Patent ID: 7151451

Claim:
A near field presence detection device provided for monitoring an area around a mouth of a reading member, in particular a card reading member, said device comprises a presence detector having a transmitter and a receiver, said transmitter and receiver being provided to operate with a carrier frequency above 18 KHz in said area, said detector being provided to detect a change of field impedance due to a foreign object placed within said area and to generate a detection signal thereupon, said detector being further provided to generate a preliminary alarm signal if a signal level of said detection signal generated by said detector exceeds a predetermined threshold range, said detector being connected to a verification unit provided for determining upon receipt of said preliminary alarm signal if said preliminary alarm signal subsists within a predetermined period (t 2 −t 1 ) and for producing an effective alarm signal if said preliminary alarm signal subsists within said predetermined period (t 2 −t 1 ), characterized in that said detector is connected to an integrator, said integrator being provided for receiving said detection signal and for integrating said detection signal over a time period Δt in order to obtain an integrated value ({overscore (V)}) for said detection signal, said verification unit comprising an input for receiving said integrated value and being provided for determining said threshold range on the basis of said integrated value.