This is the simplest 125 kHz RFID tag emulator on Arduino.
Antenna and resonant capacitor taken from RDM6300 shield.
Everything is connected according to the following scheme:
ID-card is defined in the definition: CARD_ID 0x0001020304
An article about this emulator In Russian: https://habr.com/ru/company/ruvds/blog/533474/ In English: https://habr.com/ru/company/ruvds/blog/570114/