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Burglar alarm security circuit arrangement

  • US 4,243,979 A
  • Filed: 12/07/1977
  • Issued: 01/06/1981
  • Est. Priority Date: 12/13/1976
  • Status: Expired due to Term
First Claim
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1. A circuit arrangement for a security device, which circuit arrangement receives ultrasonic and radio Doppler-frequency signals, comprising:

  • a first input for receiving an ultrasonic Doppler-frequency signal;

    a second input for receiving a radio Doppler-frequency signal;

    a frequency converter connected to one of said inputs for normalizing the frequency of the respective received signal to the frequency of the other received signal;

    a product detector connected to said frequency converter and to the other of said inputs for providing the difference frequency of the signals applied thereto;

    a low pass filter connected to said product detector and having a frequency-dependent attenuation characteristic;

    a threshold value detector connected to said low pass filter for providing output signals in response to signals above a threshold value; and

    a control voltage circuit connected to one of said inputs and to said threshold value detector to derive a control signal having a frequency which is dependent upon the frequency received at said one input,said threshold value detector including means responsive to said control voltage signal to adjust the threshold value in accordance with the frequency-dependent attenuation characteristics of said low pass filter.

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