Method and apparatus for detecting motion

Claim 1

APPARATUS FOR DETECTING MOTION OF AN INTRUDER IN A PREDETERMINED LIMITED SPACE, COMPRISING AN OSCILLATOR ARRANGED TO PRODUCE AN ALTERNATING CURRENT HAVING A SELECTED FREQUENCY, A TRANSDUCER FOR CONVERTING SAID ALTERNATING CURRENT INTO RADIANT ENERGY AND ARRANGED TO TRANSMIT SAID RADIANT ENERGY INTO SAID SPACE, MEANS INTERCOUPLING SAID OSCILLATOR AND SAID TRANSDUCER, KEYING MEANS ARRANGED PERIODICALLY TO SUPPRESS TRANSMISSION OF SAID RADIANT ENERGY AFTER A RADIATION INTERVAL OF PREDETERMINED LENGTH, RECEIVING AND DETECTING MEANS ARRANGED TO RECEIVE SAID RADIANT ENERGY REFLECTED FROM OBJECTS IN SAID SPACE, TO MIX SAID RECEIVED ENERGY AND SAID TRANSMITTED ENERGY AND TO DEFLECT MODULATION COMPONENTS IN THE MIXED ENERGY RESULTING FROM THE DOPPLER SHIFT IN FREQUENCY PRODUCED BY MOTION OF AN OBJECT IN SAID SPACE, AND ALARM SIGNALLING MEANS COUPLED TO SAID DETECTING MEANS AND ARRANGED TO PRODUCE AN ALARM SIGNAL UPON DETECTION OF MODULATION COMPONENTS OF PREDETERMINED LEVEL, SAID RADIATION INTERVAL BEING SELECTED SO THAT ENERGY CANNOT BE RADIATED, BE REFLECTED FROM AN OBJECT SUBSTANTIALLY OUTSIDE SAID SPACE, AND BE RECEIVED BEFORE THE END OF SAID RADIATION INTERVAL, THE PERIODICITY OF SAID SUPPRESSION OF RADIANT ENERGY TRANSMISSION BEING SELECTED SO THAT ENERGY RADIATED DURING ONE RADIATION INTERVAL AND RECEIVED DURING A SUBSEQUENT RADIATION INTERVAL WILL HAVE ATTENUATED SUFFICIENTLY THAT MODULATION COMPONENTS PRESET THEREIN WILL BE BELOW SAID PREDETERMINED LEVEL.