GB675198A - Improvements relating to photoelectric indicators - Google Patents

Abstract

675,198. Photo-electric smoke detectors. TRIST & CO., Ltd., R., and CLARKE, A. G. March 2, 1950 [March 14, 1949], No. 6978/49. Class 40 (iii). In a photo-electric warning system, e.g. for smoke detection, signals which build up slowly to a voltage below a predetermined magnitude due for example to dust deposits on a lens are eliminated by applying the detector output to a potential divider of two sections, both of high D.C. resistance, and tapping the signal to the indicating device across one of the sections, the other having a low impedance to rapidly fluctuating voltages. As shown, cells 1 and 2 connected across polarizing battery 3 form a normally balanced circuit. Reduction of the illumination of one of the cells, e.g. by the presence of smoke, produces a potential difference between points P and Q which is applied across the potential divider, one section of which consists of a resistor R 1 (100 megohms) and the other of a parallel resistor R2 (650 megohms) and condenser Cl (1 microfarad). The control electrode-cathode circuit of a cold cathode tube 16 is connected across resistor R1. A gradual reduction of the illumination of cell 2 drives the control electrode negative (as described in Specification 636,341. but does not trigger the tube unless the unbalance voltage rises above a predetermined level (as in the case of a smouldering fire). More rapid changes, however, trigger the tube through large condenser C1. When the cold cathode tube conducts, a condenser C3, previously charged through a resistor 20, discharges through the tube, operating a relay 17, closing the circuit of a warning device 26. Relay 17 is maintained closed by a small holding current from a low voltage supply and may be reset by a press button 27.