Web server on ESP32: How to update and display sensor values?

In many IoT Applications we monitor sensor data and we want to display it in wab page. Web page requires frequent refresh to get the update from ESP32. To solve this problem you have two options, first is refresh page with HTML Tag: ex. refresh at every 30 seconds.

<meta http-equiv=”refresh” content=”30″>
Disadvantage of using HTML refresh tag is, it flickers the screen and loads complete page again and again.

In this tutorial I will show you how to display ADC data and update without refreshing web page. You can do a lot of things with this. At the end we will see some advance applications of this. To make this possible we need to use javaScript Ajax.

Tools you need

only ESP32, Laptop and USB cable

What you will learn?

  1. ESP32 Ajax web pageĀ  to display and update sensor data without refresh.
  2. Getting data from ESP32 without page refresh and update it in web page dynamically. such as ADC values or temperature readings.

HTML and Java Script, AJAX basics

AJAX is about updating parts of a web page, without reloading the whole page.

What is AJAX?

AJAX = Asynchronous JavaScript and XML.

AJAX is a technique for creating fast and dynamic web pages.

AJAX allows web pages to be updated asynchronously by exchanging small amounts of data with the server behind the scenes. This means that it is possible to update parts of a web page, without reloading the whole page.

Classic web pages, (which do not use AJAX) must reload the entire page if the content should change.

Examples of applications using AJAX: Google Maps, Gmail, Youtube, and Facebook tabs.

How ESP32 AJAX Works ?

In ESP32 NodeMCU we create two pages on server. First that loads as normal webpage and second webpage is behind the scene i.e. AJAX.

AJAX is Based on Internet Standards

AJAX is based on internet standards, and uses a combination of:

  • XMLHttpRequest object (to exchange data asynchronously with a server)
  • JavaScript/DOM (to display/interact with the information)
  • CSS (to style the data)
  • XML (often used as the format for transferring data)

Read More on Ajax at https://www.w3schools.com/asp/asp_ajax_intro.asp

ESP ajax example

Create Web Page with AJAX

How to create a web page in ESP and uploading it read this post ?

Make your HTML page and test it in browser. Then make it header file as shown in below code. DON’T JUST CHANGE THE EXTENSION TO .h

Create index.h file near to your final sketch. Copy and paste below code. Program is commented well. You can read more on HTML and AJAX at w3schools.com

index.h File

const char MAIN_page[] PROGMEM = R"=====(
<!DOCTYPE html>
    max-width: 400px;
     min-height: 250px;
     background: #02b875;
     padding: 30px;
     box-sizing: border-box;
     color: #FFF;
     box-shadow: 0px 2px 18px -4px rgba(0,0,0,0.75);

<div class="card">
  <h4>The ESP32 Update web page without refresh</h4><br>
  <h1>Sensor Value:<span id="ADCValue">0</span></h1><br>
  <br><a href="https://circuits4you.com">Circuits4you.com</a>

setInterval(function() {
  // Call a function repetatively with 2 Second interval
}, 2000); //2000mSeconds update rate

function getData() {
  var xhttp = new XMLHttpRequest();
  xhttp.onreadystatechange = function() {
    if (this.readyState == 4 && this.status == 200) {
      document.getElementById("ADCValue").innerHTML =
  xhttp.open("GET", "readADC", true);

ESP32 Web Server Program Main INO file

 * ESP32 AJAX Demo
 * Updates and Gets data from webpage without page refresh
 * https://circuits4you.com
#include <WiFi.h>
#include <WiFiClient.h>
#include <WebServer.h>

#include "index.h"  //Web page header file

WebServer server(80);

//Enter your SSID and PASSWORD
const char* ssid = ".........";
const char* password = ".........";

// This routine is executed when you open its IP in browser
void handleRoot() {
 String s = MAIN_page; //Read HTML contents
 server.send(200, "text/html", s); //Send web page
void handleADC() {
 int a = analogRead(A0);
 String adcValue = String(a);
 server.send(200, "text/plane", adcValue); //Send ADC value only to client ajax request

// Setup

void setup(void){
  Serial.println("Booting Sketch...");

//ESP32 As access point
  WiFi.mode(WIFI_AP); //Access Point mode
  WiFi.softAP(ssid, password);
//ESP32 connects to your wifi -----------------------------------
  WiFi.mode(WIFI_STA); //Connectto your wifi
  WiFi.begin(ssid, password);

  Serial.println("Connecting to ");

  //Wait for WiFi to connect
  while(WiFi.waitForConnectResult() != WL_CONNECTED){      
  //If connection successful show IP address in serial monitor
  Serial.print("Connected to ");
  Serial.print("IP address: ");
  Serial.println(WiFi.localIP());  //IP address assigned to your ESP
  server.on("/", handleRoot);      //This is display page
  server.on("/readADC", handleADC);//To get update of ADC Value only
  server.begin();                  //Start server
  Serial.println("HTTP server started");

// This routine is executed when you open its IP in browser
void loop(void){


Make WiFi network configuration changes as per your network. Upload the program and test it. Open serial monitor to observe the IP address and other actions.


Open Serial monitor and get the IP address

Enter the IP address in web browser

ESP32 display and update sensor data on web page

In many application you may want to use jQuery and Bigger size HTML, CSS files and images. You can get clear idea from these two advanced posts

  1. SPIFFS (ESP File System) to load webpages in Flash memory
  2. Upload images in ESP8266 and display it in web page
  3. Using jQuery advance web server on ESP8266

4 thoughts on “Web server on ESP32: How to update and display sensor values?

  1. Thank you for the great tutorial. I have a problem that after few hours the web server stops responding. The loop is still runing but no web. I read some sources that it is needed to call client.stop() but they use Wifiserver and wifi.h. I am not sure for WebServer object.

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