My new weather station [1] has several sensors for wind, rain, and temperature, as well as a USB cable and evaluation software (Figure 1), albeit a desktop-only Windows application. However, I wanted to record the measured data and, if possible, access it on my cell phone while I was on the move.

Figure 1: The electronic TFA Nexus weather station records wind, rain, and outdoor temperature and outputs the data via USB. (Image: TFA Dostmann)

Positioning a Windows PC running 24/7 next to my weather station was not an option; instead, I set my sights on the Raspberry Pi. Thus far, I had used it as a tiny media center with an equally tiny speaker, but by using its USB connection, my Pi could read the data output from the weather station. The power requirement is negligible compared with that of a full-size PC, and the Pi is silent. The question then arose: How can I acquire the data via USB if the station only comes with a Windows program?

Pi Configuration

Your Raspberry Pi can be freshly installed with the official recommended Debian Raspbian "wheezy" image from the Download page on the Raspberry Pi website [2]. The tool set necessary to complete this project comprises a C program, Ruby, the Sinatra web application library, a database, and graphing software, all of which are introduced in the "Ingredient" sections of this article. Figure 2 shows how all the components interact and fit together.

Figure 2: The components of the Raspberry Pi weather station project.

Note that you do not have to install a web server. The Ruby script in Listing 4 is the complete web server and web application; Sinatra loads a simple web server for the application when the script is executed (see "Ingredient 3").

Ingredient 1: A Small C Program

A small program written by John Sebastian named te923tool [3] can read the data output by many weather stations based on HIDEKI. With some minor niggles, the program also works for my TFA Nexus. I only needed to compile the source code on my Raspberry Pi. For this to work, you need the developer tools gcc , make, and libusb-dev.

After unpacking the archive, calling make should give you the te923con program. A test with the te923con -h command not only informed me as to whether the program was running, it also provided detailed help for the program. If all is well, a call without any parameters returns the measured data:

$ sudo ./te923con
1363468817:20.40:50:i:i:i:i:i:i:i:i:-1.90:60:1008.0:i:5:0:14:0.4:0.6:-3.8:115

You will need to use sudo here because access to the USB port requires root privileges.

Notice the values between the colons. The Help function explains that an "i" stands for values that could not be assigned. Although the first field is not explained, it is Unix time (i.e., the number of seconds since the Unix epoch, or January 1, 1970). Calling

date -d @1363468817 +%c

in the terminal converts this number into a legible date: Sat 16 Mar 2013 22:20:17 CET.

In a cron job, I run te923con and redirect the output to a text file named weather.csv, which a Ruby script then uses to populate a database.

Ingredient 2: Ruby

Ruby is the perfect way to process data and feed it to a web server. The numerous libraries that can be installed via Ruby's own package management system, RubyGem, include the ActiveRecord database interface and Sinatra web server. These two libraries later provide the core of the application. Ruby can be installed on the Raspberry Pi from the Debian repositories; however, for a more recent version that is easier to manage, you need the Ruby version management system rbenv [4].

Because rbenv is installed from a GitHub.com source, you need to install the git package up front. Some Ruby libraries are also compiled before the installation on the target system. To be able to install additional libraries later, you will also want to install the zlib1g-dev, libsqlite3-dev, libssl-dev, and libreadline6-dev libraries as Debian package sources.

For instructions on installing rbenv, check out the program website; then, you can run

rbenv install 2.0.0-p0

to install the current version of Ruby. On the Raspberry Pi, that can take a while. Before you can use Ruby, you need to call

rbenv global 2.0.0-p0

to define this version as the default for the user. A test with ruby -v shows that the installation worked.