RFID Reader Detector and Tilt-Sensitive RFID Tag

„Want to detect the presence of RFID readers? Want to control when a RFID tag is active or readable? We describe how to do both using bits of copper and card, and some readily available electronics hardware.“

http://www.instructables.com/id/RFID_Reader_Detector_and_Tilt_Sensitive_RFID_Tag/