What is WebSocket?
WebSocket enables bidirectional communication in real time over the web.
WebSocket can be run together with a normal HTTP server. You can click a button in a web browser, and enable a GPIO on your Raspberry Pi which turns on a light in your house. All in real time, and with communication going both ways!
In this chapter, we will set up a web server with WebSocket. Then create a browser UI to interact with our earlier example of turning a LED on and off with a button.
What Do I Need?
For this tutorial you need a Raspberry Pi. In our examples we use a a Raspberry Pi 3, but this tutorial should work for most versions.
For this you need:
- A Raspberry Pi with Raspian, internet, SSH, with Node.js installed
- The onoff module for Node.js
- The socket.io module for Node.js
- 1 x Breadboard
- 1 x 68 Ohm resistor
- 1 x 1k Ohm resistor
- 1 x Through Hole LED
- 1 x Push Button
- 4 x Female to male jumper wires
- 1 x Male to Male jumper wires
Click the links in the list above for descriptions of the different components.
Note: The resistor you need can be different from what we use depending on the type of LED you use. Most small LEDs only need a small resistor, around 200-500 ohms. It is generally not critical what exact value you use, but the smaller the value of the resistor, the brighter the LED will shine.
Compared to our earlier example, the only new thing we need is to set up a web server, and install the socket.io module.
Webserver for Raspberry Pi and Node.js
Following the earlier chapters in this Node.js tutorial, lets set up a web server that can serve HTML files.
In our "nodetest" directory create a new directory we can use for static html files:
pi@w3demopi:~/nodetest $
mkdir public
Now lets set up a webserver. Create a Node.js file that opens the requested file and returns the content to the client. If anything goes wrong, throw a 404 error.
pi@w3demopi:~/nodetest $
nano webserver.js
let http = require('http').createServer(handler); //require http server, and
create server with function handler()
let fs = require('fs'); //require filesystem module
http.listen(8080); //listen to port 8080
function handler (req, res) { //create server
fs.readFile(__dirname + '/public/index.html', function(err, data) { //read
file index.html in public folder
if (err)
{
res.writeHead(404,
{'Content-Type': 'text/html'}); //display 404 on error
return res.end("404 Not Found");
}
res.writeHead(200, {'Content-Type': 'text/html'}); //write HTML
res.write(data); //write data from index.html
return res.end();
});
}
Go to the folder "public":
pi@w3demopi:~/nodetest $
cd public
And create a HTML file, index.html:
pi@w3demopi:~/nodetest/public $
nano index.html
<!DOCTYPE html>
<html>
<body>
<h1>Control LED light</h1>
<input
id="light" type="checkbox">LED
</body>
</html>
This file will not have any functionality yet. For now it is just a placeholder. Lets see if the webserver is working:
pi@w3demopi:~/nodetest/public $ cd ..
pi@w3demopi:~/nodetest $ node webserver.js
Open the website in a browser using http://[RaspberryPi_IP]:8080/:
The webserver should now be up and running, and we can move on to the WebSocket part.
Install socket.io for Node.js
With the webserver set up, update your Raspberry Pi system packages to their latest versions.
Update your system package list:
pi@w3demopi:~ $ sudo apt-get update
Upgrade all your installed packages to their latest version:
pi@w3demopi:~ $ sudo apt-get dist-upgrade
Note: Doing this regularly will keep your Raspberry Pi installation up to date.
To download and install newest version of socket.io, use the following command:
pi@w3demopi:~ $
npm install socket.io --save
Adding WebSocket to our Webserver
Now we can use WebSocket in our application. Lets update our index.html file:
<!DOCTYPE html>
<html>
<body>
<h1>Control LED
light</h1>
<p><input type="checkbox" id="light"></p>
<script src="https://cdnjs.cloudflare.com/ajax/libs/socket.io/2.0.3/socket.io.js"></script>
<!-- include socket.io client side script -->
<script>
let socket = io();
//load socket.io-client and connect to the host that serves the page
window.addEventListener("load", function(){ //when page loads
let
lightbox = document.getElementById("light");
lightbox.addEventListener("change", function() { //add event listener for
when checkbox changes
socket.emit("light", Number(this.checked));
//send button status to server (as 1 or 0)
});
});
socket.on('light',
function (data) { //get button status from client
document.getElementById("light").checked = data; //change checkbox according
to push button on Raspberry Pi
socket.emit("light", data); //send
push button status to back to server
});
</script>
</body>
</html>
And our webserver.js file:
let http = require('http').createServer(handler); //require http server, and
create server with function handler()
let fs = require('fs'); //require filesystem module
let io
= require('socket.io')(http) //require socket.io module and pass the http
object (server)
http.listen(8080); //listen to port 8080
function handler (req, res) { //create server
fs.readFile(__dirname + '/public/index.html', function(err, data) { //read
file index.html in public folder
if (err)
{
res.writeHead(404,
{'Content-Type': 'text/html'}); //display 404 on error
return res.end("404 Not Found");
}
res.writeHead(200, {'Content-Type': 'text/html'}); //write HTML
res.write(data); //write data from index.html
return res.end();
});
}
io.sockets.on('connection', function (socket) {// WebSocket Connection
let lightvalue = 0; //static variable for current status
socket.on('light',
function(data) { //get light switch status from client
lightvalue = data;
if (lightvalue) {
console.log(lightvalue); //turn LED on or off, for now we will just show it
in console.log
}
});
});
Lets test the server:
pi@w3demopi:~ $
node webserver.js
Open the website in a browser using http://[RaspberryPi_IP]:8080/:
Now the server should output all the changes to the checkbox to the console on the Raspberry Pi.
The client is sending the changes to the server, and the server is responding.
Lets add the push button controlled LED from a previous chapter.
Adding Hardware, and sending a response to the Client
Lets update our webserver.js file again. We will use a lot of the code from the Pushbutton controlled LED chapter.
let http = require('http').createServer(handler); //require http server, and
create server with function handler()
let fs = require('fs'); //require filesystem module
let io
= require('socket.io')(http) //require socket.io module and pass the http
object (server)
let Gpio = require('onoff').Gpio; //include onoff to
interact with the GPIO
let LED = new Gpio(4, 'out'); //use GPIO pin 4 as
output
let pushButton = new Gpio(17, 'in', 'both'); //use GPIO pin 17 as
input, and 'both' button presses, and releases should be handled
http.listen(8080); //listen to port 8080
function handler (req, res)
{ //create server
fs.readFile(__dirname
+ '/public/index.html', function(err, data) { //read file index.html in
public folder
if (err) {
res.writeHead(404, {'Content-Type': 'text/html'}); //display 404 on error
return res.end("404 Not Found");
}
res.writeHead(200, {'Content-Type': 'text/html'}); //write HTML
res.write(data); //write data from index.html
return res.end();
});
}
io.sockets.on('connection', function (socket) {// WebSocket Connection
let lightvalue = 0; //static variable for current status
pushButton.watch(function (err, value) { //Watch for hardware interrupts on
pushButton
if (err) { //if an error
console.error('There was an error', err); //output error message to console
return;
}
lightvalue = value;
socket.emit('light', lightvalue); //send button status to client
});
socket.on('light', function(data) { //get light switch status
from client
lightvalue = data;
if (lightvalue != LED.readSync()) { //only change LED if status has changed
LED.writeSync(lightvalue); //turn LED on or off
}
});
});
process.on('SIGINT', function () { //on ctrl+c
LED.writeSync(0); // Turn LED off
LED.unexport(); // Unexport LED
GPIO to free resources
pushButton.unexport(); // Unexport Button
GPIO to free resources
process.exit(); //exit completely
});
Lets test the server:
pi@w3demopi:~ $ node webserver.js
Open the website in a browser using http://[RaspberryPi_IP]:8080/:
Now the server should output all the changes to the checkbox to the console on the Raspberry Pi.
The client is sending the changes to the server, and the server is responding.
End the program with Ctrl+c.
