| 摘要 |
A smoke detector of the type utilizing a photo-responsive device to detect light reflected from smoke particles illuminated by a light source and actuate an alarm, in which a second photo-responsive device is utilized to control the intensity of the light source so that it is continuously exposed to a light intensity from the light source such that its thermal co-efficient of resistance is maintained at a value substantially equal to that of the first photo-resistive device when said first photo-resistive device is exposed to the amount of light that causes an alarm. In one embodiment of the invention identical photo-cells are connected into circuitry which includes two level detectors with the same transfer curve and voltage gain, and means causing the two photo-responsive devices to have the desired resistance at the alarm point. Changes in ambient temperature and line voltage are therefore prevented from affecting the alarm point. In another embodiment of the invention the second photo-responsive device views the light source from a distance such that intervening smoke particles reduce the amount of light falling thereon, and thereby causes an increase in the intensity of the light source. Smoke having a low reflectivity and high obscuration such as black smoke therefore causes an increase in output of the light source to thereby increase the amount of light reflected therefrom.
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