Reading a temperature sensor using PHP on a Raspberry Pi

It’s easy to add a temperature sensor to a Raspberry Pi. In this example I’ll explain how to set it up and access the data using PHP.

The DS18b20 is a great digital temperature sensor. It only needs three wires and a resistor to get it working on the Raspberry Pi.

The red wire is +3.3v, the black is ground, and yellow is data.

The resistor is connected between red and yellow to pull up the voltage on the data line.

Red is connected to pin 1 on the GPIO, black to pin 6, and yellow to pin 7.

Connecting a temperature sensor to a Raspberry Pi

Reading the temperature

Now the circuit is ready, we can access the data. We need to enable the relevant modules on the Raspberry Pi to do this.

modprobe w1-gpio
modprobe w1-therm

If we now look in the /sys/bus/w1/devices/ directory, we should see a directory starting with 28. This is where we can find the temperature data. Inside this directory is a file called w1_slave. This is the file we read get the data. When we read it, it actually asks the sensor for the data and return it. This means there is a slight delay before the data returns.

pi@Nowscreen:~ $ cat /sys/bus/w1/devices/28-031683a865ff/w1_slave
95 01 4b 46 7f ff 0c 10 65 : crc=65 YES
95 01 4b 46 7f ff 0c 10 65 t=25312

The temperature is the value t=25312. We divide this by 1000 to get the temperature of 25.312 degrees celcius.

Reading the temperature with PHP

The first thing we need to do is to find the directory where the w1_slave file is. We can use globbing to help here.

$base_dir = '/sys/bus/w1/devices/';
$device_folder = glob($base_dir . '28*')[0];
$device_file = $device_folder . '/w1_slave';

Now we need to read in the data. We can use the file method as this returns each line of the file in an array.

$data = file($device_file, FILE_IGNORE_NEW_LINES);

Now we extract the temperature. We check the first line is correct by checking for the value “YES” at the end of the line. If this is present we get the value for “t=” at the end of the second line. Finally we divide the value by 1000, and return it.

$temperature = null;
if (preg_match('/YES$/', $data[0])) {
    if (preg_match('/t=(\d+)$/', $data[1], $matches, PREG_OFFSET_CAPTURE)) {
        $temperature = $matches[1][0] / 1000;
    }
}

Now we can display the temperature.

if ($temperature) {
    echo "Temperature is ${temperature}C\n";
} else {
    echo "Unable to get temperature\n";
}

Final PHP temperature sensor code

Here’s the finished code. I’ve also included two system calls to modprobe to ensure the necessary modules are loaded before reading.

<?php

exec('modprobe w1-gpio');
exec('modprobe w1-therm');

$base_dir = '/sys/bus/w1/devices/';
$device_folder = glob($base_dir . '28*')[0];
$device_file = $device_folder . '/w1_slave';

$data = file($device_file, FILE_IGNORE_NEW_LINES);

$temperature = null;
if (preg_match('/YES$/', $data[0])) {
    if (preg_match('/t=(\d+)$/', $data[1], $matches, PREG_OFFSET_CAPTURE)) {
        $temperature = $matches[1][0] / 1000;
    }
}

if ($temperature) {
    echo "Temperature is ${temperature}C\n";
} else {
    echo "Unable to get temperature\n";
}

Controlling a LED on a Raspberry Pi with PHP

I wanted to make a LED light up using PHP on a Raspberry Pi. Most of the examples I’ve seen are for Python, so I wanted to see if was possible or not.

It is actually pretty easy to do, so I thought I’d share my work.

The first thing to do is to wire up the LED to the Raspberry Pi. I used a breadboard so there is no soldering required.

Connect the LED to GPIO pin 18, along with a 330ohm resistor, to GND. We need the resistor to limit the current through the LED otherwise it could burn out.

That’s the LED attached to Raspberry Pi, now we need to control it with PHP code.

PHP LED Control Code

Before we can write any PHP, we need to enable the gpio module on the Raspberry Pi.

sudo modprobe w1-gpio

We need a third party PHP library to be able to talk to the GPIO pins. I’m using php-gpio.

We need to install this with Composer before we can use it.

get http://getcomposer.org/composer.phar
php composer.phar require ronanguilloux/php-gpio

We can now start our PHP code by using the autoloader to bring in the GPIO library.

require 'vendor/autoload.php';

use PhpGpio\Gpio;
$gpio = new GPIO();

Now to control the LED we need to make sure GPIO pin 18 is set to “out”.

$gpio->setup(18, 'out');

To turn the LED on we write the value “1” to pin 18.

$gpio->output(18, 1);

To turn the LED off we write the value “0” to pin 18.

$gpio->output(18, 0);

Finally, we need to make sure we clean up after ourselves.

$gpio->unexportAll();

The Final Code

Bringing this all together, we can write a simple PHP script to turn the LED on, wait 1 second, and turn it off again.

<?php

require 'vendor/autoload.php';

use PhpGpio\Gpio;
$gpio = new GPIO();
$gpio->setup(18, 'out');

echo "LED on\n";
$gpio->output(18, 1);

sleep(1);

echo "LED off\n";
$gpio->output(18, 0);

$gpio->unexportAll();

The final code needs to be run as root to be able to access gpio.