134.2 kHz RFID reader for Arduino
I have been using an Arduino UNO to decode FDX-B with some success in conjunction with the EM4095 chip tuned for 134.2kHz. I purchased a populated board from Elektor, see the article  link: http://outlet.elektor.com/contents/en-us/50-00023%20manual.pdf. 

BTW you can buy a cat flap from: http://www.sureflap.co.uk/ , a bit expensive, it uses a patented technique to extend the detection range.

As for my progress, I can currently detect the cats' tags as well as a test tag and locate the header in the data stream, working on the CRC and data payload formatting.