ESP32 - Web Plotter

In this tutorial, we will learn how to create a web plotter that looks similar to the Serial Plotter in Arduino IDE. With this setup, you can easily check real-time data from an ESP32 using a web browser on your smartphone or PC. It'll display the data in a graph, just like you see it on the Serial Plotter in Arduino IDE.

ESP32 web plotter

Hardware Used In This Tutorial

1×ESP-WROOM-32 Dev Module
1×USB Cable Type-C
1×(Recommended) ESP32 Screw Terminal Adapter

Or you can buy the following sensor kit:

1×DIYables Sensor Kit 30 types, 69 units
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How Web Plotter Works

  • The ESP32 code creates a web server and WebSocket server.
  • When an users accesses the webpage on the ESP32 board via a web browser, the Web server on ESP32 board returns the web content (HTML, CSS, JavaScript) back to the web browser.
  • The JavaScript code draws a graph on web browser that looks like Serial Plotter
  • When an user clicks the connect button on the web, the JavaScript code creates a WebSocket connection to the WebSocket server on the ESP32 board
  • ESP32 sends data via the WebSocket connection to the web browser in format that silimar to the Serial Plotter (see the next part)
  • The JavaScript code on web browser receives the data and plot the data on the graph.

The data format that ESP32 sends to web plotter

To plot multiple variables, we need to separate variables from each other by “\t” or " " character. The last value MUST be terminated by “\r\n” characters.

In detail:

  • The first variable
webSocket.broadcastTXT(variable_1);
  • The middle variables
webSocket.broadcastTXT(" "); // a tab '\t' or space ' ' character is printed between the two values. webSocket.broadcastTXT(variable_2); webSocket.broadcastTXT(" "); // a tab '\t' or space ' ' character is printed between the two values. webSocket.broadcastTXT(variable_3);
  • The last variable
webSocket.broadcastTXT(" "); // a tab '\t' or space ' ' character is printed between the two values. webSocket.broadcastTXT(variable_4); webSocket.broadcastTXT("\r\n"); // the last value is terminated by a carriage return and a newline characters.

For more detail, please refer to ESP32 - Serial Plotter tutorial

ESP32 Code - Web Plotter

The webpage's content (HTML, CSS, JavaScript) are stored separately on an index.h file. So, we will have two code files on Arduino IDE:

  • An .ino file that is ESP32 code, which creates a web sever and WebSocket server
  • An .h file, which contains the webpage's content.

Quick Instructions

  • If this is the first time you use ESP32, see how to setup environment for ESP32 on Arduino IDE.
  • Connect the ESP32 board to your PC via a micro USB cable
  • Open Arduino IDE on your PC.
  • Select the right ESP32 board (e.g. ESP32 Dev Module) and COM port.
  • Open the Library Manager by clicking on the Library Manager icon on the left navigation bar of Arduino IDE.
  • Search “ESPAsyncWebServer”, then find the ESPAsyncWebServer created by lacamera.
  • Click Install button to install ESPAsyncWebServer library.
ESP32 ESPAsyncWebServer library
  • You will be asked to install the dependency. Click Install All button.
ESP32 ESPAsyncWebServer dependencies library
  • Search “WebSockets”, then find the WebSockets created by Markus Sattler.
  • Click Install button to install WebSockets library.
ESP32 WebSockets library
  • On Arduino IDE, create new sketch, Give it a name, for example, esp32io.com.ino
  • Copy the below code and open with Arduino IDE
/* * This ESP32 code is created by esp32io.com * * This ESP32 code is released in the public domain * * For more detail (instruction and wiring diagram), visit https://esp32io.com/tutorials/esp32-web-plotter */ #include <WiFi.h> #include <ESPAsyncWebServer.h> #include <WebSocketsServer.h> #include "index.h" const char* ssid = "YOUR_WIFI_SSID"; // CHANGE IT const char* password = "YOUR_WIFI_PASSWORD"; // CHANGE IT AsyncWebServer server(80); WebSocketsServer webSocket = WebSocketsServer(81); // WebSocket server on port 81 int last_update = 0; void setup() { Serial.begin(9600); delay(1000); // Connect to Wi-Fi WiFi.begin(ssid, password); while (WiFi.status() != WL_CONNECTED) { delay(1000); Serial.println("Connecting to WiFi..."); } Serial.println("Connected to WiFi"); // Initialize WebSocket server webSocket.begin(); //webSocket.onEvent(webSocketEvent); // Serve a basic HTML page with JavaScript to create the WebSocket connection server.on("/", HTTP_GET, [](AsyncWebServerRequest* request) { Serial.println("Web Server: received a web page request"); String html = HTML_CONTENT; // Use the HTML content from the index.h file request->send(200, "text/html", html); }); server.begin(); Serial.print("ESP32 Web Server's IP address: "); Serial.println(WiFi.localIP()); } void loop() { webSocket.loop(); if (millis() - last_update > 500) { last_update = millis(); String variable_1 = String(random(0, 100)); String variable_2 = String(random(0, 100)); String variable_3 = String(random(0, 100)); String variable_4 = String(random(0, 100)); // TO SERIAL PLOTTER Serial.print(variable_1); Serial.print(" "); // a tab '\t' or space ' ' character is printed between the two values. Serial.print(variable_2); Serial.print(" "); // a tab '\t' or space ' ' character is printed between the two values. Serial.print(variable_3); Serial.print(" "); // a tab '\t' or space ' ' character is printed between the two values. Serial.println(variable_4); // the last value is terminated by a carriage return and a newline characters. // TO WEB PLOTTER webSocket.broadcastTXT(variable_1); webSocket.broadcastTXT(" "); // a tab '\t' or space ' ' character is printed between the two values. webSocket.broadcastTXT(variable_2); webSocket.broadcastTXT(" "); // a tab '\t' or space ' ' character is printed between the two values. webSocket.broadcastTXT(variable_3); webSocket.broadcastTXT(" "); // a tab '\t' or space ' ' character is printed between the two values. webSocket.broadcastTXT(variable_4); webSocket.broadcastTXT("\r\n"); // the last value is terminated by a carriage return and a newline characters. } }
  • Modify the WiFi information (SSID and password) in the code to match your own network credentials.
  • Create the index.h file On Arduino IDE by:
    • Either click on the button just below the serial monitor icon and choose New Tab, or use Ctrl+Shift+N keys.
    Arduino IDE 2 adds file
    • Give the file's name index.h and click OK button
    Arduino IDE 2 adds file index.h
    • Copy the below code and paste it to the index.h.
    /* * This ESP32 code is created by esp32io.com * * This ESP32 code is released in the public domain * * For more detail (instruction and wiring diagram), visit https://esp32io.com/tutorials/esp32-web-plotter */ const char *HTML_CONTENT = R"=====( <!DOCTYPE html> <html> <head> <title>ESP32 - Web Plotter</title> <meta name="viewport" content="width=device-width, initial-scale=0.7"> <style> body {text-align: center; height: 750px; } h1 {font-weight: bold; font-size: 20pt; padding-bottom: 5px; color: navy; } h2 {font-weight: bold; font-size: 15pt; padding-bottom: 5px; } button {font-weight: bold; font-size: 15pt; } #footer {width: 100%; margin: 0px; padding: 0px 0px 10px 0px; bottom: 0px; } .sub-footer {margin: 0 auto; position: relative; width:400px; } .sub-footer a {position: absolute; font-size: 10pt; top: 3px; } </style> <script> var COLOR_BACKGROUND = "#FFFFFF"; var COLOR_TEXT = "#000000"; var COLOR_BOUND = "#000000"; var COLOR_GRIDLINE = "#F0F0F0"; //var COLOR_LINE = ["#33FFFF", "#FF00FF", "#FF0000", "#FF8C00", "#00FF00"]; //var COLOR_LINE = ["#0000FF", "#FF0000", "#00FF00", "#FF8C00", "#00FF00"]; //var COLOR_LINE = ["#33FFFF", "#FF0000", "#00FF00", "#FF8C00", "#00FF00"]; var COLOR_LINE = ["#0000FF", "#FF0000", "#009900", "#FF9900", "#CC00CC", "#666666", "#00CCFF", "#000000"]; var LEGEND_WIDTH = 10; var X_AXIS_TITLE_HEIGHT = 40; var Y_AXIS_TITLE_WIDTH = 40; var X_AXIS_VALUE_HEIGHT = 40; var Y_AXIS_VALUE_WIDTH = 50; var PLOT_AREA_PADDING_TOP = 30; var PLOT_AREA_PADDING_RIGHT = 30; var X_GRIDLINE_NUM = 5; var Y_GRIDLINE_NUM = 4; var WSP_SIZE_TYPE = 1; /* 0: Fixed size, 1: full screen */ var WSP_WIDTH = 400; var WSP_HEIGHT = 200; var MAX_SAMPLE = 50; // in sample var X_AXIS_MIN = 0; var X_AXIS_MAX = MAX_SAMPLE; var Y_AXIS_AUTO_SCALE = 1; /* 0: y axis fixed range, 1: y axis auto scale */ var Y_AXIS_MIN = -5; var Y_AXIS_MAX = 5; var X_AXIS_TITLE = "X"; var Y_AXIS_TITLE = "Y"; var plot_area_width; var plot_area_height; var plot_area_pivot_x; var plot_area_pivot_y; var sample_count = 0; var buffer = ""; var data = []; var ws; var canvas; var ctx; function init() { canvas = document.getElementById("graph"); canvas.style.backgroundColor = COLOR_BACKGROUND; ctx = canvas.getContext("2d"); canvas_resize(); setInterval(update_view, 1000 / 60); } function connect_onclick() { if(ws == null) { ws = new WebSocket("ws://" + window.location.host + ":81"); document.getElementById("ws_state").innerHTML = "CONNECTING"; ws.onopen = ws_onopen; ws.onclose = ws_onclose; ws.onmessage = ws_onmessage; ws.binaryType = "arraybuffer"; } else ws.close(); } function ws_onopen() { document.getElementById("ws_state").innerHTML = "<span style='color: blue'>CONNECTED</span>"; document.getElementById("bt_connect").innerHTML = "Disconnect"; } function ws_onclose() { document.getElementById("ws_state").innerHTML = "<span style='color: gray'>CLOSED</span>"; document.getElementById("bt_connect").innerHTML = "Connect"; ws.onopen = null; ws.onclose = null; ws.onmessage = null; ws = null; } function ws_onmessage(e_msg) { e_msg = e_msg || window.event; // MessageEvent console.log(e_msg.data); buffer += e_msg.data; buffer = buffer.replace(/\r\n/g, "\n"); buffer = buffer.replace(/\r/g, "\n"); while(buffer.indexOf("\n") == 0) buffer = buffer.substr(1); if(buffer.indexOf("\n") <= 0) return; var pos = buffer.lastIndexOf("\n"); var str = buffer.substr(0, pos); var new_sample_arr = str.split("\n"); buffer = buffer.substr(pos + 1); for(var si = 0; si < new_sample_arr.length; si++) { var str = new_sample_arr[si]; var arr = []; if(str.indexOf("\t") > 0) arr = str.split("\t"); else arr = str.split(" "); for(var i = 0; i < arr.length; i++) { var value = parseFloat(arr[i]); if(isNaN(value)) continue; if(i >= data.length) { var new_line = [value]; data.push(new_line); // new line } else data[i].push(value); } sample_count++; } for(var line = 0; line < data.length; line++) { while(data[line].length > MAX_SAMPLE) data[line].splice(0, 1); } if(Y_AXIS_AUTO_SCALE) auto_scale(); } function map(x, in_min, in_max, out_min, out_max) { return (x - in_min) * (out_max - out_min) / (in_max - in_min) + out_min; } function get_random_color() { var letters = '0123456789ABCDEF'; var _color = '#'; for (var i = 0; i < 6; i++) _color += letters[Math.floor(Math.random() * 16)]; return _color; } function update_view() { if(sample_count <= MAX_SAMPLE) X_AXIS_MAX = sample_count; else X_AXIS_MAX = 50; ctx.clearRect(0, 0, WSP_WIDTH, WSP_HEIGHT); ctx.save(); ctx.translate(plot_area_pivot_x, plot_area_pivot_y); ctx.font = "bold 20px Arial"; ctx.textBaseline = "middle"; ctx.textAlign = "center"; ctx.fillStyle = COLOR_TEXT; // draw X axis title if(X_AXIS_TITLE != "") { ctx.fillText(X_AXIS_TITLE, plot_area_width / 2, X_AXIS_VALUE_HEIGHT + X_AXIS_TITLE_HEIGHT / 2); } // draw Y axis title if(Y_AXIS_TITLE != "") { ctx.rotate(-90 * Math.PI / 180); ctx.fillText(Y_AXIS_TITLE, plot_area_height / 2, -Y_AXIS_VALUE_WIDTH - Y_AXIS_TITLE_WIDTH / 2); ctx.rotate(90 * Math.PI / 180); } ctx.font = "16px Arial"; ctx.textAlign = "right"; ctx.strokeStyle = COLOR_BOUND; for(var i = 0; i <= Y_GRIDLINE_NUM; i++) { var y_gridline_px = -map(i, 0, Y_GRIDLINE_NUM, 0, plot_area_height); y_gridline_px = Math.round(y_gridline_px) + 0.5; ctx.beginPath(); ctx.moveTo(0, y_gridline_px); ctx.lineTo(plot_area_width, y_gridline_px); ctx.stroke(); ctx.strokeStyle = COLOR_BOUND; ctx.beginPath(); ctx.moveTo(-7 , y_gridline_px); ctx.lineTo(4, y_gridline_px); ctx.stroke(); var y_gridline_value = map(i, 0, Y_GRIDLINE_NUM, Y_AXIS_MIN, Y_AXIS_MAX); y_gridline_value = y_gridline_value.toFixed(1); ctx.fillText(y_gridline_value + "", -15, y_gridline_px); ctx.strokeStyle = COLOR_GRIDLINE; } ctx.strokeStyle = COLOR_BOUND; ctx.textAlign = "center"; ctx.beginPath(); ctx.moveTo(0.5, y_gridline_px - 7); ctx.lineTo(0.5, y_gridline_px + 4); ctx.stroke(); for(var i = 0; i <= X_GRIDLINE_NUM; i++) { var x_gridline_px = map(i, 0, X_GRIDLINE_NUM, 0, plot_area_width); x_gridline_px = Math.round(x_gridline_px) + 0.5; ctx.beginPath(); ctx.moveTo(x_gridline_px, 0); ctx.lineTo(x_gridline_px, -plot_area_height); ctx.stroke(); ctx.strokeStyle = COLOR_BOUND; ctx.beginPath(); ctx.moveTo(x_gridline_px, 7); ctx.lineTo(x_gridline_px, -4); ctx.stroke(); var x_gridline_value; if(sample_count <= MAX_SAMPLE) x_gridline_value = map(i, 0, X_GRIDLINE_NUM, X_AXIS_MIN, X_AXIS_MAX); else x_gridline_value = map(i, 0, X_GRIDLINE_NUM, sample_count - MAX_SAMPLE, sample_count);; ctx.fillText(x_gridline_value.toString(), x_gridline_px, X_AXIS_VALUE_HEIGHT / 2 + 5); ctx.strokeStyle = COLOR_GRIDLINE; } //ctx.lineWidth = 2; var line_num = data.length; for(var line = 0; line < line_num; line++) { // draw graph var sample_num = data[line].length; if(sample_num >= 2) { var y_value = data[line][0]; var x_px = 0; var y_px = -map(y_value, Y_AXIS_MIN, Y_AXIS_MAX, 0, plot_area_height); if(line == COLOR_LINE.length) COLOR_LINE.push(get_random_color()); ctx.strokeStyle = COLOR_LINE[line]; ctx.beginPath(); ctx.moveTo(x_px, y_px); for(var i = 0; i < sample_num; i++) { y_value = data[line][i]; x_px = map(i, X_AXIS_MIN, X_AXIS_MAX -1, 0, plot_area_width); y_px = -map(y_value, Y_AXIS_MIN, Y_AXIS_MAX, 0, plot_area_height); ctx.lineTo(x_px, y_px); } ctx.stroke(); } // draw legend var x = plot_area_width - (line_num - line) * LEGEND_WIDTH - (line_num - line - 1) * LEGEND_WIDTH / 2; var y = -plot_area_height - PLOT_AREA_PADDING_TOP / 2 - LEGEND_WIDTH / 2; ctx.fillStyle = COLOR_LINE[line]; ctx.beginPath(); ctx.rect(x, y, LEGEND_WIDTH, LEGEND_WIDTH); ctx.fill(); } ctx.restore(); } function canvas_resize() { canvas.width = 0; // to avoid wrong screen size canvas.height = 0; if(WSP_SIZE_TYPE) { // full screen document.getElementById('footer').style.position = "fixed"; var width = window.innerWidth - 20; var height = window.innerHeight - 20; WSP_WIDTH = width; WSP_HEIGHT = height - document.getElementById('header').offsetHeight - document.getElementById('footer').offsetHeight; } canvas.width = WSP_WIDTH; canvas.height = WSP_HEIGHT; ctx.font = "16px Arial"; var y_min_text_size = ctx.measureText(Y_AXIS_MIN.toFixed(1) + "").width; var y_max_text_size = ctx.measureText(Y_AXIS_MAX.toFixed(1) + "").width; Y_AXIS_VALUE_WIDTH = Math.round(Math.max(y_min_text_size, y_max_text_size)) + 15; plot_area_width = WSP_WIDTH - Y_AXIS_VALUE_WIDTH - PLOT_AREA_PADDING_RIGHT; plot_area_height = WSP_HEIGHT - X_AXIS_VALUE_HEIGHT - PLOT_AREA_PADDING_TOP; plot_area_pivot_x = Y_AXIS_VALUE_WIDTH; plot_area_pivot_y = WSP_HEIGHT - X_AXIS_VALUE_HEIGHT; if(X_AXIS_TITLE != "") { plot_area_height -= X_AXIS_TITLE_HEIGHT; plot_area_pivot_y -= X_AXIS_TITLE_HEIGHT; } if(Y_AXIS_TITLE != "") { plot_area_width -= Y_AXIS_TITLE_WIDTH; plot_area_pivot_x += Y_AXIS_TITLE_WIDTH; } ctx.lineWidth = 1; } function auto_scale() { if(data.length >= 1) { var max_arr = []; var min_arr = []; for(var i = 0; i < data.length; i++) { if(data[i].length >= 1) { var max = Math.max.apply(null, data[i]); var min = Math.min.apply(null, data[i]); max_arr.push(max); min_arr.push(min); } } var max = Math.max.apply(null, max_arr); var min = Math.min.apply(null, min_arr); var MIN_DELTA = 10.0; if((max - min) < MIN_DELTA) { var mid = (max + min) / 2; max = mid + MIN_DELTA / 2; min = mid - MIN_DELTA / 2; } var range = max - min; var exp; if (range == 0.0) exp = 0; else exp = Math.floor(Math.log10(range / 4)); var scale = Math.pow(10, exp); var raw_step = (range / 4) / scale; var step; potential_steps =[1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 6.0, 8.0, 10.0]; for (var i = 0; i < potential_steps.length; i++) { if (potential_steps[i] < raw_step) continue; step = potential_steps[i] * scale; Y_AXIS_MIN = step * Math.floor(min / step); Y_AXIS_MAX = Y_AXIS_MIN + step * (4); if (Y_AXIS_MAX >= max) break; } var count = 5 - Math.floor((Y_AXIS_MAX - max) / step); Y_AXIS_MAX = Y_AXIS_MIN + step * (count - 1); ctx.font = "16px Arial"; var y_min_text_size = ctx.measureText(Y_AXIS_MIN.toFixed(1) + "").width; var y_max_text_size = ctx.measureText(Y_AXIS_MAX.toFixed(1) + "").width; Y_AXIS_VALUE_WIDTH = Math.round(Math.max(y_min_text_size, y_max_text_size)) + 15; plot_area_width = WSP_WIDTH - Y_AXIS_VALUE_WIDTH - PLOT_AREA_PADDING_RIGHT; plot_area_pivot_x = Y_AXIS_VALUE_WIDTH; } } window.onload = init; </script> </head> <body onresize="canvas_resize()"> <h1 id="header">ESP32 - Web Plotter</h1> <canvas id="graph"></canvas> <br> <div id="footer"> <div class="sub-footer"> <h2>WebSocket <span id="ws_state"><span style="color: gray">CLOSED</span></span></h2> </div> <button id="bt_connect" type="button" onclick="connect_onclick();">Connect</button> </div> </body> </html> )=====";
    • Now you have the code in two files: esp32io.com.ino and index.h
    • Click Upload button on Arduino IDE to upload code to ESP32.
    • Open the Serial Monitor
    • Check out the result on Serial Monitor.
    COM6
    Send
    Connecting to WiFi... Connected to WiFi ESP32 Web Server's IP address IP address: 192.168.0.2
    Autoscroll Show timestamp
    Clear output
    9600 baud  
    Newline  
    • Take note of the IP address displayed, and enter this address into the address bar of a web browser on your smartphone or PC.
    • You will see the webpage it as below:
    ESP32 plotter web browser
    • Click the CONNECT button to connect the webpage to ESP32 via WebSocket.
    • You will see the plotter plots data as the below image.
    ESP32 web graph
    • You can open the Serial Plotter on Arduino IDE to compare with the Web Plotter on the web browser.

    ※ NOTE THAT:

    • If you modify the HTML content in the index.h and does not touch anything in esp32io.com.ino file, when you compile and upload code to ESP32, Arduino IDE will not update the HTML content.
    • To make Arduino IDE update the HTML content in this case, make a change in the esp32io.com.ino file (e.g. adding empty line, add a comment....)

    Line-by-line Code Explanation

    The above ESP32 code contains line-by-line explanation. Please read the comments in the code!

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