How Waterloo made his own proximity cards – “Lots of companies use proximity cards to control physical access. An employee holds their card within a few inches of the reader; the reader receives a unique id from the card and transmits it to some central computer that tells it whether or not to open the door. This is rather magical, considering that the tag is credit card-thin and contains no battery. The trick is the same as for RFID tags. The reader constantly transmits a rather strong carrier; the tag derives its power and clock from this carrier, kind of like a crystal radio.” Link.