Public telephone security device, for chip card credit consumption checking, modulates carrier generated in public phone with variable impedance connected across line in control unit

摘要

Transmission of encrypted metering pulses to a public phone is achieved by connection of a variable impedance, preferably an FET, across the line so that an out of band carrier generated in the public phone is reflected with a phase determined by the signal applied to the FET gate. This signal is based on the random number generated in the control unit and represents metering pulses which have been received from the exchange and encrypted by it.