| 摘要 |
A transmitter emits a pulsed radio signal and a digitally encoded radio signal near each exit of a protected premises. Monitored articles carry a receiver having a first circuit for receiving the pulsed signal, a power source, and a power switch for coupling the power source to a second circuit on detection of the pulsed signal. The second circuit receives the digitally encoded signal as multiple, serially-transmitted code bits. It also contains a clock for generating timing signals synchronized by the pulsed signal in order to distinguish a plurality of code bit intervals. A processor, an associated memory, and an address counter compare the digitally encoded signal with a stored bit pattern. On receiving an improper code bit, the processor causes the power switch to decouple the power source from the second circuit to conserve power. If the received signal matches the stored bit pattern, power is maintained to the second circuit. Alarm circuitry is triggered by overflow from the address counter. The address counter is reset each time the pulsed signal is received and thus produces no overflow, but when the article is removed from the protected premises, the address counter is allowed to overflow, triggering the alarm circuit.
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