RFID readers aren’t very expensive to buy. But if you are real hobbyist and like building instead of purchasing stuff, then Attiny13 based RFID reader might be interesting to experiment with. It was build by Vassilis Serasidis – author of many great projects. As always he tries to keep things simple, cheap and easy to follow. The RFID reader he builds works with standard 125kHz tags that uses EM4100 protocol.
There is a good explanation on how the circuit works in physical level. It includes how reader transmits data and receives from passive tag. Reader is assembled on prototyping board using through hole components and hand made coil on the back. You will also get some understanding how to calibrate coil to have great range and reliability. Currently reader outputs the 10-digit Tag serial number via Serial interface 2400 bps 8N1.