GB918563A - Electrical protection systems - Google Patents

Abstract

918,563. Intruder alarms. AMERICAN DISTRICT TELEGRAPH CO. April 14, 1961 [May 25, 1960], No. 13493/61. Class 118. In an intruder alarm of the kind wherein the change of capacitance to ground of an object, which occurs on the approach of an intruder thereto, changes the frequency of an oscillator of which the capacitance is an element, the oscillator output is supplied to a resonant circuit the resonant frequency of which differs from the normal operating frequency of the oscillator, and a signal voltage is derived from the resonant circuit which voltage varies in amplitude in dependence upon the difference between the oscillator frequency and the resonant frequency, it being arranged that either a change in magnitude of the signal voltage of a predetermined amount, or a rate of change thereof of a predetermined value causes an alarm to be given. As shown, the capacitances to ground of the safe 10 and the filing cabinet 11 are included by way of door switches 15 and 12 as frequency determining elements of the oscillator including transistor 39 and transformer TF1. Part of the radio-frequency (e.g. 50 kc./s.) output of the oscillator is applied via link winding 48 to a series resonant circuit including capacitor 49 and coil TF2, the oscillator frequency being arranged by adjustment of capacitor 47 to be somewhat less than that of the resonant circuit. Part of the oscillator output applied to the resonant circuit is rectified by rectifier 51 and produces a D.C. voltage across resistor 54 and capacitor 55 which voltage, termed the detector voltage, charges the capacitor 53. On the approach of an intruder the oscillator frequency becomes lower, the detector voltage drops and capacitor 53 discharges via the base of transistor 50, the amplified collector current flowing in the main winding of relay X. If the rate of change of the oscillator frequency is sufficiently high, relay X operates, whereupon closure of 65-66 signals the control station over 31, 43. To detect the slow approach of an intruder (rate of change of frequency not high enough for the discharge current of 53 to operate X) a small voltage (the supervisory voltage) is supplied across resistor 54 by potential divider 56, 57, which voltage is of opposite polarity to the detector voltage. When the detector voltage becomes less than the supervisory voltage rectifier 52 conducts, and relay X is operated. Power is supplied from the central station, bridge rectifier 38 ensuring that line 58 is positive and 59 negative independent of the polarity of 31 and 43. The sensitivity may be varied by connecting the capacitances of the objects to be protected to different ones of the taps T1, T2; T4, T5; T6, T7. Setting-up test.-The application of increased line voltage from the central station breaks down Zener diode 67 and brings up the relay Y. Operation of its contact 30 connects capacitor 69 into the oscillator circuit so as to simulate the approach of an intruder. Relay X comes up and rings the local bell 70 to signal a successful test, the bell contact 46 rhythmically interrupting the line to the central station. Bell 70 does not operate on normal line voltage. The central station operator momentarily opens the line to drop out X and Y, and then restores the normal line voltage. If the safe 10 or cabinet 11 is open during the test the capacitance of the relevant object will not be in circuit and the change in capacity will not be enough to cause relay X to operate.