Intrusion detector

Claim 1

An intrusion detection system for reliably detecting intrusions into an enclosed area comprising: detecting means for providing signals indicative of intrusions into said enclosed area; means for determining significant signals indicative of intrusions above a fixed threshold level; first means for measuring the time length of each of said significant signals, and for accumulating the measured time lengths of successive significant signals; means responsive to the accumulated time lengths in said first means reaching a first preselected time length for providing an intrusion-indicating signal; second means for measuring the time length during which no significant signals are received; means connected to said second means for resetting said second means, so that the time length measured thereby is reset to zero upon receipt of a significant signal; and means interconnecting said first and second means for resetting said first means, so that the time length accumulated therein is reset to zero when the measured time length in said second means reaches a second preselected time length, whereby an intrusion-indicating signal is provided only when the accumulated time lengths in said first means reach said first preselected time length prior to the time length during which no significant signals are received, which is measured by said second means reaching said second preselected time length.