RFID tags working

I’ve got the RFID tags working for the free drinks at Hull CS50. The first time you touch your card on the reader you get a green light. The second and subsequent times you do this you get a red light. There’s also a magic “reset” card (which I really mustn’t lose) which you can use to reset the card storage and let everyone order drinks again.

It was suggested that we just use drinks tickets. But where’s the technical fun in that?