Interface Multiple DS18B20s with ESP32 & Display Values Using Web Server

Have you ever wanted to have sensors scattered all around your house and garden reporting their temperature regularly to a central server? Then, this IoT project might be the solid launching point for you!

This project uses ESP32 as the control device that easily connects to existing WiFi network & creates a Web Server. When any connected device accesses this web server, ESP32 reads in temperature from multiple DS18B20 Temperature sensors & sends it to the web browser of that device with a nice interface. Excited? Let’s get started!

Multiple DS18B20s On Single Bus

One of the biggest features of DS18B20 is that multiple DS18B20s can coexist on the same 1-Wire bus. As each DS18B20 has a unique 64-bit serial code burned in at the factory, it’s easier to differentiate them from one another.

This feature can be a huge advantage when you want to control many DS18B20s distributed over a large area. In this tutorial we are going to do the same.

Wiring Multiple DS18B20 Sensors to ESP32

Connecting DS18B20 sensors to ESP32 is fairly simple.

Start by connecting all the DS18B20s in parallel i.e. common all the VDD pins, GND pins & signal pins. Then connect VDD to the 3.3V out on ESP32, GND to ground and connect signal pin to digital pin 15 on ESP32.

Next, you’ll need to add one 4.7k pull-up resistor for whole bus between the signal and power pin to keep the data transfer stable.

Wiring Multiple DS18B20 Temperature Sensors to ESP32

Preparing the Arduino IDE

There’s an add-on for the Arduino IDE that allows you to program the ESP32 using the Arduino IDE. Follow below tutorial to prepare your Arduino IDE to work with the ESP32, if you haven’t already.

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Installing Library For DS18B20

The Dallas 1-Wire protocol is somewhat complex, and requires a bunch of code to parse out the communication. To hide away this unnecessary complexity we will install  DallasTemperature.h  library so that we can issue simple commands to get temperature readings from the sensor.

To install the library navigate to the Arduino IDE > Sketch > Include Library > Manage Libraries… Wait for Library Manager to download libraries index and update list of installed libraries.

Filter your search by typing ‘ds18b20’. There should be a couple entries. Look for DallasTemperature by Miles Burton. Click on that entry, and then select Install.

This Dallas Temperature library is a hardware-specific library which handles lower-level functions. It needs to be paired with One Wire Library to communicate with any one-wire device not just DS18B20. Install this library as well.

Finding Addresses Of DS18B20s On Bus

We know that each DS18B20 has a unique 64-bit address assigned to it to differentiate them from one another. First, we’ll find that address to label each sensor accordingly. The address can then be used to read each sensor individually.

The following sketch detects all the DS18B20s present on the bus and prints their one-wire address on the serial monitor.

You can wire just one sensor at a time to find its address (or successively add a new sensor) so that you’re able to identify each one by its address. Then, you can label each sensor.

#include <OneWire.h>
#include <DallasTemperature.h>

// Data wire is plugged into port 15 on the ESP32
#define ONE_WIRE_BUS 15

// Setup a oneWire instance to communicate with any OneWire devices
OneWire oneWire(ONE_WIRE_BUS);

// Pass our oneWire reference to Dallas Temperature.
DallasTemperature sensors(&oneWire);

// variable to hold device addresses
DeviceAddress Thermometer;

int deviceCount = 0;

void setup(void)
{
  // start serial port
  Serial.begin(115200);

  // Start up the library
  sensors.begin();

  // locate devices on the bus
  Serial.println("Locating devices...");
  Serial.print("Found ");
  deviceCount = sensors.getDeviceCount();
  Serial.print(deviceCount, DEC);
  Serial.println(" devices.");
  Serial.println("");
  
  Serial.println("Printing addresses...");
  for (int i = 0;  i < deviceCount;  i++)
  {
    Serial.print("Sensor ");
    Serial.print(i+1);
    Serial.print(" : ");
    sensors.getAddress(Thermometer, i);
    printAddress(Thermometer);
  }
}

void loop(void)
{ }

void printAddress(DeviceAddress deviceAddress)
{ 
  for (uint8_t i = 0; i < 8; i++)
  {
    Serial.print("0x");
    if (deviceAddress[i] < 0x10) Serial.print("0");
    Serial.print(deviceAddress[i], HEX);
    if (i < 7) Serial.print(", ");
  }
  Serial.println("");
}

Now, open the serial monitor. You should get something as follows.

Copy all the addresses as we need them in out next sketch.

Create ESP32 Web Server using WiFi Station (STA) mode

Now, we are going to configure our ESP32 into Station (STA) mode, and create a web server to serve up web pages to any connected client under existing network.

If you want to learn about creating a web server with ESP32 in AP/STA mode, check this tutorial out.

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Before you head for uploading the sketch, you need to make some changes to make it work for you.

• You need to modify the following two variables with your network credentials, so that ESP32 can establish a connection with existing network.

  const char* ssid = "YourNetworkName";  // Enter SSID here
  const char* password = "YourPassword";  //Enter Password here

• Before serving up a web page ESP32 reads the temperature from each DS18B20 by its address so, you need to change the addresses of DS18B20s with the one you’ve found in previous sketch.

  uint8_t sensor1[8] = { 0x28, 0xEE, 0xD5, 0x64, 0x1A, 0x16, 0x02, 0xEC };
  uint8_t sensor2[8] = { 0x28, 0x61, 0x64, 0x12, 0x3C, 0x7C, 0x2F, 0x27 };
  uint8_t sensor3[8] = { 0x28, 0x61, 0x64, 0x12, 0x3F, 0xFD, 0x80, 0xC6 };

Once you are done, go ahead and try the sketch out.

#include <WiFi.h>
#include <WebServer.h>
#include <OneWire.h>
#include <DallasTemperature.h>

// Data wire is plugged into port 15 on the ESP32
#define ONE_WIRE_BUS 15

// Setup a oneWire instance to communicate with any OneWire devices (not just Maxim/Dallas temperature ICs)
OneWire oneWire(ONE_WIRE_BUS);

// Pass our oneWire reference to Dallas Temperature.
DallasTemperature sensors(&oneWire);

uint8_t sensor1[8] = { 0x28, 0xEE, 0xD5, 0x64, 0x1A, 0x16, 0x02, 0xEC  };
uint8_t sensor2[8] = { 0x28, 0x61, 0x64, 0x12, 0x3C, 0x7C, 0x2F, 0x27  };
uint8_t sensor3[8] = { 0x28, 0x61, 0x64, 0x12, 0x3F, 0xFD, 0x80, 0xC6  };

/*Put your SSID & Password*/
const char* ssid = "YourNetworkName";  // Enter SSID here
const char* password = "YourPassword";  //Enter Password here

WebServer server(80);             

float tempSensor1, tempSensor2, tempSensor3;
 
void setup() {
  Serial.begin(115200);
  delay(100);
  
  sensors.begin();              

  Serial.println("Connecting to ");
  Serial.println(ssid);

  //connect to your local wi-fi network
  WiFi.begin(ssid, password);

  //check wi-fi is connected to wi-fi network
  while (WiFi.status() != WL_CONNECTED) {
  delay(1000);
  Serial.print(".");
  }
  Serial.println("");
  Serial.println("WiFi connected..!");
  Serial.print("Got IP: ");  Serial.println(WiFi.localIP());

  server.on("/", handle_OnConnect);
  server.onNotFound(handle_NotFound);

  server.begin();
  Serial.println("HTTP server started");

}
void loop() {
  server.handleClient();
}

void handle_OnConnect() {
  sensors.requestTemperatures();
  tempSensor1 = sensors.getTempC(sensor1); // Gets the values of the temperature
  tempSensor2 = sensors.getTempC(sensor2); // Gets the values of the temperature
  tempSensor3 = sensors.getTempC(sensor3); // Gets the values of the temperature
  server.send(200, "text/html", SendHTML(tempSensor1,tempSensor2,tempSensor3)); 
}

void handle_NotFound(){
  server.send(404, "text/plain", "Not found");
}

String SendHTML(float tempSensor1,float tempSensor2,float tempSensor3){
  String ptr = "<!DOCTYPE html> <html>\n";
  ptr +="<head><meta name=\"viewport\" content=\"width=device-width, initial-scale=1.0, user-scalable=no\">\n";
  ptr +="<title>ESP32 Temperature Monitor</title>\n";
  ptr +="<style>html { font-family: Helvetica; display: inline-block; margin: 0px auto; text-align: center;}\n";
  ptr +="body{margin-top: 50px;} h1 {color: #444444;margin: 50px auto 30px;}\n";
  ptr +="p {font-size: 24px;color: #444444;margin-bottom: 10px;}\n";
  ptr +="</style>\n";
  ptr +="</head>\n";
  ptr +="<body>\n";
  ptr +="<div id=\"webpage\">\n";
  ptr +="<h1>ESP32 Temperature Monitor</h1>\n";
  ptr +="<p>Living Room: ";
  ptr +=tempSensor1;
  ptr +="&deg;C</p>";
  ptr +="<p>Bedroom: ";
  ptr +=tempSensor2;
  ptr +="&deg;C</p>";
  ptr +="<p>Kitchen: ";
  ptr +=tempSensor3;
  ptr +="&deg;C</p>";
  ptr +="</div>\n";
  ptr +="</body>\n";
  ptr +="</html>\n";
  return ptr;
}

Accessing the Web Server

After uploading the sketch, open the Serial Monitor at a baud rate of 115200. And press the EN button on ESP32. If everything is OK, it will output the dynamic IP address obtained from your router and show HTTP server started message.

Next, load up a browser and point it to the IP address shown on the serial monitor. The ESP32 should serve up a web page showing temperatures from all the DS18B20s.

Detailed Code Explanation

The sketch starts by including following libraries.

• WiFi.h library provides ESP32 specific WiFi methods we are calling to connect to network.
• WebServer.h library has some methods available that will help us setting up a server and handle incoming HTTP requests without needing to worry about low level implementation details.
• DallasTemperature.h library is a hardware-specific library which handles lower-level functions. It needs to be paired with One Wire Library, in order to make it work.
• OneWire.h library communicates with any one-wire device not just DS18B20.

#include <WiFi.h>
#include <WebServer.h>
#include <OneWire.h>
#include <DallasTemperature.h>

Next we create the instances needed for the temperature sensor and variables to store temperature readings.  The temperature sensor is connected to GPIO15.

// Data wire is plugged into port 15 on the ESP32
#define ONE_WIRE_BUS 15

// Setup a oneWire instance to communicate with any OneWire devices
OneWire oneWire(ONE_WIRE_BUS);

// Pass our oneWire reference to Dallas Temperature.
DallasTemperature sensors(&oneWire);

float tempSensor1, tempSensor2, tempSensor3;

Next, we enter the addresses that are found previously for each temperature sensor.  In our case, we have the following.

uint8_t sensor1[8] = { 0x28, 0xEE, 0xD5, 0x64, 0x1A, 0x16, 0x02, 0xEC  };
uint8_t sensor2[8] = { 0x28, 0x61, 0x64, 0x12, 0x3C, 0x7C, 0x2F, 0x27  };
uint8_t sensor3[8] = { 0x28, 0x61, 0x64, 0x12, 0x3F, 0xFD, 0x80, 0xC6  };

As we are configuring ESP32 in Station (STA) mode, it will join existing WiFi network. Hence, we need to provide it with your network’s SSID & Password. Next we start web server at port 80.

/*Put your SSID & Password*/
const char* ssid = "YourNetworkName";  // Enter SSID here
const char* password = "YourPassword";  //Enter Password here

WebServer server(80);

Inside Setup() Function

Inside Setup() Function we configure our HTTP server before actually running it. First of all, we initialize serial communication with PC and initialize DallasTemperature object using begin()  function. It initializes the bus and detects all the DS18B20s present on it. Each sensor is then assigned with an index and set bit resolution to 12-bit.

Serial.begin(115200);
delay(100);
  
sensors.begin(); 

Now, we need to join the WiFi network using WiFi.begin() function. The function takes SSID (Network Name) and password as a parameter.

Serial.println("Connecting to ");
Serial.println(ssid);

//connect to your local wi-fi network
WiFi.begin(ssid, password);

While the ESP32 tries to connect to the network, we can check the connectivity status with WiFi.status() function.

//check wi-fi is connected to wi-fi network
  while (WiFi.status() != WL_CONNECTED) {
  delay(1000);
  Serial.print(".");
  }

Once the ESP32 is connected to the network, the sketch prints the IP address assigned to ESP32 by displaying WiFi.localIP() value on serial monitor.

Serial.println("");
  Serial.println("WiFi connected..!");
  Serial.print("Got IP: ");  Serial.println(WiFi.localIP());

In order to handle incoming HTTP requests, we need to specify which code to execute when a URL is hit. To do so, we use on method. This method takes two parameters. First one is a URL path and second one is the name of function which we want to execute when that URL is hit.

The code below indicates that when a server receives an HTTP request on the root (/) path, it will trigger the handle_OnConnect function. Note that the URL specified is a relative path.

server.on("/", handle_OnConnect);

We haven’t specified what the server should do if the client requests any URL other than specified with server.on. It should respond with an HTTP status 404 (Not Found) and a message for the user. We put this in a function as well, and use server.onNotFound to tell it that it should execute it when it receives a request for a URL that wasn’t specified with server.on

server.onNotFound(handle_NotFound);

Now, to start our server, we call the begin method on the server object.

server.begin();
Serial.println("HTTP server started");

Inside Loop() Function

To handle the actual incoming HTTP requests, we need to call the handleClient() method on the server object.

server.handleClient();

Next, we need to create a function we attached to root (/) URL with server.on Remember?

At the start of this function, we get the temperature reading from each sensor. In order to respond to the HTTP request, we use the send method. Although the method can be called with a different set of arguments, its simplest form consists of the HTTP response code, the content type and the content.

In our case, we are sending the code 200 (one of the HTTP status codes), which corresponds to the OK response. Then, we are specifying the content type as “text/html“, and finally we are calling SendHTML() custom function which creates a dynamic HTML page containing temperature readings.

void handle_OnConnect() {
  sensors.requestTemperatures();
  tempSensor1 = sensors.getTempC(sensor1);
  tempSensor2 = sensors.getTempC(sensor2);
  tempSensor3 = sensors.getTempC(sensor3);
  server.send(200, "text/html", SendHTML(tempSensor1,tempSensor2,tempSensor3)); 
}

Likewise, we need to create a function to handle 404 Error page.

void handle_NotFound(){
  server.send(404, "text/plain", "Not found");
}

Displaying the HTML Web Page

SendHTML()  function is responsible for generating a web page whenever the ESP32 web server gets a request from a web client. It merely concatenates HTML code into a big string and returns to the server.send() function we discussed earlier. The function takes temperature readings as a parameter to dynamically generate the HTML content.

The first text you should always send is the <!DOCTYPE> declaration that indicates that we’re sending HTML code.

String SendHTML(float tempSensor1,float tempSensor2,float tempSensor3){
  String ptr = "<!DOCTYPE html> <html>\n";

Next, the <meta> viewport element makes the web page responsive in any web browser, while title tag sets the title of the page.

ptr +="<head><meta name=\"viewport\" content=\"width=device-width, initial-scale=1.0, user-scalable=no\">\n";
ptr +="<title>ESP32 Temperature Monitor</title>\n";

Styling the Web Page

Next, we have some CSS to style the web page appearance. We choose the Helvetica font, define the content to be displayed as an inline-block and aligned at the center.

ptr +="<style>html { font-family: Helvetica; display: inline-block; margin: 0px auto; text-align: center;}\n";

Following code then sets color, font and margin around the body, H1 and p tags.

ptr +="body{margin-top: 50px;} h1 {color: #444444;margin: 50px auto 30px;}\n";
ptr +="p {font-size: 24px;color: #444444;margin-bottom: 10px;}\n";
ptr +="</style>\n";
ptr +="</head>\n";
ptr +="<body>\n";

Setting the Web Page Heading

Next, heading of the web page is set; you can change this text to anything that suits your application.

ptr +="<div id=\"webpage\">\n";
ptr +="<h1>ESP32 Temperature Monitor</h1>\n";

Displaying Temperature readings on Web Page

To dynamically display temperature readings, we put those values in paragraph tag. To display degree symbol, we use HTML entity &deg;

ptr +="<p>Living Room: ";
ptr +=tempSensor1;
ptr +="&deg;C</p>";
ptr +="<p>Bedroom: ";
ptr +=tempSensor2;
ptr +="&deg;C</p>";
ptr +="<p>Kitchen: ";
ptr +=tempSensor3;
ptr +="&deg;C</p>";
ptr +="</div>\n";
ptr +="</body>\n";
ptr +="</html>\n";
return ptr;
}

Styling Web Page to Look More Professional

Programmers like us are often intimidated by design – but a little effort can make your web page look more attractive and professional. Below screenshot will give you a basic idea of what we are going to do.

Pretty amazing, Right? Without further ado, let’s apply some style to our previous HTML page. To start with, copy-paste below code to replace SendHTML() function from the sketch above.

String SendHTML(float tempSensor1,float tempSensor2,float tempSensor3){
  String ptr = "<!DOCTYPE html>";
  ptr +="<html>";
  ptr +="<head>";
  ptr +="<title>ESP32 Temperature Monitor</title>";
  ptr +="<meta name='viewport' content='width=device-width, initial-scale=1.0'>";
  ptr +="<link href='https://fonts.googleapis.com/css?family=Open+Sans:300,400,600' rel='stylesheet'>";
  ptr +="<style>";
  ptr +="html { font-family: 'Open Sans', sans-serif; display: block; margin: 0px auto; text-align: center;color: #444444;}";
  ptr +="body{margin-top: 50px;} ";
  ptr +="h1 {margin: 50px auto 30px;} ";
  ptr +=".side-by-side{display: table-cell;vertical-align: middle;position: relative;}";
  ptr +=".text{font-weight: 600;font-size: 19px;width: 200px;}";
  ptr +=".temperature{font-weight: 300;font-size: 50px;padding-right: 15px;}";
  ptr +=".living-room .temperature{color: #3B97D3;}";
  ptr +=".bedroom .temperature{color: #F29C1F;}";
  ptr +=".kitchen .temperature{color: #26B99A;}";
  ptr +=".superscript{font-size: 17px;font-weight: 600;position: absolute;right: -5px;top: 15px;}";
  ptr +=".data{padding: 10px;}";
  ptr +=".container{display: table;margin: 0 auto;}";
  ptr +=".icon{width:82px}";
  ptr +="</style>";
  ptr +="</head>";
  ptr +="<body>";
  ptr +="<h1>ESP32 Temperature Monitor</h1>";
  ptr +="<div class='container'>";
  ptr +="<div class='data living-room'>";
  ptr +="<div class='side-by-side icon'>";
  ptr +="<svg enable-background='new 0 0 65.178 45.699'height=45.699px id=Layer_1 version=1.1 viewBox='0 0 65.178 45.699'width=65.178px x=0px xml:space=preserve xmlns=http://www.w3.org/2000/svg xmlns:xlink=http://www.w3.org/1999/xlink y=0px><polygon fill=#3B97D3 points='8.969,44.261 8.969,16.469 7.469,16.469 7.469,44.261 1.469,44.261 1.469,45.699 14.906,45.699 ";
  ptr +="14.906,44.261 '/><polygon fill=#3B97D3 points='13.438,0 3,0 0,14.938 16.438,14.938 '/><polygon fill=#3B97D3 points='29.927,45.699 26.261,45.699 26.261,41.156 32.927,41.156 '/><polygon fill=#3B97D3 points='58.572,45.699 62.239,45.699 62.239,41.156 55.572,41.156 '/><path d='M61.521,17.344c-2.021,0-3.656,1.637-3.656,3.656v14.199H30.594V21c0-2.02-1.638-3.656-3.656-3.656";
  ptr +="c-2.02,0-3.657,1.636-3.657,3.656v14.938c0,2.021,1.637,3.655,3.656,3.655H61.52c2.02,0,3.655-1.637,3.655-3.655V21";
  ptr +="C65.177,18.98,63.54,17.344,61.521,17.344z'fill=#3B97D3 /><g><path d='M32.052,30.042c0,2.02,1.637,3.656,3.656,3.656h16.688c2.019,0,3.656-1.638,3.656-3.656v-3.844h-24";
  ptr +="L32.052,30.042L32.052,30.042z'fill=#3B97D3 /><path d='M52.396,6.781H35.709c-2.02,0-3.656,1.637-3.656,3.656v14.344h24V10.438";
  ptr +="C56.053,8.418,54.415,6.781,52.396,6.781z'fill=#3B97D3 /></g></svg>";
  ptr +="</div>";
  ptr +="<div class='side-by-side text'>Living Room</div>";
  ptr +="<div class='side-by-side temperature'>";
  ptr +=(int)tempSensor1;
  ptr +="<span class='superscript'>°C</span></div>";
  ptr +="</div>";
  ptr +="<div class='data bedroom'>";
  ptr +="<div class='side-by-side icon'>";
  ptr +="<svg enable-background='new 0 0 43.438 35.75'height=35.75px id=Layer_1 version=1.1 viewBox='0 0 43.438 35.75'width=43.438px x=0px xml:space=preserve xmlns=http://www.w3.org/2000/svg xmlns:xlink=http://www.w3.org/1999/xlink y=0px><g><path d='M25.489,14.909H17.95C13.007,14.908,0,15.245,0,20.188v3.688h43.438v-3.688";
  ptr +="C43.438,15.245,30.431,14.909,25.489,14.909z'fill=#F29C1F /><polygon fill=#F29C1F points='0,31.25 0,35.75 2.5,35.75 4.5,31.25 38.938,31.25 40.938,35.75 43.438,35.75 43.438,31.25 ";
  ptr +="43.438,25.375 0,25.375  '/><path d='M13.584,11.694c-3.332,0-6.033,0.973-6.033,2.175c0,0.134,0.041,0.264,0.105,0.391";
  ptr +="c3.745-0.631,7.974-0.709,10.341-0.709h1.538C19.105,12.501,16.613,11.694,13.584,11.694z'fill=#F29C1F /><path d='M30.009,11.694c-3.03,0-5.522,0.807-5.951,1.856h1.425V13.55c2.389,0,6.674,0.081,10.444,0.728";
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  ptr +="</div>";
  ptr +="<div class='side-by-side text'>Bedroom</div>";
  ptr +="<div class='side-by-side temperature'>";
  ptr +=(int)tempSensor2;
  ptr +="<span class='superscript'>°C</span></div>";
  ptr +="</div>";
  ptr +="<div class='data kitchen'>";
  ptr +="<div class='side-by-side icon'>";
  ptr +="<svg enable-background='new 0 0 48 31.5'height=31.5px id=Layer_1 version=1.1 viewBox='0 0 48 31.5'width=48px x=0px xml:space=preserve xmlns=http://www.w3.org/2000/svg xmlns:xlink=http://www.w3.org/1999/xlink y=0px><circle cx=24.916 cy=15.75 fill=#26B99A r=15.75 /><path d='M14.917,15.75c0-5.522,4.478-10,10-10c2.92,0,5.541,1.26,7.369,3.257l1.088-1.031";
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  ptr +="C16.011,20.945,14.917,18.477,14.917,15.75z'fill=#FFFFFF /><path d='M45.766,2.906c-1.232,0-2.232,1-2.232,2.234v11.203c0,0,2.76,0,3,0v12H48v-12V2.906";
  ptr +="C48,2.906,46.035,2.906,45.766,2.906z'fill=#26B99A /><path d='M6.005,2.917v5.184c0,0.975-0.638,1.792-1.516,2.083V2.917H3.021v7.267c-0.878-0.29-1.516-1.107-1.516-2.083";
  ptr +="V2.917H0v5.458c0,1.802,1.306,3.291,3.021,3.592v16.376H4.49v-16.38c1.695-0.318,2.979-1.8,2.979-3.588V2.917H6.005z'fill=#26B99A /></svg>";
  ptr +="</div>";
  ptr +="<div class='side-by-side text'>Kitchen</div>";
  ptr +="<div class='side-by-side temperature'>";
  ptr +=(int)tempSensor3;
  ptr +="<span class='superscript'>°C</span></div>";
  ptr +="</div>";
  ptr +="</div>";
  ptr +="</body>";
  ptr +="</html>";
  return ptr;
}

If you try to compare this function with the previous one, you’ll come to know that they are similar except these changes.

• We have used Google commissioned Open Sans web font for our web page. Note that you cannot see Google font, without active internet connection on the device. Google fonts are loaded on the fly.

  ptr +="<link href='https://fonts.googleapis.com/css?family=Open+Sans:300,400,600' rel='stylesheet'>";

• The icons used to display temperature readings are actually a Scalable Vector Graphics (SVG) defined in <svg> tag. Creating SVG doesn’t require any special programming skills. You can use Google SVG Editor for creating graphics for your page. We have used these SVG icons.

Improvement to the Code – Auto Page Refresh

One of the improvements you can do with our code is refreshing the page automatically in order to update the sensor value.

With the addition of a single meta tag into your HTML document, you can instruct the browser to automatically reload the page at a provided interval.

<meta http-equiv="refresh" content="2" >

Place this code in the the <head> tag of your document, this meta tag will instruct the browser to refresh every two seconds. Pretty nifty!

Dynamically load Sensor Data with AJAX

Refreshing a web page isn’t too practical if you have a heavy web page. A better method is to use Asynchronous Javascript And Xml (AJAX) so that we can request data from the server asynchronously (in the background) without refreshing the page.

The XMLHttpRequest object within JavaScript is commonly used to execute AJAX on webpages. It performs the silent GET request on the server and updates the element on the page. AJAX is not a new technology, or different language, just existing technologies used in new ways. Besides this, AJAX also makes it possible to

• Request data from a server after the page has loaded
• Receive data from a server after the page has loaded
• Send data to a server in the background

Here is the AJAX script that we’ll be using. Place this script just before you close </head> tag.

ptr +="<script>\n";
ptr +="setInterval(loadDoc,1000);\n";
ptr +="function loadDoc() {\n";
ptr +="var xhttp = new XMLHttpRequest();\n";
ptr +="xhttp.onreadystatechange = function() {\n";
ptr +="if (this.readyState == 4 && this.status == 200) {\n";
ptr +="document.body.innerHTML =this.responseText}\n";
ptr +="};\n";
ptr +="xhttp.open(\"GET\", \"/\", true);\n";
ptr +="xhttp.send();\n";
ptr +="}\n";
ptr +="</script>\n";

The script starts with <script> tag. As AJAX script is nothing but a javascript, we need to write it in <script> tag. In order for this function to be repeatedly called, we will be using the javascript setInterval() function. It takes two parameters – a function to be executed and time interval (in milliseconds) on how often to execute the function.

ptr +="<script>\n";
ptr +="setInterval(loadDoc,1000);\n";

The heart of this script is a loadDoc() function. Inside this function, an XMLHttpRequest()  object is created. This object is used to request data from a web server.

ptr +="function loadDoc() {\n";
ptr +="var xhttp = new XMLHttpRequest();\n";

The xhttp.onreadystatechange() function is called every time the readyState changes. The readyState property holds the status of the XMLHttpRequest. It has one of the following values.

• 0: request not initialized
• 1: server connection established
• 2: request received
• 3: processing request
• 4: request finished and response is ready

The status property holds the status of the XMLHttpRequest object. It has one of the following values.

• 200: “OK”
• 403: “Forbidden”
• 404: “Page not found”

When readyState is 4 and status is 200, the response is ready. Now, the content of body  (holding temperature readings) is updated.

ptr +="xhttp.onreadystatechange = function() {\n";
ptr +="if (this.readyState == 4 && this.status == 200) {\n";
ptr +="document.body.innerHTML =this.responseText}\n";
ptr +="};\n";

The HTTP request is then initiated via the open() and send() functions.

ptr +="xhttp.open(\"GET\", \"/\", true);\n";
ptr +="xhttp.send();\n";
ptr +="}\n";