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US 4,225,791 AGranted
Optical smoke detector circuit
AssigneeHoneywell Inc.
InventorArlon D. Kompelien
Life of this patent
Estimating the term…
Abstract
An improved circuit for an optical smoke detector utilizing an active band pass filter which has a bias feedback to the phototransistor detecting element to bias the phototransistor to a static conductivity state of high gain or high sensitivity without regard to whether the phototransistor is in a high ambient light level or a low (dark) ambient light level.
Claims4
Optical smoke detector apparatus comprising:
- a source of light pulses having a pulse rate of f.sub.o within a smoke sensing chamber;
- phototransistor light pulse sensing means in said chamber, said phototransistor having collector, emitter and base electrodes, said phototransistor being energized by a DC source connected to said collector electrode;
- a resistance connected from said emitter electrode to a reference potential, said phototransistor providing electrical pulses at said emitter corresponding to the sensed light pulses;
- a band-pass active filter tuned to the pulse rate frequency f.sub.o, and having an input terminal connected to said emitter electrode to receive electrical pulse signals from said phototransistor, and having an output terminal for transmitting the electrical pulse signals to utilizing apparatus and;
- means for maintaining constant the phototransistor gain comprising a phototransistor bias and low pass current feedback path connected from said band pass filter to said base electrode to maintain substantially constant the DC emitter current over wide ranges of ambient light to which the phototransistor is exposed.
2 more elements
Optical smoke detector apparatus comprising:
- a smoke sensing chamber having openings in said chamber to allow access of the air in the room being monitored into said chamber;
- a light emitting diode in said chamber;
- means for pulse energizing said light emitting diode at a pulse rate frequency f.sub.o to produce light pulses;
- a phototransistor in said chamber for sensing said light pulses when there is smoke in said chamber, said phototransistor having collector, emitter and base electrodes;
- an active band pass filter tuned to the pulse rate frequency f.sub.o, said filter having an input terminal connected to said emitter electrode to receive electrical pulse signals from said phototransistor, said filter also having an output terminal;
- bias circuit means connected from said filter to said base electrode to bias said phototransistor into a relatively constant conductive state even when no ambient light falls on said phototransistor so that high gain and sensitivity of said phototransistor is maintained irrespective of the level of ambient light falling on said phototransistor.
3 more elements
Dependencies are inferred from the claim text.