Patent GB-1042876-A (GB)
Title: Improvements in or relating to electric alarm systems
Inventor: HONE JOHN GRAHAM ELLIOTT
IPC: G08B23/00
Filed: 1961-10-17
Abstract: 1,042,876. Protective systems. GRESHAM LION GROUP Ltd. Oct. 17. 1962 [Oct. 17 1961], No. 37264/61. Heading H2K. [Also in Division G4] A monitoring circuit which gives both a visible and an audible alarm if contacts M1 are open (Fig. 4), includes a binary circuit T1, T2, having two stable states; a transistor T4 controlling the energization of lamp SL and transistor T7 controlling the provision of an audio output at 13 to external alarm equipment, the arrangement being such that the pulse signal continuously applied at 11 comes flashing of lamp SL and an audio output if contacts M1 are open. Contacts M1 closed.-If " accept " switch ACB is closed T1 conducts and T2 is cut-off. Each negative pulse of the voltage at 11 drives current through R8, D2 and T8, but since the latter provides an effective short-circuit no current will flow through R25 and the voltage at C will be less than the breakdown voltage of Zener diode D4. T3, T4, T7 are therefore cut off. Circuit T1 T2 is changed via T8, R5, D1 Contacts M1 open.-T8 cuts off, the clamp on circuit T1, T2 is released, and the voltage at A can follow that at 11. During each negative pulse, the voltage at C will be larger than the breakdown voltage of D4 and the current through transistors T3, T4 will cause flashing of lamp SL and that through T7 an audio output at 13. Operation of switch ACB will changeover T1 T2, so that T1 is cut off and T2 conducts. Point C is now continuously above the Zener breakdown voltage so that current flows continuously to T3, T4 and lamp SL is permanently alight. T7 is cut off and there will therefore be no audio output at 13. Opening of test switch TPB simulates the opening of contacts M1. A number of circuits for monitoring a number of contacts may be fed from the same supply have the switches ACB, TPB in common and operate common alarms. In a further embodiment (Fig. 5, not shown). contacts M1 are of the normally-open type. In the embodiment of Fig. 6 a continuous alarm indication is provided even though contacts M1 are only opened momentarily. This is achieved by the provision of two binary circuits T1, T2 and T10, T11. If contacts M1 open, T9 will conduct and circuit T1, T2 will change to the " on " state where T1 is off and T2 is on, T10, T11 remaining in its original state. The flashing lamp and the audio signal will be provided as before. Should contacts M1 re-open, T9 will cut off but T1, T2 will remain in its " on " state, and the lamp continue to flash. If switch ACB is operated, T10, T11 will switch over to the " on " state and SL will be continuously lit as before. A reset switch RPB is provided, but is ineffective if it is operated before contacts M1 reclose. A " contact normally open" embodiment is described in Fig. 7 (not shown). In yet another embodiment (Fig. 1, not shown), an OR gate is included if a number of circuits are provided having a common alarm to ensure energization of the alarm for a fault in any channel. The equivalent " contacts normally open " embodiment is described in Fig. 2 (not shown). An arrangement providing a flashing light signal of a first colour and an audio signal on an alarm condition, a steady first colour signal and no audio signal on acceptance of the alarm, on clearance a flashing light signal of a second colour and an audio signal and, on acceptance of the cleared alarm, a steady second colour signal and no audio signal, are described.