GB1222351A - Improvements in or relating to fire-detection apparatus - Google Patents

Abstract

1,222,351. Photo-electric smoke detectors. D.C. ADAMS. Feb. 29, 1968 [Dec. 7, 1966], No. 54833/68. Heading G1A. [Also in Division G4N] A smoke sensing device comprises two photodetectors one of which is arranged to receive light directly from a source through a smoke detecting zone while the other is arranged to receive light diffused by the smoke, a lamp,failure alarm responsive to the absence of current through the source and a transistor amplifier. The two detectors may be arranged in the two arm of a potentiometer network connected across a voltage source. As shown a first photo-resistor P1 is arranged to receive light transmitted through the smoke detecting zone 1 fron a lamp LP1 and a second photo-resistor P2 is arranged to receive light diffused by smoke in the zone. The two photo-resistors are connected in series with an adjustment resistor R1 across power supply lines 10 and 13. The voltage on these lines which supply a number of similar devices is stabilized at each device by a feed resistor R4 and a Zener diode ZD. A transistor T is arranged to amplify the differential output to the two photo-resistors one of which responds mainly to white smoke while the other responds mainly to black smoke, a thermistor R3 being included to compensate for temperature changes. The output of the transistor is again amplified and operates an alarm relay RW2 closing normally open contacts 15 if the smoke density reaches adetermined level. Closure of the contacts 15 energizes an alarm relay AR at the central station and lights a local indicator lamp LP2. A normally energized relay RW1 is series with the lamp LP1 releases if this lamp fails thus operating contacts 16 and energizing a lamp failure relay FR at the central station. The lamp failure relay has normally closed contacts 18 in series with the alarm relay which is slow to operate and thus disables the alarm relay in event of lamp failure. In a modification the contacts 16 are in series with the contacts 15. In a further modified device the lamp LP1 additionally functions as the feed resistor R4. In another modification the two photo-resistors are separately connected each in series with a resistor as separate potential dividers across the voltage source and the two outputs are applied to the base of the transistor via diodes. In a yet further form similar to that last described the diodes are replaced by further transistors functioning as amplifiers.