Arduino Internet Voltmeter with Dial Gauge, Logging and Graph

Created on: 19 March 2016

In this tutorial an Arduino Internet Voltmeter is made by using an Arduino and Ethernet shield set up as a web server to host a web page that displays voltage measured on one of the analog pins in a dial gauge on the web page.

An IoT (Internet of Things) web service called ThingSpeak is used to log the voltage and plot it to a graph. The graph of the voltage plotted over time is then displayed on the web page next to the dial gauge.

Voltage on the dial gauge is updated every 200ms for a near real-time view of the voltage. The same voltage is plotted to the ThingSpeak graph every 20 seconds.

An SD card is used to store the web page that is hosted by the Arduino. JavaScript using Ajax sends the voltage from the Arduino to the web page. Ajax is also used to send the voltage from the web page to ThingSpeak for logging and plotting to a graph. The graph is placed on the Arduino hosted web page which is updated from ThingSpeak.

The video below shows the Arduino Internet Voltmeter in action.

Can't see the video? View on YouTube →

Arduino Internet Voltmeter Hardware

An Arduino Uno, Ethernet shield and micro SD card are used in this project, but any Arduino with Ethernet shield or Arduino that has Ethernet capabilities should work.

An Ethernet cable will be needed to connect the Arduino to your network.

To vary the voltage on analog pin A2, a potentiometer can be used. The potentiometer is connected to the Arduino as shown in the analog read serial example, except that the middle pin of the potentiometer is connected to analog pin A2.

A voltage between 0V and 5V can be measured by the Arduino Internet volmeter.

Making the Arduino Internet Voltmeter

Before continuing, you will need to learn how ThingSpeak works by creating a ThingSpeak account and sending data to it from the Arduino.

Arduino Sketch

The Arduino sketch used in this tutorial is the sketch from the Arduino Web Server Gauge Displaying Analog Value tutorial and is used unmodified, just load it to the Arduino. The sketch is included below for your convenience.

Change these settings to suit your network and Arduino board. After making changes, click the Generate Code button to apply the changes to the Arduino sketch below. Click the Select Code button to select the code for copying.


: : : : :
MAC address found on sticker on bottom of Ethernet shield


. . .
IP address that the Arduino will be at

After making changes in any of the fields above, click this button to update the code below

Select the code below before copying

  Program:      eth_websrv_SD_Ajax_gauge

  Description:  Arduino web server web page displays Arduino
                analog value on a dial gauge.
                The web page is stored on the SD card.
                Ajax is used to update the analog value on the
                web page.
  Hardware:     Arduino Uno and official Arduino Ethernet
                shield. Should work with other Arduinos and
                compatible Ethernet shields.
                2Gb micro SD card formatted FAT16.
                Potentiometer interfaced to A2 analog input.
  Software:     Developed using Arduino 1.0.5 software
                Should be compatible with Arduino 1.0 +
                SD card contains web page called index.htm
  References:   - WebServer example by David A. Mellis and 
                  modified by Tom Igoe
                - SD card examples by David A. Mellis and
                  Tom Igoe
                - Ethernet library documentation:
                - SD Card library documentation:
                - Gauge from:

  Date:         27 March 2013
  Modified:     19 June 2013
  Author:       W.A. Smith,

#include <SPI.h>
#include <Ethernet.h>
#include <SD.h>

// size of buffer used to capture HTTP requests
#define REQ_BUF_SZ   50

// MAC address from Ethernet shield sticker under board
byte mac[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };
IPAddress ip(192, 168, 0, 20); // IP address, may need to change depending on network
EthernetServer server(80);  // create a server at port 80
File webFile;               // the web page file on the SD card
char HTTP_req[REQ_BUF_SZ] = {0}; // buffered HTTP request stored as null terminated string
char req_index = 0;              // index into HTTP_req buffer

void setup()
    // disable Ethernet chip
    pinMode(10, OUTPUT);
    digitalWrite(10, HIGH);

    Serial.begin(9600);       // for debugging
    // initialize SD card
    Serial.println("Initializing SD card...");
    if (!SD.begin(4)) {
        Serial.println("ERROR - SD card initialization failed!");
        return;    // init failed
    Serial.println("SUCCESS - SD card initialized.");
    // check for index.htm file
    if (!SD.exists("index.htm")) {
        Serial.println("ERROR - Can't find index.htm file!");
        return;  // can't find index file
    Serial.println("SUCCESS - Found index.htm file.");
    Ethernet.begin(mac, ip);  // initialize Ethernet device
    server.begin();           // start to listen for clients

void loop()
    EthernetClient client = server.available();  // try to get client

    if (client) {  // got client?
        boolean currentLineIsBlank = true;
        while (client.connected()) {
            if (client.available()) {   // client data available to read
                char c =; // read 1 byte (character) from client
                // buffer first part of HTTP request in HTTP_req array (string)
                // leave last element in array as 0 to null terminate string (REQ_BUF_SZ - 1)
                if (req_index < (REQ_BUF_SZ - 1)) {
                    HTTP_req[req_index] = c;          // save HTTP request character
                // last line of client request is blank and ends with \n
                // respond to client only after last line received
                if (c == '\n' && currentLineIsBlank) {
                    // send a standard http response header
                    client.println("HTTP/1.1 200 OK");
                    // remainder of header follows below, depending on if
                    // web page or XML page is requested
                    // Ajax request - send XML file
                    if (StrContains(HTTP_req, "ajax_inputs")) {
                        // send rest of HTTP header
                        client.println("Content-Type: text/xml");
                        client.println("Connection: keep-alive");
                        // send XML file containing input states
                    else {  // web page request
                        // send rest of HTTP header
                        client.println("Content-Type: text/html");
                        client.println("Connection: keep-alive");
                        // send web page
                        webFile ="index.htm");        // open web page file
                        if (webFile) {
                            while(webFile.available()) {
                                client.write(; // send web page to client
                    // display received HTTP request on serial port
                    // reset buffer index and all buffer elements to 0
                    req_index = 0;
                    StrClear(HTTP_req, REQ_BUF_SZ);
                // every line of text received from the client ends with \r\n
                if (c == '\n') {
                    // last character on line of received text
                    // starting new line with next character read
                    currentLineIsBlank = true;
                else if (c != '\r') {
                    // a text character was received from client
                    currentLineIsBlank = false;
            } // end if (client.available())
        } // end while (client.connected())
        delay(1);      // give the web browser time to receive the data
        client.stop(); // close the connection
    } // end if (client)

// send the XML file containing analog value
void XML_response(EthernetClient cl)
    int analog_val;
    cl.print("<?xml version = \"1.0\" ?>");
    // read analog pin A2
    analog_val = analogRead(2);

// sets every element of str to 0 (clears array)
void StrClear(char *str, char length)
    for (int i = 0; i < length; i++) {
        str[i] = 0;

// searches for the string sfind in the string str
// returns 1 if string found
// returns 0 if string not found
char StrContains(char *str, char *sfind)
    char found = 0;
    char index = 0;
    char len;

    len = strlen(str);
    if (strlen(sfind) > len) {
        return 0;
    while (index < len) {
        if (str[index] == sfind[found]) {
            if (strlen(sfind) == found) {
                return 1;
        else {
            found = 0;

    return 0;

HTML Web Page

The HTML code below must be saved to a file called index.htm on a micro SD card and inserted into the micro SD card slot on the Ethernet shield or Arduino Ethernet board.

The HTML page was taken from the same Arduino Web Server Gauge Displaying Analog Value tutorial as the above Arduino sketch, but is modified to display voltage and to send the voltage to ThingSpeak for plotting to a graph. See the instructions for modifying this code to insert your ThingSpeak Write API Key and ThingSpeak graph.

<!DOCTYPE html>
        <title>Arduino Internet Voltmeter</title>
		var data_val = 0;		// raw data from Arduino analog input (0 to 1023)
		var volts = 0;			// voltage calculated from Arduino analog raw data value
		var num_updates = 0;	// number of 200ms periods used to calculate time to send data to ThingSpeak
		// gauge code
		eval(function(p,a,c,k,e,r){e=function(c){return(c<a?'':e(parseInt(c/a)))+((c=c%a)>35?String.fromCharCode(c+29):c.toString(36))};if(!''.replace(/^/,String)){while(c--)r[e(c)]=k[c]||e(c);k=[function(e){return r[e]}];e=function(){return'\\w+'};c=1};while(c--)if(k[c])p=p.replace(new RegExp('\\b'+e(c)+'\\b','g'),k[c]);return p}('t W=v(f){W.2t.3T(A);A.B={Z:2u,19:1q,1h:1q,1J:U,1b:D,K:0,V:[\'0\',\'20\',\'40\',\'2A\',\'2B\',\'D\'],2r:10,2C:M,1E:U,2q:{2D:3,2E:2},2H:M,1c:{2j:10,2m:3y,2O:\'3v\'},J:{2P:\'#4h\',V:\'#31\',2r:\'#3k\',1J:\'#37\',1E:\'#37\',3e:\'#31\',1t:{2s:\'1e(3Y, 3d, 3d, 1)\',3c:\'1e(1Y, 5I, 5E, .9)\'}},1o:[{1n:20,1D:2A,1F:\'#3j\'},{1n:2A,1D:2B,1F:\'#36\'},{1n:2B,1D:D,1F:\'#5n\'}]};t g=0,1p=A,N=0,1S=0,1G=U;A.5d=v(a){N=f.1c?g:a;t b=(f.1b-f.K)/D;1S=a>f.1b?1S=f.1b+b:a<f.K?f.K-b:a;g=a;f.1c?3l():A.1g();C A};A.3m=v(a){N=g=a;A.1g();C A};A.4T=v(){g=N=1S=A.B.K;A.1g();C A};A.4R=v(){C g};A.13=v(){};v 2k(a,b){Q(t i 4P b){z(1H b[i]=="1W"&&!(4O.4y.2V.4p(b[i])===\'[1W 4n]\')&&i!=\'Z\'){z(1H 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		// function called periodically to get analog value from Arduino using Ajax
		function GetArduinoInputs()
			nocache = "&nocache=" + Math.random() * 1000000;
			var request = new XMLHttpRequest();
			request.onreadystatechange = function()
				if (this.readyState == 4) {
					if (this.status == 200) {
						if (this.responseXML != null) {
							document.getElementById("input3").innerHTML =
								data_val = this.responseXML.getElementsByTagName('analog')[0].childNodes[0].nodeValue;
								// calculate voltage
								volts = data_val * 5 / 1023;
								// only send data to ThingSpeak every 20 seconds or 100 x 200ms
								if (num_updates >= 100) {
									num_updates = 0;
									// send voltage to ThingSpeak
									ThingSpeakSend("<INSERT THINGSPEAK API KEY HERE", volts);	// insert your ThingSpeak Write API Key here
			}"GET", "ajax_inputs" + nocache, true);
			setTimeout('GetArduinoInputs()', 200);	// send the request for Arduino analog data every 200ms
		// function to send data to ThingSpeak
		function ThingSpeakSend(api_write_key, voltage)
			var ts_req = new XMLHttpRequest();
			// GET request string - modify if more fields are needed
			var req_str = "" + api_write_key + "&field1=" + voltage;
			ts_req.onreadystatechange = function()
				// not doing anything with response from ThingSpeak
			// send the data to ThingSpeak"GET", req_str, true);
    <body onload="GetArduinoInputs()">
        <h1>Arduino Internet Voltmeter</h1>
        <p>Analog (A2): <span id="input3">...</span></p>
        <canvas id="an_gauge_1" data-major-ticks="0 1 2 3 4 5" data-type="canv-gauge" data-min-value="0" data-max-value="5" data-onready="setInterval( function() { Gauge.Collection.get('an_gauge_1').setValue(volts);}, 200);"></canvas>
        <!-- insert ThingSpeak chart code here -->

Inserting Your ThingSpeak Write API Key Into the HTML File

You will need to create a ThingSpeak account, create a channel and then get the ThingSpeak Write API Key for the channel. Insert the ThingSpeak Write API Key into the above code where it is marked as shown below.

Insert your ThingSpeak Write API Key into the HTML

Inserting a ThingSpeak Graph into the HTML Page.

Once you have created a channel to send the voltage across in your ThingSpeak account, open the channel at the ThingSpeak website by clicking Channels → My Channels and then Private under the channel that you are using.

At the top right of the graph that is displayed, click the speech bubble icon (shown below) to open a dialog box and get the HTML code for the graph.

Getting the ThingSpeak HTML Graph Code

Select and copy the graph code in the dialog box that pops up. The dialog box is shown below.

Copy the ThingSpeak graph code

Paste the graph code into the above HTML web page code that will be copied to the micro SD card. The place to past the code in the HTML file is shown below.

Inserting the graph code into the HTML file

Testing the Project

After the index.htm file has been modified, copy it to the micro SD card and insert it into the slot in the Arduino Ethernet shield. Connect the shield to your network and power the Arduino and Ethernet shield.

Use a web browser to surf to the IP address set in the Arduino sketch. The voltage gauge and graph should appear on the web page that loads. You must be logged into your ThingSpeak account in order for the graph on the web page to be updated.