RFID cards are commonly used in many areas like access control, security, and for other identification purposes. One of simplest cards to read are HID ISOProx that don’t use any encryption. Their carrier frequency is 125 kHz.
Hardest part in this reader project is the reader itself as it has to power the tag with magnetic field and sense the transmitted signal. For this series resonance of 150 kHz circuit – antenna is used. Signal from antenna is passed through low pass and band pass circuits and then to PIC comparator, where RFID FSK pulses are detected and decoded. Latest ASM source can be seen here. Probably RFID reader could use more advanced microcontroller than PIC16F628A for more efficient signal reading by using interrupt based compare capture module as this one already uses it for 150 kHz carrier signal generation.