Patent GB-882413-A (GB)
Title: Signalling circuit
IPC: G08B13/22
Filed: 1960-06-16
Abstract: 882,413. Electric burglar alarm systems. AMERICAN DISTRICT TELEGRAPH CO. June 16, 1960 [June 18, 1959], No. 21168/60. Class 118. In a burglar alarm system the usual electro-mechanical " drop " which responds when the line current falls below, or rises above, a normal range of values, is replaced by a circuit arrangement including two double-base transistors 25 and 35, responding when the line current falls and rises respectively. The slider of potential divider 14 is adjusted so that when the current in line 10, 11 equals the minimum of the normal range the Zener diode 20 conducts; transistor 18 therefore bottoms and capacitor 23 charges rapidly via diode 24. The potential on the lower plate of capacitor 23 is fed to the base B2 of the double-base transistor 25, B1 of which is grounded, and the emitter of which is connected to a 12-volt tapping 27 on battery 12. The B1-B2 voltage thus exceeds twice the emitter-B1 voltage and the double-base transistor 25 does not conduct. When the line current drops below this value, diode 20 and transistor 18 become non-conductive and capacitor 23 discharges through resistors 21 and 22. When the voltage across 23 reaches a value of 24 volts, the double-base transistor 25 conducts and the lamp 28 is lit. An audible alarm may be given if desired. Should the line current rise into the normal range before capacitor 23 has discharged sufficiently to allow transistor 25 to conduct, it is recharged rapidly and no indication of the momentary break is given. The circuit is reset by manual opening of switch 26. The tap of potential divider 15 is adjusted so that the Zener diode 31 conducts only when the line current exceeds the maximum of the normal range. The double-base transistor 35 has the full battery voltage of 30 volts connected across B1-B2 and will not conduct until the emitter-B1 voltage equals 15 volts. This voltage being normally 12 volts (tap 27), transistor 35 is non-conductive. However, when the line current rises above the maximum of the range, diode 31 and transistor 30 conduct and capacitor is charged. After about 0.3 second the potential across 34 allows the double-base transistor 35 to conduct, whereafter the load transfers to tap 27 and the lamp 38 is lit, an audible alarm being sounded if desired. If the line current returns to the normal range within the 0.3 second charging period of 34, no indication is given. Transistor 43 follows transistor 18 and flashes the neon lamps 46, 47 in response to rapid fluctuations of the line current between the normal range and a lower range, thus to respond to a " hold-up "signal. Testing arrangements.-A three-position ganged switch S is provided having contact sets LS and TS and including the switch 13, all of which are normally in the positions shown. Upon operation of switch S to a first test position the LS contacts and switch 13 change their condition, contacts LS1 preventing conduction of transistor 35 so that the ground lamp 39 will not light if the line current rises above the normal range upon manual closure of switch 13<SP>1</SP>, contacts LS2 maintaining capacitor 23 charged to prevent conduction of transistor 25 and operation of break lamp 28, and contacts LS3 connecting up lamps 46, 47 which remain alight during testing to indicate that no protection is being afforded. In the second test position of switch S the LS contacts and switch 13 remain in their first test position conditions and the TS contacts change their conditions. Contacts TS1 connect the line 10 to test equipment (not shown) which applies test potentials thereto, while contacts TS2 open to allow transistor 25 to conduct thus lighting the break lamp 28 until the capacitor 23 is allowed to recharge and the reset switch 26 is operated.