We describe an animal entry control system, for example for a cat flap, using RFID (radio frequency identification). The system comprises an RFID reader to register the presence and identification number of a transponder (15) injected under the skin of an animal; and a door (8) mounted on a hinge (10) and controlled by the RFID tag reader. The RFID reader comprises a resonant circuit including a tuning circuit to control the RFID reader such that a drive frequency of the RFID reader matches both a resonant frequency of the RFID reader and a resonant frequency of the transponder. Embodiments of the system are thus tolerant to a degree of detuning, for example from a metallic or magnetic material in the vicinity of the antenna.
