Smoke detection system and method

A smoke detector is provided in which two light sources are employed in conjunction with a single photo responsive device and control circuit elements adapted for sequential operation of the light sources. One light source is directed at the photo responsive device to pre-bias the device while the other is directed at a smoke chamber to which the device is exposed. If smoke is present in the chamber, the light from the second source will strike the smoke particles and be reflected against the photo responsive device changing the output thereof. The light sources in one embodiment are timed to operate in a series of pulses and measurements are made on the declining output of the photo responsive device, such that the presence of smoke in the chamber will produce a longer decay time than will occur in the absence of smoke. A timer connected to the circuit measures the decay time and is adapted to generate a signal under the appropriate conditions.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to smoke detection and more particularly 
is directed towards a new and improved method and system for detecting the 
presence of an aerosol such as smoke using a small number of simple 
components arranged and operated in a highly efficient manner and with low 
power requirements. 
2. Description of the Prior Art 
Most optical smoke detectors now in use employ reflected light principles 
in which a light source is directed into an examination zone in which 
smoke is adapted to pass in the event of a fire. A photocell is also 
directed at the zone and is adapted to respond to light reflected from any 
smoke particles illuminated by the light source. 
Reflected light smoke detectors of the sort described above typically 
function in either of two operating modes. In one mode, the light source 
is operated on a continuous basis which makes them impractical for battery 
powered units or for line powered units employing standby battery power. 
While detectors of this type are relatively simple to fabricate and can be 
highly sensitive, they are somewhat slow to react because of the response 
characteristics of the photo resistive cells generally used in this 
application. 
The second mode of operation is one in which the light is pulsed in order 
to reduce current drain. While the pulsing type of smoke detector reduces 
current demand, photodetectors of much faster response are required with a 
concurrent increase in cost. Also, the light source employed in the pulse 
type unit must be capable of higher output than one of a steady state 
detector. Further costs are incurred by the necessity of including an 
amplifier to compensate for the reduced photo gain of the high speed 
detector used in this type of system. 
Accordingly, it is an object of the present invention to provide 
improvements in smoke detectors, especially low current drain smoke 
detectors. 
Another object of this invention is to provide a battery powered smoke 
detector of simple, low cost construction characterized by high 
sensitivity and quick response. 
A further object of this invention is to provide a novel method and 
associated system for detecting smoke. 
SUMMARY OF THE INVENTION 
This invention features the method of detecting smoke using a pair of light 
sources and a photo responsive device, comprising the steps of operating 
the light sources in a predetermined sequence while aiming one of the 
light sources directly at the photo responsive device to pre-bias the 
device and the other at a zone visible to the device whereby the presence 
of smoke particles in the zone will cause light from the other source to 
be reflected against the device. The output of the device is a waveform 
which will have one characteristic if no smoke is present and a different 
characteristic is smoke is present. The different characteristics are 
measured by timing different points of the waveform and generating a 
signal when the time exceeds a predetermined value. 
The system includes a photo responsive device directed towards a zone 
through which smoke will flow and a pair of light sources, one directed at 
the device in order to pre-bias the device and the other directed through 
the zone which is viewed by the device. In one embodiment a control 
circuit operates the light sources in a timed sequence which selectively 
turns the light sources on and off so as to generate a waveform output 
from the device. A timer is connected to the circuitry for measuring 
different points along a declining waveform which will have one value when 
no smoke is present and if smoke is present, it will have a different 
value whereby the timer will detect the difference and generate a signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings and to FIG. 1 in particular, the reference 
character 10 generally indicates a smoke detection system organized about 
a housing 12 defining a chamber 14 which is open to ambient atmosphere but 
normally protected from ambient light conditions by known means. The 
chamber 14 is designed to provide an examination zone in which aerosols, 
such as smoke particles, may pass for detection purposes. In FIG. 1 a 
cloud of smoke particles is indicated at 16 and is adapted to be 
illuminated by a light source 18 which, in the preferred embodiment, is a 
light emitting diode. A second light source 20, which also may be a light 
emitting diode, is provided in association with the housing 12, but is 
directed at the cell of a photo responsive device 22 aimed at the interior 
of the chamber and adapted to responsd to any light reflected from smoke 
particles illuminated by the light source 18. 
The light source 20 is connected to the output of an amplifier 24 receiving 
inputs from a ramp generator 26 and the photocell 22. The photocell 22 
also provides an input to an amplifier 28 which is a level detector with 
hysteresis. A switch 30 is connected to the outputs of the amplifier 28 
and the ramp generator 26 for reasons that will presently appear. A timer 
32 is connected to the output of the light source 18 and to the amplifier 
28 and is adapted to provide an actuating signal which can be used to 
signal an alarm device 34. 
The circuit operates in the following manner. Assuming that power, D.C., 
rectified line or battery, is applied in the manner illustrated in FIG. 2, 
the switch 30 is open. The ramp generator 36, which is formed by the 
resistor 36 and the capacitor 38, generates a voltage V1 which first 
begins to rise. The voltage V1 is compared with the output voltage V2 of 
the photo responsive device 22. Voltages V1 and V2 are applied to the 
amplifier 24, as shown. If the voltage V2 is lower than the voltage V1, 
then the amplifier 24 will energize the light emitting diode 20 which will 
then shine directly against the face of the photo device 22, causing the 
voltage output V2 of the photo device to rise thereby pre-biasing it. If 
the voltage V2 should then rise above the level of the voltage V1, the 
amplifier 24 will deenergize the light emitting diode 20. The amplifier 24 
thereby maintains the voltage V2 at the increasing level of the voltage 
V1. 
During this phase of the operation of the circuit, the voltage V2 will 
continue to rise until the trip level of the amplifier 28 is reached. As 
previously indicated, the amplifier 28 is a level detector with 
hysteresis. When the trip level of the amplifier 28 is reached, it will 
cause the switch 30 to close, and energize the light emitting diode 18. 
When the switch 30 closes, the voltage V1 will quickly drop below the 
level of the voltage V2 which will cause the light emitting diode 20 to be 
deenergized. The amplifier 28 will remain in this state until the voltage 
V2 drops below the reset level of the amplifier 28. Typically, this action 
will take place in about 100 MS. 
If a sufficient amount of smoke 16 or other aerosol is present in the 
chamber 14, light from the light emitting diode 18 will reflect from the 
smoke particles in the cloud 16 with some of the reflected light falling 
upon the photo responsive device 22. The reflected light picked up by the 
device 22 will cause the voltage V2 to drop more slowly than would be the 
case if no smoke were present and no reflected light were impinging on the 
device 22. 
The timer 32 functions to measure the two levels of the declining waveform 
output of the device 22, and, in this instance, if the increase in time of 
the declining voltage V2 exceeds 200 MS, then the timer 32 will actuate 
the alarm 34. If it does not exceed 200 MS the amplifier 28 resets itself 
and the entire sequence is repeated. 
The circuit of the sort shown in FIGS. 1 and 2 is characterized by very low 
power consumption and hence is ideally suited for battery powered 
application. Low power consumption is achieved by the short operating time 
limits of the light sources. The light emitting diode 20 is positioned to 
shine directly against the photo responsive device 22 so that only 
microamps of current are required to provide sufficient output to drive 
the device to the desired level. Similarly, the light emitting diode 18 
functions only for a relatively short period of time with a worst case 
operating interval typically being on the order of 0.2 seconds every 10 
seconds, requiring an average current of 1/50 of normal. In this instance 
1/50 of 5 MA would be involved which is 100.mu..alpha.. 
The technique employed allows the use of a relatively inexpensive and less 
sensitive photo responsive device than would otherwise be required. The 
technique also takes advantage of the decay time of the photo responsive 
device which is faster than the rise time. Typically, the response 
characteristic of a photo responsive device, such as a cadmium 
sulfo-selenide device, would have an ascent time of 800 milliseconds, but 
a descent time of only 250 milliseconds. By utilizing the decay portion of 
the photodector output, higher detection speed is achieved since the photo 
responsive device has been brought up to the alarm point by the light 
emitting diode 20 prior to the energization of the light emitting diode 
18. Thus, no time is lost waiting for the device 22 to reach its peak 
output. 
Referring now more particularly to FIG. 3, 4 and 5, the voltages V1 and V2 
are plotted to demonstrate the different decay characteristics of the 
photo responsive device output, first when no smoke is present in the 
chamber and secondly, when smoke is present in the chamber. The waveforms 
are shown slightly offset for the sake of clarity and no such time 
difference exists in practice. In FIG. 3 it will be noted that the 
waveform characteristic of the voltage V1 repeats itself since it is 
controlled by the fixed parameters of the ramp generator 26. V2, however, 
which is subject to change depending upon the presence or absence of smoke 
in the chamber, will display different decay characteristics. In the first 
decay waveform of V2, indicated by T1, no smoke is present and the decay 
displays a curve with a relatively sharp drop. However, in the next 
sampling cycle, assuming smoke is present at T2, the photo device will 
decay at a slower rate producing a more gradual curve, as shown. 
In FIG. 4, T1 has been magnified and it will be seen that the voltages VA1 
and VA2 are represented as the voltages at the output of the amplifier 24 
and the amplifier 28, respectively, as shown in the drawings. The 
amplifier 28, when activated, drives voltages VA1 and VA2 to ground at 
time Ta, voltage V1 resets to ground and voltage V2 resets at time Tb, the 
time lapse between Ta and Tb being approximately 100 MS. 
In FIG. 5 there is shown a magnified curve representing T2 on the FIG. 3 
diagram. In FIG. 5 it will be seen that the decay curve of the voltage V2 
extends between times Tc to Td from a peak to the voltage level 0.495. In 
this instance Td-Tc=200 MS, sufficient to generate a signal, since the 
timer has measured an excess of a predetermined value. 
Referring now to FIG. 6, there is illustrated a modification of the 
invention and, in this embodiment, another technique is shown for 
utilizing different points of a waveform for smoke detection purposes. In 
this embodiment amplifiers 40 and 42 form a window detector, the inputs to 
which are from a resistive network which includes a photo responsive 
device 44 having an output V2'. The amplifiers 40 and 42 provide an input 
to a gate 46 adapted to drive a timer 48 which in turn is connected to an 
alarm 50. A light emitting diode 52 is connected between the gate 46 and 
the timer 48 while another light emitting diode 54 is controlled by an 
external timer 56 which provides additional inputs to the gate 46. 
The amplifiers 40 and 42 form a window detector and operate in such a 
manner that the photo responsive device 44 is maintained continuously 
above the highest threshold level providing a pre-biased condition similar 
to that in the principal embodiment. The external timer 56 or other 
control means, functions to deenergize the light source 54 whereby the 
voltage output of the device 44, represented by V2', will decrease until 
the upper threshold is reached. At this point, the gate 46 will switch and 
remain on until the lower level of the window is reached. At the lower 
level, the external timer 56 is reset and the gate 46 is inhibited. 
While the invention has been described with particular reference to the 
illustrated embodiments, numerous modifications thereto will appear to 
those skilled in the art. For example, instead of cyclically pre-biasing 
the photo responsive device with a pulsing light source, the device could 
be maintained at a high level by a steady light source which may be 
periodically turned off as by a control until when a second light source 
is turned on to illuminate any smoke that is present and the decay curve 
measured. Also, instead of measuring the decay curve, the system could be 
modified to measure different points along another part of the curve.