GB1481884A - Burglar alarm - Google Patents

Abstract

1481884 Burglar alarms BERNA INDUSTRIES Ltd 5 April 1976 [9 April 1975] 14621/75 Heading G4N A burglar alarm which produces an audible alarm when the presence of a burglar is detected by disturbance of a microwave field includes a timer 25 for terminating the audible alarm after a predetermined interval and carrying a visual indicator to be operated at the end of this interval. As shown, the alarm consists of a microwave doppler module 20 whose output signal is processed at 21. Upon detection of a burglar, the processor 21 cause a latch circuit 18 to be set which in turn causes an audible alarm to be given by a buffer 23, an audio generator 22 and a power amplifier and speaker 24. The latch 18 also causes the timer 25 to be set, and after the predetermined interval the timer 25 causes the latch circuit 18 and itself to be reset and also sets a latch circuit 19 which then operates a red l.e.d. driver 28 to give the visual indication. The alarm also comprises a key operated switch consisting of normally open switches 10 and 11 and normally closed switch 12. Upon setting of the alarm, the switches 10, 11 and 12 assume their opposite positions and the circuitry is powered by the means via a power supply 14. A battery 13 is simultaneously charged. Should the mains supply fail, a green l.e.d. driver 16 is de-energized the circuitry is automatically powered by the battery 13. A timer 17 inhibits the latches 18 and 19 for a predetermined period after the alarm is set, thus allowing the operator to leave the premises without setting off the alarm. The output from latch 18 may also activate a transmitter in communication with a receiver at a remote station. The receiver can thus set off additional audible alarms or automatic telephone dialling apparatus. The communication link between transmitter and receiver may be by injection of coded r.f. signals into the mains circuit.