Patent ID: 6731198
Filing Date: 2004-05-04
Classification: G06K

Abstract:
A security system comprising(a) a base station, and (b) at least one transponder having a first transmitter device, a dialogue taking place between said base station and said transponder wherein the base station sends data to the transponder on a carrier frequency in the LF (low frequency) range and the transponder sends back data to the base station on a carrier frequency in the RF (radio frequency) range, said transponder comprising a second transmitter device for data transmission to a destination selected from the group consisting of the base station and a read/write device, said second transmitter device sending on a frequency selected from the group consisting of an electromagnetic carrier frequency in the LF range, an audible carrier frequency in the ultrasonic range and an optical carrier frequency in the infrared range while the base station and the transponder transmit on different carrier frequencies in the LF range; wherein the base station, because of received identification or attendance signals of the transponders sends out status messages of the transponders identified or registered attendant at intervals and the transponders evaluate the status messages; and wherein the transponder, which on evaluation of the status messages discovers the absence of its identity or attendance messages, immediately or after a predetermined number of repetitions of the status messages upon absence of its identity or attendance messages, sends out data to the base station in the LF range by means of the second transmitter device.