GB936270A - Electrical protection and burglar alarm system - Google Patents

Abstract

936,270. Burglar alarms. AMERICAN DISTRICT TELEGRAPH CO. April 6, 1962 [May 31, 1961], No. 13293/62. Class 118. [Also in Group XL (c)] A burglar alarm operates on the principle of detecting vibrations caused by attempts at entering the premises and energizes piezoelectric microphones 12, 13 fixed to the walls connected through a transistor amplifier 21, 24, 25, 26 to a unijunction transistor 63 so biased as to energize an alarm relay 64 only if the detected vibrations are of duration sufficient to indicate that they are not accidental. This is achieved by the provision of a transistor 45 (normally biased to cut-off), and a capacitor 61, the amount of charge on which determines when transistor 63 becomes conductive. If the amplitude of the detected vibrations is sufficient transistor 45 conducts and energizes relay 47. Armature 51 of relay 47 then makes with contact 52 and capacitor 61 begins to charge up through one of resistors 58, 59, 60 and provided the vibrations are of sufficient duration the charge on capacitor 61 will be high enough to make transistor 63 conductive. The alarm signal may be transmitted to a central station 65. If an alarm is detected contact 64.1 of relay 64 opens, opening the central station circuit and resulting in the opening of " ground drop 69 alerting the central station operator. Transistor 63 locks relay 64 until power is removed from the line at the central station. After opening of 69 a further " ground drop " 70 is operated as another warning due to the energization of relay 82 when contact 64.1 opens. A rectifier 100 produces the delay between operation of 69 and 70. Restoration of the circuit is effected by opening switch 71. A testing circuit comprises a buzzer 102 having interruptor contacts 103 simulating vibrations and associated with one of the microphones. Test switch 71 armature is switched to 105 at the central station, this connecting battery 68 and drops 69, 70 from line 66, 67 and connecting test battery 106 and the primary of transformer 107. The voltage of battery 106 is higher than that of 68 and sufficient to energize relay 83 and open contact 83.1. If the system is in operating condition the vibrations produced by buzzer 102 will cause energization of transistor 63, opening of alarm relay contact 64.1 and energization of relay 82. Contact 82.2 closes, test bell coils 98, 99 are energized and the test bells ring in addition to the flashing of a neon 108 at the central station due to the intermittent opening of test bell contact 89. A varistor 101 across 64.1 quenches any arcs on opening of this contact and a Zener diode 104 is connected across the buzzer coil.