GB1006362A - An improved ultra-sonic detector - Google Patents

Abstract

1,006,362. Fire and burglar alarms. R. R. CLARK. July 26, 1962 [July 26, 1961], No. 26982/61. Heading G4N. [Also in Division H3] A fire or burglar alarm system comprises an emitter of ultrasonic waves including a local transistor oscillator 10 connected to an ultrasonic output transducer 20, and a receiver comprising transistor amplifiers 50, 80 and a switching transistor 101 having a normally energized relay 110 in its collector circuit. Any disturbance of the standing wave sound field, due to a fire or an intruder, causes transistor 101 to switch off and de-energize relay 110 to operate an alarm. Band pass amplifier 50 which has a centre frequency tuned to that of the transmitter output is characterized in that it has automatic gain control (see Division H3) provided by the transistor 53 output via diode 55 and resistor capacity circuit 59 which is connected via a clamping diode 56 to the transistor 52 input. Operation.-If the standing sound wave field is not disturbed, a sound signal of constant amplitude and of the local oscillator frequency only will be received, resulting in a D.C. potential only across the capacitor 71 of filter 70 (i.e. the output of the first detector 61). Disturbance of the standing wave field, however, results in the reception of varying amplitude and fequency sound waves, which produce low frequency A.C. signals at the first detector output. These signals are amplified at 80, and rectified at 91, 92, the resulting D.C. voltage being appliedto the base of transistor 101, to overcome the forward bias via resistor 102, and cut off the transistor. Alarm relay 110 includes " latching " contacts 116 which form a " reset " control. Capacitative coupling for transistor 101 is provided by capacitor 103 to discriminate against stray signals. A test signal may be applied at 123.