drj113 from instructables has been inspired by RFID card with keypad on it so he decided to build his own. With Arduino compatible Atmega microcontroller he generated the 125kHz RF field by using simple circuit built of inductor couple caps and diode bridge.
Key has a keypad assembled directly on PCB where you can enter access number that is found on regular tags and this way emulate it. Test results didn’t disappoint – key emulator worked at once when swept near reader.
We recommend EasyEDA for circuit design and PCB prototype