hi recently i bought a mfrec522 rfid with tags..i can compare the uid of a particular card to other cards..so that i can give acces to that particular card..im trying to use mfrc522 rfid as sensor..whenever i put the card on the reader it read continuously uid of the card(which i want to give acess),but i want arduino should give me an acknowledgment continuously whenever card is not present on that reader..so that i can put the mfrc522 reader on the fix bar of a door and the tag is on the door...whenever someone try to open that door it will give a alarm..plzz give me the code