Public telephone security device, to prevent interference with smart card credit consumption, senses call charge acknowledgement signals transmitted by payphone using sub-audible frequency line current modulation

摘要

The external security device (11) has a current sensing circuit (110) which detects call charge acknowledgement signals from the public telephone to guard against fraudulent usage. These are transmitted using modulation of DC flowing in the line within permissible limits, e.g. a few mA, at an infrasonic frequency such as 10 Hz. The signals are produced by a variable impedance (Z) in the public telephone.