GB941093A - Improvements in and relating to detection systems - Google Patents

Abstract

941,093. Intruder and fire alarms; audible signalling. ELECTRONIC APPLICATIONS (COMMERCIAL) Ltd. March 7,1962 [Dec. 8, 1960], No. 42310/60. Headings G4F and G4N. An intruder or fire alarm system comprises a transmitter transmitting radio, sonic or ultrasonic waves into the space to be protected, a receiver in the space being responsive to any disturbance of the waves, characterized in that the receiver and transmitter active elements are transistors and is that their power supplies are derived from batteries connected in a charging circuit operated from the mains. Ultrasonic waves are used in the particular embodiment and the transmitter (Fig. 2) includes a barium titanate crystal (not shown) connected to the primary and secondary of transformers 35, 37 respectively through a switch 38. A reference oscillation is supplied to the receiver for terminal 29. The battery power supply (Fig. 3, not shown) is connected at 46, 47. The receiver (Fig. 1) is connected to the receiving transducer (not shown) at 10, 11 and consists of a three stage amplifier comprising transistors 12, 13, 14, a high pass filter 16, a rectifier 16 (or a Class B biased transistor), a low pass filter 17, to a further amplifier including transistors 18,19. The receiver output is rectified by 20, 21 and applied to a transistor 22 having relay contacts 24 in its collector circuit. When no signals are received, the collector current is sufficient to maintain the relay energized but when a signal is received due to the presence of a fire or intruder the collector current falls, the relay is de-energized and the alarm operates. A test circuit is provided from an input 31 through switch 32 to the base of transistor 12 and automatic gain control is incorporated in the amplifier 12,13,14.