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RF/ultrasonic separation distance alarm

  • US 5,640,144 A
  • Filed: 10/19/1995
  • Issued: 06/17/1997
  • Est. Priority Date: 10/13/1995
  • Status: Expired due to Fees
First Claim
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1. An alarm system for preventing loss or theft of at least one article, comprising:

  • a transmitter located in a protection device or tag attached to or incorporated in said at least one article; and

    a monitoring unit provided with a receiver for receiving the signal from said transmitter and triggering an alarm when the distance between said receiver and transmitter exceeds a predetermined value,said transmitter being an ultrasonic transmitter and said receiver being an ultrasonic receiver, and said monitoring unit further comprising an RF transmitter for periodically transmitting coded RF signals, and each protection device including an RF receiver and a logic unit for activating said ultrasonic transmitter in response to said coded RF signals.

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