Arduino Ethernet Web Server with Relay

This post shows how to build an Arduino Ethernet web server that controls a relay that is attached to a lamp.

You can access your web server with any device that has a browser and it’s connected to the same network.

Recommended resources:

If you like Arduino projects and you want to build more, I recommend getting my Arduino Step-by-step Projects – Build 25 Projects course.

Note: if you’re not comfortable dealing with mains voltage, but you still want to try to do the project,  you can replace the relay module with an LED, for example. The code and the schematics are very similar.

Ethernet shield

The Arduino Ethernet shield connects your Arduino to the internet in a simple way. Just mount this module onto your Arduino board, connect it to your network with an RJ45 cable and follow a few simple steps to start controlling your projects through the web.

Note: you must connect an Ethernet cable from your router to your Ethernet shield as shown in the following figure.

Pin usage

When the Arduino is connected to an Ethernet shield, you can’t use Digital pins from 10 to 13, because they are being used in order to establish a communication between the Arduino and the Ethernet shield.

Relay module

A relay is an electrically operated switch. It means that it can be turned on or off, letting the current going through or not. The relay module is the one in the figure below.

This particular relay module comes with two relays (those blue cubes).

About mains voltage, relays have 3 possible connections:

  • COM: common pin
  • NO: normally open – there is no contact between the common pin and the normally open pin. So, when you trigger the relay, it connects to the COM pin and power is provided to the load (a lamp, in our case).
  • NC: normally closed – there is contact between the common pin and the normally closed pin. There is always contact between the COM and NC pins, even when the relay is turned off. When you trigger the relay, the circuit is opened and there is no power provided to the load.

Relating this project, it is better to use a normally open circuit, because we want to light up the lamp occasionally. Read this tutorial to learn more about using a relay module with the Arduino board.

The connections between the relay and the Arduino are really simple:

  • GND: goes to ground
  • IN1: controls the first relay. Should be connected to an Arduino digital pin
  • IN2: controls the second relay. Should be connected to an Arduino digital pin
  • VCC: goes to 5V

Parts required

Here’s a complete list of the components you need for this project:

You can use the preceding links or go directly to to find all the parts for your projects at the best price!


Copy the following code to your Arduino IDE and before uploading it to your Arduino board read the “Configuring your network” section below.

 * Rui Santos 
 * Complete Project Details

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

// Enter a MAC address and IP address for your controller below.
// The IP address will be dependent on your local network:
byte mac[] = { 
  0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };
IPAddress ip(192,168,1, 111);

// Initialize the Ethernet server library
// with the IP address and port you want to use 
// (port 80 is default for HTTP):
EthernetServer server(80);

// Relay state and pin
String relay1State = "Off";
const int relay = 7;

// Client variables 
char linebuf[80];
int charcount=0;

void setup() { 
  // Relay module prepared 
  pinMode(relay, OUTPUT);
  digitalWrite(relay, HIGH);
  // Open serial communication at a baud rate of 9600
  // start the Ethernet connection and the server:
  Ethernet.begin(mac, ip);
  Serial.print("server is at ");

// Display dashboard page with on/off button for relay
// It also print Temperature in C and F
void dashboardPage(EthernetClient &client) {
  client.println("<!DOCTYPE HTML><html><head>");
  client.println("<meta name=\"viewport\" content=\"width=device-width, initial-scale=1\"></head><body>");                                                             
  client.println("<h3>Arduino Web Server - <a href=\"/\">Refresh</a></h3>");
  // Generates buttons to control the relay
  client.println("<h4>Relay 1 - State: " + relay1State + "</h4>");
  // If relay is off, it shows the button to turn the output on          
  if(relay1State == "Off"){
    client.println("<a href=\"/relay1on\"><button>ON</button></a>");
  // If relay is on, it shows the button to turn the output off         
  else if(relay1State == "On"){
    client.println("<a href=\"/relay1off\"><button>OFF</button></a>");                                                                    

void loop() {
  // listen for incoming clients
  EthernetClient client = server.available();
  if (client) {
    Serial.println("new client");
    // an http request ends with a blank line
    boolean currentLineIsBlank = true;
    while (client.connected()) {
      if (client.available()) {
       char c =;
       //read char by char HTTP request
        if (charcount<sizeof(linebuf)-1) charcount++;
        // if you've gotten to the end of the line (received a newline
        // character) and the line is blank, the http request has ended,
        // so you can send a reply
        if (c == '\n' && currentLineIsBlank) {
        if (c == '\n') {
          if (strstr(linebuf,"GET /relay1off") > 0){
            digitalWrite(relay, HIGH);
            relay1State = "Off";
          else if (strstr(linebuf,"GET /relay1on") > 0){
            digitalWrite(relay, LOW);
            relay1State = "On";
          // you're starting a new line
          currentLineIsBlank = true;
        else if (c != '\r') {
          // you've gotten a character on the current line
          currentLineIsBlank = false;
    // give the web browser time to receive the data
    // close the connection:
    Serial.println("client disonnected");

View raw code

Configuring your network

Take a look at the configuring your network code snippet:

byte mac[] = {
0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };
IPAddress ip(192,168,1,XXX);

Important: you actually might need to replace that variable highlighted in red with appropriate values that are suitable for your network, otherwise your Arduino will not establish a connection with your network.

Replace the following line with an IP that is available and suitable for your network:

IPAddress ip(X, X, X, X);

In my case, my IP range is 192.168.1.X and with the software Angry IP Scanner I know that the IP is available in my network, because it doesn’t have any active device in my network with that exact same IP address:

IPAddress ip(192, 168, 1, 111);


Wire you circuit accordingly to the schematic below:


Your Arduino Web Sever looks like the figure below:

Here’s a demonstration showing what you have at the end of this project:

Wrapping up

With this project you built an Arduino web server that turns a relay on and off.

Now, you can use this project to control any electronics appliance you want.

This project is a simplified version of one of my projects in the  Arduino Step-by-step Projects. If you liked this project make sure you check out the course that includes 23 Arduino projects.

Thanks for reading,

Rui and Sara

Learn how to build a home automation system and we’ll cover the following main subjects: Node-RED, Node-RED Dashboard, Raspberry Pi, ESP32, ESP8266, MQTT, and InfluxDB database DOWNLOAD »
Learn how to build a home automation system and we’ll cover the following main subjects: Node-RED, Node-RED Dashboard, Raspberry Pi, ESP32, ESP8266, MQTT, and InfluxDB database DOWNLOAD »

Recommended Resources

Build a Home Automation System from Scratch » With Raspberry Pi, ESP8266, Arduino, and Node-RED.

Home Automation using ESP8266 eBook and video course » Build IoT and home automation projects.

Arduino Step-by-Step Projects » Build 25 Arduino projects with our course, even with no prior experience!

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35 thoughts on “Arduino Ethernet Web Server with Relay”

  1. Many thanks for putting this together. I’m hoping to build something very similar but just using an ESP8266. Hopefully the code adapts without too much trouble.

  2. Hello, Mr Santos
    I very much like your tutorials.
    Do you have a paperback book that includes registers,timers,direct port addressing for Arduino Uno, Nano (atmel 328).
    I mean something with weird notations like PORT D &= (compound bitwise and) ~ (bitwise not) (1 << 4) << (bitshift left) of 4 bit's
    Thanks , Sneppe Rudy

  3. Hi Rui,

    I tried this project; just replaced relay with LED.

    I am always getting for
    EthernetClient client = server.available();
    client == 0

    This is despite I am trying to connect to proper server from web browser.

    Any hint? I do understand it is thoroughly an Ethernet library issue, but please see if you can help.


      • Yes; it does work and it did work.
        The over project also worked as I was able to turn LED on/off using the browser.
        Just for clarification so that other readers are helped; I used a cross cable between by Laptop and Arduino Ethernet Shield. I had a Dlink wifi router and possibly it does not work as switch.

  4. hi rui my probleem is dat ik van de 19-arduino code-web-server
    geen schets kan maken in de ide ze zeggen dat het bestand zich een
    schetchmap met die naam moet bevinden maar ik kan hem niet
    ik hoop dat je mij kan helpen
    groeten wim

  5. can this also be used in conjunction with a program that will turn the relays on and off at specific time of the day but use the web server for an override ?

  6. This is fine and works on my home net, bug how do I phone my home when I’m away and turn on my air conditioner, so it will be cool when I get home?

      • Hello Rui
        I have tried adding declaring more outputs and buttons and have not been successful. The modified code verified and uploads without errors but the browser does connect after doing this.

      • Olá… este projeto era perfeito para uma pequena situação que tenho em mĂŁos.

        Preciso de controlar uma placa de 4 reles para ligar e desligar camaras mas depois de alterar o cĂłdigo para tentar aumentar o numero ( com sucesso ) a pagina web surge com os botões a desaparecer quando se carrega neles… pode ajudar ( pago se for preciso )

  7. Hi Rui,
    Thank you for this awesome tutorial. I have a question, can I control the relay just by putting its state in the browser.
    For example, if am searching for “\relayon” directly can I control in this way the relay, instead of going into the page and clicking on the buttons.

  8. This is a very solid and helpful beginning project for networking “things” and I thank you for your time in putting it together. I wonder about whether there is a way to do the “while do,” in a connection state, in a way that is non-blocking. If there are other bits of code, they don’t run if there is a client connected. Any thoughts? (I just couldn’t think of any.) Great job!

    • Hi Chris.
      Thank you for following your work.
      I think this question describes what you want to do:

  9. I tried exact code with just change in IP address. But Server does not connect.
    I am sure hardware connection, IP Address etc are all good.
    But still i cannot find how where i am going wrong.
    Please help me.

  10. Hi Rui
    its a nice tutorial Thankls alot
    im Confused about alot of tutrials on the web about the same project
    but your one is more clear
    But i need to read a switch & display it on the web page
    do you have this to try ?
    thanks in advance


  11. Hi Sara,
    I want to add a physical switch near the Arduino to activate the relay.
    How can I implement this on the sketch ?

  12. Hello,
    I’m new to Arduino, and found this relay control project and am having some fun. It works really well. I’ve done port forwarding on my router, so I can control it externally from the web on my mobile phone. I’ve hooked it up to my furnace so I can change the heat setting remotely, so I’d like to change the default port to a more obscure setting to keep unwanted agents from accessing it. Any attempts I make to change it from the default port, (by appending the ip address with a colon and port number), result in a message “This page isn’t working 192.168.xx.xx sent an invalid response.” ERR_INVALID_HTTP_RESPONSE
    I’ve tried this on the local LAN to rule out any ISP or Router anomalies. Does the port change need to happen somewhere else in the code, in addition to the “EthernetServer server(80);” I’m using a “HiLetgo W5100” ethernet shield I got from amazon for $15.00
    Thank you!

  13. I forgot to mention, I already tied modifying this line: from Ethernet.begin(mac, ip);
    to this Ethernet.begin(mac, ip, server); which did not resolve the issue.


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