Electronic intruder alarm apparatus

An intruder alarm apparatus including a radio-frequency radiation source for irradiating with said radio-frequency radiation a space within which an intruder is to be detected, a radio detector responsive to said radio-frequency radiation returned towards said source from objects within said space or from the boundaries thereof to develop detector signals representative of the amount of said returned radiation, and signal processing circuit means responsive to said detector signals to develop an alarm signal in response to predetermined fluctuations in said amount of said returned radiation, the improvement comprising:

• signal peak comparator means in said signal processing circuit means, said signal peak comparator means yielding a resetting signal when the amplitude of a peak in said detector signal is not substantially less than the amplitude of a preceding signal peak;
• timer means reset to an initial condition by each said resetting signal and, unless so reset, yielding an alarm control signal at an output thereof at the end of a predetermined time interval, the duration of said time interval being somewhat greater than the periodicity of an alternating current supply used to energize fluorescent tubes lighting said space;
• and alarm means predeterminedly responsive to said alarm control signal to yield an intruder alarm signal;
• whereby said alarm apparatus is immune to periodically fluctuating return of said radiation from said fluorescent tubes.