Proximity detector circuitry for elevator closures

Claim 1

IN A PROXIMITY DETECTOR CIRCUIT, IN COMBINATION, TWO CAPACITIVE TYPE BRIDGE CIRCUITS, EACH ONE OF WHICH HAS A PAIR OF ANTENNAS ELECTRICALLY CONNECTED DIRECTLY TO OPPOSITE ENDS OF THE DIAGONAL OF THE BRIDGE TO FORM TWO OF ITS OPPOSITELY DISPOSED ARMS; A SOURCE OF PERIODIC VOLTAGE CONNECTED ACROSS EACH OF SAID BRIDGES; MEANS FOR BALANCING EACH BRIDGE UNDER STATIC CONDITIONS; AMPLIFYING MEANS INDIVIDUAL TO EACH OF SAID BRIDGES, SAID AMPLIFYING MEANS INCLUDING THREE SEMI-CONDUCTOR ELEMENTS, EACH ONE OF WHICH HAS BASE, EMITTER AND COLLECTOR ELECTRODES, SAID ELEMENTS BEING SERIES COUPLED TO PROVIDE THREE STAGES OF AMPLIFICATION, THE FIRST SAID ELEMENT OF SAID SERIES HAVING ITS COLLECTOR ELECTRODE TRANSFORMER COUPLED TO THE BASEEMITTER CIRCUIT OF THE SECOND ELEMENT OF SAID SERIES, SAID SECOND ELEMENT HAVING ITS COLLECTOR ELECTRODE CONDENSER COUPLED TO THE BASE-EMITTERE CIRCUIT OF THE THIRD ELEMENT OF SAID SERIES; MEANS BIASING THE BASE ELECTRODES OF SAID ELEMENTS NEGATIVE WITH RESPECT TO THEIR RESPECTIVE EMITTER ELECTRODES; TRANSFORMER COUPLING MEANS INDIVIDUAL TO EACH OF SAID BRIDGE CIRCUITS COUPLING THE DIAGONAL OF ITS BRIDGE TO THE BASE-EMITTER CIRCUIT OF THE CORRESPONDING SAID FIRST ELEMENTS, WHEREBY UNDER CONDITIONS WHERE ITS CORRESPONDING BRIDGE IS UNBALANCED A PERIODIC SIGNAL IS PRODUCED AND APPLIED TO SUCH FIRST ELEMENT BASE-EMITTER CIRCUIT FOR AMPLIFICATION; AND SIGNAL UTILIZATION MEANS ELECTRICALLY COUPLED TO THE COLLECTOR ELECTRODES OF BOTH THE LAST ONES OF SAID SERIES COUPLED ELEMENTS FOR CONTROLLED OPERATION IN RESPONSE TO SAID AMPLIFIER SIGNALS.