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Burglar alarm device

  • US 4,275,390 A
  • Filed: 03/24/1977
  • Issued: 06/23/1981
  • Est. Priority Date: 03/31/1976
  • Status: Expired due to Term
First Claim
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1. Burglar alarm apparatus with transmitters for electromagnetic and for ultrasonic radiation respectively and with receivers for reflected electromagnetic radiation and ultrasonic radiation reflected respectively, which reflected radiations are Doppler shifted by an object moving through the radiation fields, comprising:

  • a demodulating electromagnetic radiation receiver for receiving the electromagnetic radiation and providing electrical signals corresponding to the Doppler frequency of the electromagnetic radiation;

    a demodulating ultrasonic receiver for receiving the ultrasonic radiation and providing electrical signals corresponding to the Doppler frequency of the ultrasonic radiation; and

    signal comparison means connected to both of said receivers and operable to produce a signal in response to the ratio of the Doppler frequency signals being determined within a predetermined tolerance width to indicate that the two Doppler frequency signals are reflected by the same object.

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