Patent GB-996076-A (GB)
Title: Fire centre and area of temperature difference detector
IPC: F23N5/08
Filed: 1962-04-11
Abstract: 996,076. Fire detectors. COMMONWEALTH OF AUSTRALIA. April 11, 1962 [April 14, 1961], No. 13976/62. Heading G1A. [Also in Division H4] Means (Fig. 1) to detect a fire centre on an area of temperature difference, susceptible to radiation having wavelengths between about 3Á and 14Á so as to be operable in dense smoke, comprises an optical system (mirror 9) focusing the radiation on to a radiation detector 8, via a vibrating reed chopper 12, and an electronic circuit connecting the detector to an indicator (loudspeaker 23) whereby any change in the level of radiation received by the detector is shown by the indicator. The radiation first passes through a protective wire mesh on a fibre glass screen 1, then through a removable high pass filter 4 with cut off at 5Á to cut out background radiation above 5Á that might actuate the indicator, and is then reflected by the spherical mirror 9 through the chopper 12, vibrating perpendicularly to the paper in solenoid 57, and through a pass band filter 13, having a pass band from 3Á to 14Á to cut out the sun's reflected rays, to impinge on the gold film bolometer detector 8. The detector is formed from a plurality of gold film strips to give a shape corresponding to the required field of view e.g. “‹ by 15 degrees. By using such a narrow field of view the detector means may be swung horizontally with the film strips upright to scan the ground from the horizon to say 20 feet in front of the operator and determine the horizontal position of the radiant object. The means may then be swung vertically in the said horizontal position plane, with the strips horizontal, to locate the radiant object along a second axis. The electronic circuit is arranged in spaces 43 and 44 and comprises (Fig. 5) a negative feedback stabalized amplifier 18 and 19 fed from a tuned circuit 17 excited by the bolometer 8. The A.C. signal from the amplifier is fullwave rectified in tuned arrangement 22 and the two rectified signals are smoothed and sent to control an oscillator circuit. The oscillator circuit comprises transistors 24 feeding tuned circuit 25 from which negative and positive feedback signals are sent to the base of the first transistor 24, the two voltages thereat being developed across two diodes 26 having a resistance controlled by the D.C. current flowing therethrough. The D.C. bias current and feedback are so chosen that equal voltages are normally developed across the two diodes and the oscillator will then just fail to oscillate. The normal equal voltages across the diodes also give temperature stability. The two smoothed rectified signals from 22 produce variable D.C. bias currents for the two diodes. Since the two signals are in opposite sense they unbalance the resistances of the diodes and cause the voltages across them to differ thus sending the oscillator into oscillation to feed the loudspeaker 23. The level of the signal from the bolometer which will just cause the oscillator to oscillate is controlled by a signal applied to the base of the first transistor 19 from a potentiometer 32 connected in the oscillatory circuit exciting the vibrating reed solenoid. The potentiometer control 52 is provided on the handle 50 attached to the main body of the detector. A switch 51 is also provided in the handle and a desiccant 60 is provided adjacent the loudspeaker 23 which is protected by screens 39 and 40 similar to screens 1 and 2. A delay C/R circuit 35/36 is provided to sustain oscillations due to a very short signal from the bolometer to allow a fast swing scanning rate. A sighting, wedge shaped, projection is formed on the side of the main body. Alternatively to being held by the handle in the hand, the mirror, chopper, bolometer and amplifier 18 may be mounted separately on a helmet or clothing.