I am always interested in capturing real values via hardware devices in order to generate the appropriate graphs with my monitoring system. Of course, the outside temperature in our city was at the pole position for such a project. Therefore I ordered a few temperature/humidity sensors from PCsensor (via eBay), plugged them via USB on my Raspberry Pi (Raspbian Linux), and queried them via SNMP from my MRTG/Routers2 monitoring server. Here is the whole story:

Hardware

The USB sticks can also be ordered from PCsensor directly. I am using the sensor called “TEMPerHUM” with the USB ID “1130:660c“. (Google is a really good source for finding similar projects using exactly this type of sensor!) It has a built-in temperature and humidity sensor. Here is a photo of the USB stick:

The following messages appeared after plugging the USB device in ( dmesg ):

With  lsusb the following “Foot Pedal/Thermometer” appeared:

Driver etc.

After quite a few tests with different drivers for this sensor I found a working one, called HID-TEMPerHUM. It needs “libusb-dev” as a prerequisite:

After a download of the HID-TEMPerHUM driver ( git clone https://github.com/jeixav/HID-TEMPerHUM ) and a cd into the directory ( cd HID-TEMPerHUM/ ) it must be built with make , made executable with  sudo chmod u+x temper  and should be moved to the following folder: sudo mv temper /usr/local/bin/. The driver needs root privileges. An example call looks like that:

The first value is the temperature while the second one is the humidity.

Two improvements of this procedure are: Change the ownership of the program with  sudo chown root /usr/local/bin/temper  and set the setuid bit so that every user can run this executable with the rights of the owner: sudo chmod +s /usr/local/bin/temper .

So, the first steps are done. Great. ;)

Script for MRTG

However, I wanted to query the values via SNMP from my MRTG/Routers2 installation. At first this required a small change in the output of the “temper” program. I wrote a small script called “temperMRTG.sh” which calls the original temper program, but displays the two values without a dot on two distinct lines. This is my script:

Make it executable with chmod u+x temperMRTG.sh  and move it to the same folder as the mere temper program: sudo mv temperMRTG.sh /usr/local/bin/ . A call now looks like that:

Querying the Sensor via SNMP

(Not exactly true: You are querying the Raspberry Pi on which the USB sensor is plugged in.) The final step is to add the script in the snmpd.conf file to make it accessible via SNMP. (For further information about the SNMP daemon, use my post about adding a Linux host to MRTG.) Open the snmpd.conf file sudo nano /etc/snmp/snmpd.conf and add the following line under the “EXTENDING THE AGENT” section:

A restart of the snmpd is required: sudo /etc/init.d/snmpd restart. Now “walk the NET-SNMP-EXTEND-MIB tables […] to see the resulting output”, as it is said in the snmpd.conf. Like every time, I am using the iReasoning MIB Browser for this job. This quickly revealed the following OIDs:

MRTG/Routers2 Configuration

Finally, here is my MRTG/Routers2 configuration for this sensor. It queries the two OIDs that I found on the above step with the MIB browser. Note that these OIDs might be different on your system! Also note the “/100” at the very end of the first line which shifts the comma for the values two times to the left. I am using a graph with two y-axis, one on the left (temperature) and one on the right (humidity). Refer to the sample screenshots at the end of this post for a better understanding.

Changing the RRD Files

In order to store negative values (which is not common an a mere MRTG installation since it only reports positive traffic through routers/switches and not negative temperature values), the RRD files of the temperature must be adjusted a bit. The program rrdtune does that. At first, move to the folder in which all RRD files are stored. In my case, this is “/var/mrtg/”. Locate the correct *.rrd file and read out the file with:

The interesting line looks like that:

To have a minimum value of “-100” stored, the rrd must be changed with root privileges such as:

After that when doing an “info” on more time, the line should look like that:

DONE!!! ;)

Sample Screenshots

Since the temperature was not that interesting in Germany during the last few weeks, these graphs look quite boring, i.e., no negative temperatures, or the like. However, here we go:

By the way, this is the sensor on the outside at our balcony:

Though it is not meant to be used outside a building, I have not encountered any problems with this USB sensor. Of course, it is placed under a small roof and therefore protected from direct rain and weather, which has the disadvantage that the measured temperature is a little bit different from the real temperature due to the window near to the sensor.

Links

• Linux Journal using the same perl driver: Hack and / – Temper Temper
• Newer driver for Linux: TEMPered
• (German) Linux Magazin: Perl-Skript misst und bewertet Temperaturen

Featured image “40° im Schatten 346/365” by Dennis Skley is licensed under CC BY-ND 2.0.