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R.F. SYSTEM FOR DETECTING UNAUTHORIZED TRAVEL OF ARTICLES THROUGH A SELECTED ZONE

  • US 3,818,472 A
  • Filed: 05/26/1972
  • Issued: 06/18/1974
  • Est. Priority Date: 05/26/1972
  • Status: Expired due to Term
First Claim
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1. A theft detection system for detecting the passage of protected articles through a selected area, said system comprising:

  • a. a transmitter for transmitting, when activated, radio frequency signal bursts at a predetermined frequency to said selected area;

    b. a parallel connected inductance capacitance tank circuit tuned to said predetermined frequency and adapted for affixation to articles passing through the selected area;

    c. a superheterodyne receiver for detecting, when activated, radio frequency signal bursts retransmitted thereto from said tank circuit at said predetermined frequency;

    d. control means connected to both said transmitter and said receiver and operative to alternately activate same at a chosen control frequency;

    e. alarm means comprising a main alarm signal channel including a main alarm and an error alarm signal channel including an error alarm, said alarm means including pulse shaping means connected to said control means for reception of control signals therefrom and connected also to both said alarm signal channels, said pulse shaping means incorporating timing means for sorting the radio frequency bursts detected by said receiver into said main alarm channel if such bursts are detected During a predetermined time interval and into said error alarm signal channel if such bursts are detected at times other than said predetermined interval, and counting means for counting a predetermined number of radio frequency bursts for energizing at least one of said alarms.

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