GB892872A - Intruder alarm systems - Google Patents

Abstract

892,872. Intruder alarm systems. AMERICAN DISTRICT TELEGRAPH CO. Sept. 12, 1960, No. 31296/60. Class 118. A transmitting antenna array T (Fig. 1) is fed with a radio frequency signal part of which is received by a receiving antenna array R and fed to a receiver, a direct electrical coupling between the signal source and the receiver being also provided. Changes in the capacitative coupling between the arrays T and R and in the capacitance between each array and ground, which occur on the approach of an intruder, change the average value of the received signal at a rate faster than a predetermined one and result in a alarm signal being given. As described the signal source is a tuned-anode oscillator, including tube 30 (Fig. 3) provided with regenerative feed-back via coils 31, 32, which feeds the transmitting antenna array with signals at a frequency between 100 kc/s. and 200 kc/s. A coil 37, inductively coupled to the coil 32, provides a direct transfer of energy to the receiving circuit, which consists of a parallel resonant circuit coupled to the detector tube 38, which also acts as a half-wave voltage doubler. The receiving circuit is tuned to a frequency which differs from the oscillator frequency by an amount depending on the Q of the circuit so that it operates on a steep portion of the tuning curve. The composite A.C. signal delivered to the tube 38 is rectified and doubled, resulting in a D.C. voltage being developed between terminal 44 and ground, which normally is unchanging so that no signal voltage is applied via 47, 48 to the pentode amplifier tube 46. Capacitor 47 and potentiometer 48 form a differentiating circuit with a time constant of about 20 seconds, so that slow changes in the D.C. voltage caused by weather changes and the like will not affect tube 46. More rapid changes will be amplified by tube 46 and passed via a further differentiating circuit 50, 51 (having a similar time constant) to a second amplifying tube 49. The anode current of tube 49 flows in winding 52A of a differential relay and is normally balanced by flow via potentiometer 53. Changes in the anode current operate the relay which transmits an alarm signal to a remote station (not shown). The sensitivity is adjustable by means of potentiometer 48. Capacitor 55 gives negative feed-back for steep front pulses such as are produced by lightning to prevent spurious operation of the alarm.