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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.
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.
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
1 x 68 Ohm resistor 1 x 1k Ohm resistor 1 x
1 x
4 x
1 x
Click the links in the list above for descriptions of the different components.
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 publicNow 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 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 publicAnd create a HTML file, index.html:
pi@w3demopi:~/nodetest/public $nano index.html index.html: <!DOCTYPE html> <html> <body>
Formula
< 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.
Formula
pi@w3demopi:~ $ sudo apt - get updateUpgrade all your installed packages to their latest version:
Formula
pi@w3demopi:~ $ sudo apt - get dist - upgradeDoing 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
Now we can use WebSocket in our application. Lets update our index.html file: index.html: <!DOCTYPE html> <html> <body>
Formula
< h1 > Control LEDlight</h1> <p><input type="checkbox" id="light"></p>
Formula
< 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: webserver.js:
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
}
});
});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.
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. webserver.js:
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
});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.
Formula
Ctrl + c.