To my great surprise it turns out you can nowadays get an RFID reader with 2 tags for somewhere between 7$ and 10$. Better yet, interfacing these to Arduino is well documented and several libraries are available for that purpose.
My friend who bought a couple of these asked me to see if I can read the tags directly from a Linux based ARM board (for example Raspberry Pi). It turned out to be rather easy as long as you aren’t interested in anything but the ID of the card. This can be done using rpi-rc522 library (NOT written by me). It sadly currently lacks any documentation or a Makefile so here are my notes on how to get it running.
First you have to connect the rc522 and RPi over the SPI interface. It will look something like this:
Then you have to install the bcm2835 library. Assuming that you have gcc and other essential development stuff already installed you can achieve it this way:
wget http://www.airspayce.com/mikem/bcm2835/bcm2835-1.35.tar.gz tar -zxf bcm2835-1.35.tar.gz cd bcm2835-1.35 ./configure sudo make install
After that you can compile and run the rpi-rc522 with the following commands:
sudo apt-get install subversion svn checkout http://rpi-rc522.googlecode.com/svn/trunk/ rpi-rc522-read-only cd rpi-rc522-read-only/rc522 gcc config.c rfid.c rc522.c main.c -o rc522_reader -lbcm2835 sudo cp RC522.conf /etc/ ./rc522_reader -d
You should see tag IDs in the output of the program as they are read. You can put these into the /etc/RC522.conf file and map each ID to a specific command to run there.