US Patent 7151451 Near field presence detection device

The invention relates to a near field presence detection device provided to monitor an area (11) around a mouth (2) of a reading member, in particular a card reading member (1). The device comprises a presence detector (3) including a transmitter (4) and a receiver (5), said detector is provided to detect a change of field impedance due to a foreign object in the area (11). The device is further provided to generate a preliminary alarm signal and an effective alarm signal if preliminary alarm signal subsists within a predetermined time period.