| 摘要 |
1529473 Smoke detectors CHLORIDE BATTERIJEN BV 26 Nov 1975 [27 Nov 1974 9 Oct 1975] 48613/75 Heading G1A A detector Fig. 2 comprises: first and second photo-resistive cells C1, C3 placed to receive light originating at a source 18, the first cell C1 only after reflection by a test medium; means R1, A1 responsive to a decrease in resistance of C1 to a predetermined resistance to actuate, SCR, a signalling device A; and control means R2, A2, T1 responsive to a change in the resistance of C3 to adjust the source 18 to an intensity at which C3 is continuously exposed to that intensity which causes its thermal resistance coefficient to be substantially equal to that of Cl when exposed to the light intensity at which its resistance is equal to the predetermined value. In a smoke detector Fig. 1 a block 10 houses a LED 18 directing a beam via a lens 20 through a first aperture 12, and a first photo-resistive cell C1 viewing, via a lens 22 and aperture 14, light forward-scattered by any smoke in the beam. A second photo-resistive cell C3 in the block 10 receives, via a light guide 24, light in the beam path which may have been attenuated by the smoke. A manual screw 26 regulates the amount of light received by C3. A further photosensor C2 monitors the source 18 to actuate a warning lamp L, Fig. 2 in case of failure. The cells C1, C3, Fig. 2, having matched characteristics and temperature coefficients, are connected in series with respective equal resistances R1, R2, the junction points Jl, J2 each being connected to an amplifier A1, A2 arranged to change its output state when its input crosses a potential equal to half the supply voltage. The first amplifier A1 is connected to trigger a SCR actuating an audible warning device A. The second amplifier A2 controls the LED 18 so that the light received by C3 is just sufficient to maintain its resistance equal to R2 and therefore equal to the resistance which Cl will have at the alarm point. The arrangement is especially advantageous in maintaining the sensitivity and accuracy of the detector in the presence of black smoke having high attenuation and poor reflectivity. |
