristicChange(event){
        const newValueReceived = new TextDecoder().decode(event.target.value);
        console.log("Characteristic value changed: ", newValueReceived);
        retrievedValue.innerHTML = newValueReceived;
        timestampContainer.innerHTML = getDateTime();
    }

    function writeOnCharacteristic(value){
        if (bleServer && bleServer.connected) {
            bleServiceFound.getCharacteristic(ledCharacteristic)
            .then(characteristic => {
                console.log("Found the LED characteristic: ", characteristic.uuid);
                const data = new Uint8Array([value]);
                return characteristic.writeValue(data);
            })
            .then(() => {
                latestValueSent.innerHTML = value;
                console.log("Value written to LEDcharacteristic:", value);
            })
            .catch(error => {
                console.error("Error writing to the LED characteristic: ", error);
            });
        } else {
            console.error ("Bluetooth is not connected. Cannot write to characteristic.")
            window.alert("Bluetooth is not connected. Cannot write to characteristic. \n Connect to BLE first!")
        }
    }

    function disconnectDevice() {
        console.log("Disconnect Device.");
        if (bleServer && bleServer.connected) {
            if (sensorCharacteristicFound) {
                sensorCharacteristicFound.stopNotifications()
                    .then(() => {
                        console.log("Notifications Stopped");
                        return bleServer.disconnect();
                    })
                    .then(() => {
                        console.log("Device Disconnected");
                        bleStateContainer.innerHTML = "Device Disconnected";
                        bleStateContainer.style.color = "#d13a30";

                    })
                    .catch(error => {
                        console.log("An error occurred:", error);
                    });
            } else {
                console.log("No characteristic found to disconnect.");
            }
        } else {
            // Throw an error if Bluetooth is not connected
            console.error("Bluetooth is not connected.");
            window.alert("Bluetooth is not connected.")
        }
    }

    function getDateTime() {
        var currentdate = new Date();
        var day = ("00" + currentdate.getDate()).slice(-2); // Convert day to string and slice
        var month = ("00" + (currentdate.getMonth() + 1)).slice(-2);
        var year = currentdate.getFullYear();
        var hours = ("00" + currentdate.getHours()).slice(-2);
        var minutes = ("00" + currentdate.getMinutes()).slice(-2);
        var seconds = ("00" + currentdate.getSeconds()).slice(-2);

        var datetime = day + "/" + month + "/" + year + " at " + hours + ":" + minutes + ":" + seconds;
        return datetime;
    }


</script>

</html>

View raw code

style.css

On the same folder of your index.html file, create a file called style.css with the following.

html {
    font-family: Arial, Helvetica, sans-serif;
    display: inline-block;
    text-align: center;
}
h1 {
    font-size: 1.8rem;
    color: white;
}
.topnav {
    overflow: hidden;
    background-color: #0A1128;
}
body {
    margin: 0;
}
.content {
    padding: 50px;
}
.card-grid {
    max-width: 800px;
    margin: 0 auto;
    margin-bottom: 30px;
    display: grid;
    grid-gap: 2rem;
    grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
}
.card {
    background-color: white;
    box-shadow: 2px 2px 12px 1px rgba(140,140,140,.5);
}
button {
    color: white;
    padding: 14px 20px;
    margin: 8px 0;
    border: none;
    cursor: pointer;
    border-radius: 4px;
}

.onButton{
    background-color: #1b8a94;
}

.offButton{
    background-color: #5f6c6d;
}

.connectButton{
    background-color: #24af37;
}

.disconnectButton{
    background-color: #d13a30;
}

.gray-label {
    color: #bebebe;
    font-size: 1rem;
}

.reading {
    font-size: 1.8rem;
}

View raw code

favicon

Additionally, add the following favicon to your folder.

• Download the .zip folder and unzip it to get the favicon.ico file

Demonstration

After creating all the required files in the same folder, open the index.html file on your web browser.

This is what the application will look like. It works exactly the same way as we’ve seen previously, but it looks much better.

Taking it Further – Hosting your Web BLE App

At the moment, you can only connect to the ESP32 via BLE by opening the index.html file on the web browser of your computer. If you want to open it on your smartphone or any other device you would need to copy that file to the device and then, open it on the web browser. This is not very convenient.

The best way to have access to your web app on any device is to host your files on a server. To work with BLE, the files need to be served via HTTPS.

To host our web app, we’ll use GitHub pages. If you don’t have a GitHub account, create one before proceeding.

1. On your account dashboard, click on the + icon and create a New repository.

2. The following page will load. Give a name to your repository, and make sure it is set to Public. Then, click on Create repository.

3. Then, click on Add file > Upload files to repository.

4. Then, drag your index.html, style.css, and favicon.ico files to the repository. Then, click on Commit changes.

5. Then, go to Settings > Pages and make sure you have the options highlighted in red below. Then, click the Save button.

After submitting, wait a few minutes for the web page to be available. Your web app will be in the following domain:

YOUR_GITHUB_USERNAME.github.io/YOUR_REPOSITORY_NAME

In our case, it is available on the following web page: https://ruisantosdotme.github.io/esp32-web-ble/

https://ruisantosdotme.github.io/esp32-web-ble/

Demonstration

Now, you can access your Web BLE App on any device (that supports Web BLE) with a web browser by going to that URL. Then, you can connect to the ESP32 via BLE using that device and read and write on its characteristics.

Now you can access the Web App on your smartphone or tablet and control your ESP32 BLE Device from there.

Wrapping Up

In this tutorial, you learned about the Web BLE technology. In simple terms, it is a JavaScript API that allows us to create web apps to interact with BLE devices from any web browser that supports Web BLE.

You learned how to set the ESP32 as a BLE device with a Service and Characteristics and you create a web application to interact with the ESP32 characteristics. This way, we are now able to control the ESP32 via BLE using our browser.

We hope you’ve found this tutorial useful and that it helped you start into this relatively new technology.

We have other ESP32 tutorials related to Bluetooth that you may like:

• Getting Started with ESP32 Bluetooth Low Energy (BLE) on Arduino IDE
• ESP32 BLE Server and Client (Bluetooth Low Energy)
• ESP32 Bluetooth Classic with Arduino IDE – Getting Started
• ESP32 BLE Peripheral (Server): Environmental Sensing Service

Do you want to learn more about the ESP32? Check out all our Resources:

• Learn ESP32 with Arduino IDE (eBook)
• Build Web Servers with ESP32 and ESP8266 (eBook)
• Firebase Web App with ESP32 and ESP8266 (eBook)
• SMART HOME with Raspberry Pi, ESP32, and ESP8266
• Free ESP32 Projects and Tutorials…

Thanks for reading.