Intrusion warning system

Apparatus for detecting the intrusion of a fence comprising:

• a length of shielded electrical cable, including at least one center conductor surrounded by a dielectric material, mounted on a fence to be protected;
• first circuit means, connected to said cable, for sensing the change in the electric field signal generated by stressing of the cable dielectric by the flexing of same due to movement of the fence and for producing an electrical signal corresponding to same;
• an AM detector connected to the output of said first circuit means for detecting the peaks of the produced electrical signal;
• second circuit means for producing an output signal whenever the detected signal has a duration greater than a predetermined value;
• third circuit means responsive to output signals from said second circuit means for producing an alarm when said second circuit means produces output signals for a predetermined portion of a preset time period whose duration is greater than said predetermined value, whereby protection against intrusion by climbing over the fence is provided;
• forth circuit means for producing an output signal whenever the detected signal has a duration less than said predetermined value;
• count selector circuit means for counting the output signals from said forth circuit means and for producing an output signal whenever said count selection circuit means reaches a predetermined count;
• and an alarm circuit, responsive to an output signal from said count selector circuit means, for providing an alarm, whereby protection against intrusion by cutting through the fence is provided.