Capacity intruder alarm having capacitive a.c. coupling and d.c. bias coupling in parallel between a detector and amplifier

Claim 1

A CAPACITY ALARM SYSTEM COMPRISING AN OSCILLATOR, A TUNED RESONANT CIRCUIT CONNECTED TO SAID OSCILLATOR, SAID RESONANT CIRCUIT INCLUDING ANTENNA MEANS AND AN INDUCTIVE WINDING, A FIRST TRANSISTOR CONNECTED TO SAID INDUCTIVE WINDING FOR SENSING CHANGES IN CAPACITANCE IN SAID TUNED RESONANT ANTENNA CIRCUIT, SAID TRANSISTOR INCLUDING A COLLECTOR HAVING A D.C. POTENTIAL IN VARIABLE RESPONSE TO SAID CHANGE IN CAPACITANCE, A SECOND TRANSISTOR FOR CONTROLLING ACTUATION OF AN ALARM, AND FAIL-SAFE CIRCUIT MEANS INTERCONNECTING SAID FIRST TRANSISTOR AND SAID SECOND TRANSISTOR SAID FAIL-SAFE CIRCUIT COMPRISING A RESISTOR INTERCONNECTING SAID COLLECTOR AND SAID SECOND TRANSISTOR, AND A PARALLEL OPERATING BIAS FOR SAID SECOND TRANSISTOR, AND A PARALLEL CIRCUIT MEANS INCLUDING A CAPACITOR AND SERIES CONNECTED SENSITIVITY RESISTOR EFFECTIVE TO CAUSE CURRENT FLOW IN A REVERSE DIRECTION TO SAID SECOND TRANSISTOR IN RESPONSE TO AN INCREASE IN CAPACITY IN SAID RESONANT CIRCUIT AND A DECREASE IN POTENTIAL OF SAID COLLECTOR, AND A DIODE SHUNING SAID SENSITIVITY RESISTOR, SAID DIODE BEING CONNECTED TO PROVIDE A HIGH IMPEDANCE PATH TO CURRENT DISCHARGED BY SAID CAPACITOR.