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In the previous post about Raspberry Pi I’ve showed what is needed to be done to install an OS with a mini www server on it ( as easy nodejs script).

RPI is not just a miniature version of a standard PC, where you can test applications or web pages. For this, you can use the VM (at the same time allocate more resources). Raspberry has something more – a built-in GPIO expansion port, with which we can interact with external electronic devices. Starting from a simple lighting a LED diodes, through reading the value of the devices to more advanced control possibilities.

In this post I will show you one of those options – the temperature reading from the 1-wire Dallas 18B20 sensor.

System update

Unfortunately, in the available in July Debian image, there were no 1-wire module, the kernel was in 3.1.9 version.

vokiel@rpi ~$ uname -a
Linux raspberrypi 3.1.9+ #168 PREEMPT Sat Jul 14 18:56:31 BST 2012 armv6l GNU/Linux

Work on the RPI is still going on, so the new updates, additions appear very often. In this case we need a system upgrade. There are a plenty instructions avalible on the net, such as: http://elinux.org/RPi_Kernel_Compilation, http://www.bootc.net/projects/raspberry-pi-kernel/. The easiest onve (if you do not want to compile the kernel yourselfs) is Hexxeh’s easy updater tool, the use of which boils down to three commands:

wget http://goo.gl/1BOfJ -O /usr/bin/rpi-update && chmod +x /usr/bin/rpi-update

sudo apt-get install ca-certificates

rpi-update 240 #podział pamięci na 240MB CPU / 16MB GPU

Given solution does not not update the kernel to the latest available version, but it is sufficient to run the 1-wire:

vokiel@rpi ~ $ uname -a
Linux rpi 3.2.18+ #20 Mon May 28 17:27:57 EDT 2012 armv6l GNU/Linux

1-wire module

Forum Thread on RPI forum about 1-wire is quite extensive: http://www.raspberrypi.org/phpBB3/viewtopic.php?t=6649, one is important for us – enabling the kernel module.

Let’s see first if we have the w1 (1-wire) device visible:

vokiel@rpi ~ $ ls /sys/bus/
amba  hid  mmc  platform  scsi  sdio  spi  usb

As we see – we don’t, so we need to add the module, and reboot the Raspberry

root@rpi:~# modprobe w1-gpio

After reboot everything seems to be ok:

vokiel@rpi ~ $ ls /sys/bus/
amba  hid  mmc  platform  scsi  sdio  spi  usb  w1

Our thermometer will appear in the /sys/bus/w1/devices/ directory, under it’s own id, for example: 28-00000249bf39. But for this to happen, we need to get on with a small soldering.

Electronics

Dallas 18B20

Dallas 18B20

In the picture on the right you can see how the Dallas 18B20 looks like. It is very small and compact, suitable for miniaturization of the RPI.

Next diagram shows pin marks: 0V (GND), DQ, V + – ground, the signal. power. We need a resistor with a resistance of 4.7kΩ, which need to be inserted between 2-3 pin.

Example schema: (To be sure of right connection, check manufacturer’s datasheet.)

http://nathan.chantrell.net/

To finish the project, set of cables will be needed yet. A good source might be an old pc, from which we can take out front panel to the motherboard connection cables. The advantage of such is that, they are ready to connect them to the GPIO pins. Another useful thing can be shrink tubs, to have the termometer not look so tragic like mine – isolated with the tape.

Since this is not an electronics blog, I will not be showing here the soldering process, it is not complicated, or even interesting. Ready, connected set looks as follows:

The doubled plug (blue + grey wire) is doubled just because I’ve lacked of single;), gray wire in this case is unnecessary.

Temperature reading

By default w1-wire for RPi operates on GPIO4 (pin 7) – the blue wire on the photo above. I suggest you refer to the system of pins and their markings, convenient schedule is available at http://www.raspberrypi-spy.co.uk/2012/06/simple-guide-to-the-rpi-gpio-header-and-pins/. The white wire visible in the picture, is connected to P1 – Power 3.3 V, black on the back – to the GND.

If everything is connected correctly, you should see the device in the system:

vokiel@rpi ~ $ ls -l /sys/bus/w1/devices/
razem 0
lrwxrwxrwx 1 root root 0 wrz 16 08:16 28-00000249bf39 -> ../../../devices/w1_bus_master1/28-00000249bf39
lrwxrwxrwx 1 root root 0 wrz 16 13:04 w1_bus_master1 -> ../../../devices/w1_bus_master1

We can see couple files and folder in the device directory:

vokiel@rpi ~ $ cd /sys/bus/w1/devices/28-00000249bf39
vokiel@rpi /sys/bus/w1/devices/28-00000249bf39 $ ls -la
razem 0
drwxr-xr-x 3 root root    0 wrz 16 08:16 .
drwxr-xr-x 4 root root    0 wrz 16 08:16 ..
lrwxrwxrwx 1 root root    0 wrz 16 13:52 driver -> ../../../bus/w1/drivers/w1_slave_driver
-r--r--r-- 1 root root 4096 wrz 16 13:52 id
-r--r--r-- 1 root root 4096 wrz 16 13:52 name
drwxr-xr-x 2 root root    0 wrz 16 13:52 power
lrwxrwxrwx 1 root root    0 wrz 16 13:52 subsystem -> ../../../bus/w1
-rw-r--r-- 1 root root 4096 wrz 16 13:52 uevent
-r--r--r-- 1 root root 4096 wrz 16 08:16 w1_slave

The most important for us is the w1_slave, from which we will be reading current tempreature. It can be done jus by using cat program:

vokiel@rpi /sys/bus/w1/devices/28-00000249bf39 $ cat w1_slave 
c3 01 4b 46 7f ff 0d 10 2f : crc=2f YES
c3 01 4b 46 7f ff 0d 10 2f t=28187

As it is not difficult to guess, t=28187 is the current temperature from the sensor (28.187 °C). Of course, this figure is not very suitable for further use. Ways to read the temperature in a more readable format is as many as programmers, my looks as follows:

vokiel@rpi /sys/bus/w1/devices/28-00000249bf39 $ \
> cat ./w1_slave | grep t= | cut -f2 -d= | awk '{print $1/1000}'
28.125

Node

Since we can read the temperature from the sensor, it’s time to combine it with nodejs.

For start we will need child_process module by which we call systems command and pick the result of its action.

#!/usr/bin/node
 
var exec = require('child_process').exec;
 
var tempId = '28-00000249bf39',
    dispTempInterval,
    dispTempIntervalClock = 10; //sec
 
var dispTemp = function(){
    exec( "cat /sys/bus/w1/devices/" + tempId + "/w1_slave | grep t= | cut -f2 -d= | awk '{print $1/1000}'", function( error, stdout, stderr ){
        if ( error != null ){
          console.log( "Error: " + error);
        }
 
        var temp = parseFloat(stdout).toFixed(2);
        console.log ( "Current temperature: " + temp + "°C" );
    });
}
 
console.log( 'Started node-temp');
console.log( '-----------------');
 
/* Start interval */
dispTempInterval = setInterval( function(){
  dispTemp();
}, dispTempIntervalClock*1000 );

We lauch the script and every 10 seconds will se current temperature readings, which could be saved into a database to draw some pretty chart.

Started node-temp
-----------------
Current temperature: 22.50°C
Current temperature: 22.50°C
Current temperature: 24.19°C

After some small modyfications ( github – rpi-node-temp ) the effect looks a little better. I’ve added one threshold that separates “safe” temperature from the critical and colors them respectively. Touching RPi chip with the thermometer shows the effect of colours:

rpi-node-temp