US 2278920 A
Description (OCR text may contain errors)
April 7, 1942. F. c. EVANS ET AL- PHOTOELECTRIC SMOKE DETECTION SYSTEM Filed MarCh 15, 1959 [DETECTOR COMPENSATOR INVENTOR F. c. EVANS K.O. DONELIAN BY M 'ATTORNEY Patented Apr. 7, 19.42
Francis 0. Evans, Dongan Hills, and Khatchik 0.
Donelian, New York, N. Y., assignors to American District Telegraph Company, Jersey City,
I N. J., a corporation of New Jersey Application March 15, 1939, Serial N0. 261,878
This invention relates generally to light-sensitive detecting or control systems adapted primarily for purposes of operating alarm devices or elec- The intrical release devices of various types. vention is particularly adapted for use as a fire detection system for closing dampers and stopping fans in air conditioning or ventilating systems wherein it is necessary to detect fire conditions when existing either in the air conditioning system or in or near the premises served by such system.
Smoke detecting systems used for fire detection must be unusually sensitive and also reliable to the extent that ordinary fluctuations in the commercial supply voltage will not cause false alarms or undesired control of the air conditioning system. The smoke detector must also be under constant supervision from a central control point whereby trouble conditions may be discovered promptly to prevent accidental periods of 'inoperativeness which might allow fires to occur without causing an alarm. -In prior smoke detectors of the photoelectric type, it has been necessary to utilize complicated and expensive apparatus for the purpose of compensating for variations in supply voltages. This invention provides an extremely sensitive and simple circult wherein the above mentioned requirements are met.
a The primary object of this invention is to pro-' vide a photoelectric smoke detecting system of high sensitivity wherein the sensitivity ismaintained over a high range of voltage fluctuations in the electrical supply circuit and wherein constant trouble supervision is maintained.
, 'Another object of this invention is to provide a photoelectric smoke-detecting circuit which prevents false alarms or false operations of load circuits due to voltage fluctuations.
Still another object of this invention is to provide a light-sensitive circuit wherein trouble con- 1 ditions maybe supervised from a remote point.
A further object of this invention is to provide indications of trouble in the amplifier associated with the photoelectric smoke detecting circuit.
A still. further object of this invention is to provide indications of trouble in the photoelectric cell branch of the smoke detecting system.
Still another object of this invention is to provide a supervisory circuit for indicating trouble conditions within the light source which is used in connection with a smoke detecting device or failure of the circuit of said light source.
A further object of this invention is to provide a supervisory circuit comprising a single trouble responsive element for detecting troubles in certain component parts of a smoke detecting system.
Another object of this invention is to provide 5 an amplifier and associated photoelectric cell circuit which is suitable for supervision by a single trouble responsive unit.
Still a further object of this invention is to v provide a smoke detection circuit which can be supervised by a single trouble responsive element.
Still another object of this invention is to provide a new and improved sensitive smoke detecting circuit wherein fluctuations in supply voltage may be compensated, alarm conditions and trouble conditions are distinguished from one another "and which is simple in arrangement.
Further objects and advantages of this invention will be evident from consideration of the iollowing'description of. the preferred embodiment shown in the accompanying drawing wherem. v The figure oi the drawing shows a circuit diagram of the smoke detector system embodying I this invention In order to utilize photoelectric smoke detecting systems for alarm purposes they must be highly sensitive or responsive to small variations of received light radiation over a wide range of voltage fluctuations in the electrical supply cirrm cultand they must also be arranged for constant supervision whereby indications of trouble may be transmitted to a central control point. The circuit embodying this invention and meeting the above requirements cbnsists of a bridge circuit 3,; containing-the photoelectric cells 2 and 3 which are adapted to receive light from a light source I. In the preferred application of this circuit it is used for smoke detection but it is adapted for use in any application where changes in light 40 may be used for signalling or control purposes.
The cell 2 acts as a detector for detecting smoke as it passes before the light source I, while the cell 3 is arranged to receive unintercepted light from source I, or light unaffected by smoke or other factors affecting the cell 2, and acts as a compensator for compensating fluctuations in supply voltage. The cells are connected in a bridge circuit with the condensers 8 and 8 as shown in the drawing. The bridge. comprising the cells 2 and 3 and condensers 6 and 8 is normally in a balanced condition whereby changes in supply voltage affecting the amount of light emitted by the light source I do not disturb thebalanced-condition and consequently no change in output occurs.
tube in which is in turn connected to an amplifler id. The normal preset output of this amplifier l4 controls the alarm relay 28 and the trouble relay 2! whereby changes in amplifier output result either in alarm signals or trouble signals. I
Since under normal conditions light from the source 5 ialls on both the cells 2 and 3, they are conductive and the potentials occurring across each cell will be determined by the light radiations received by each. Consequently the grid ii of the amplifier it will be forced to assume the potential occurring at junction point 50 of the two photocells. The bias voltage on grid 12 is provided by means of potentiometer 25 and is so adjusted as to, maintain the circuit in its most sensitive condition. Should smoke or any other suspended matter reduce the amount of light falling on cell 2, the conductivity of the detecting photocell 2 will be decreased thus increasing the impedance of that photocell and driving the grid it to a more negative bias with a consequent reduction in. the plate output of tube it). Inspection of the circuit diagram will show that the second stage it of the amplifier is operated backto-back with respect to the first stage it) and that,
the output from amplifier I6 is used directly for controlling the control grid it of tube 54. Hence the decrease in the plateoutput in the tube it due to an interruption of the light beam from source 3 causes the grid to to be driven to a more positive bias on the next half cycle and increases the current output from plate 1? very sharply. Alarm relay 2c is adjustedto become energized on plate currents above the normal preset output of plate ll whereby the increase in output resulting from interception of the light beam from source l causes relay 2!! to energize and close its contacts to transmit an alarm signal to alarm 22.
As mentioned above, the voltage fluctuations in the electrical supply circuit are balanced out in .the bias of grid l2 becomes more positive, resulting in a more negative bias in tube 14 and a decrease in the output from plate I! whereby relay 2| becomes deenerglzed to transmit a trouble alarm. ,Obviously it either of the tubes ID or 14 should fail due to filament burnout, the energizing current of relay 2! would cease and a trouble condition would result in energization of trouble indicator 24. This operation results from the fact that the filaments of tubes ill and it are connected in series with the voltage source whereby failure of the filament in tube it opens the series circuit and extinguishes the filament of tube 14 whereby electron emission therein will cease. The same is true when the filament of tube H fails.
From the above description it is evident that this invention provides a highly sensitive smoke detecting arrangement which maintains its sensitivity over a wide range of voltage fluctuation in the supply circuit. It is further evident that a single trouble relay is enabled to supervise component parts of the smoke detector. circuit whereby trouble alarms will result upon a failure or trouble condition in the circuit.
Various modifications may be made in the system embodying this invention without departing from the spirit and scope thereof and we desire therefore that only such limitations shall be placed thereon as are imposed by the prior art and as set forth in the appended claims.
1. In a photoelectric circuit, a source of light, a
' balanced photoelectric circuit responsive to vari- 1 ations of light from said source comprising a compensator cell arranged to receive uninter rupted light from said source, a detector cell connected in series with the compensator cell and arranged to receive light from said source and.
detect the passage of foreign matter across a supervised area between said source and cell, a pair or condensers connected in series across said cells for limiting the impedance increase in said balanced circuit when the light source fails, and
a connection Irom a common point between said the same direction and in proportion, thereby keeping the bridge circuit which is connected to the grid E2 of amplifier lil'in a balanced condition. When both photocells are dark, the condensers 8 and 8 connected in parallel with the photocells 2 and 3 maintain the grid l2 of the tube H3 at a preset bias or potential which is slightly positive with respect to the normal bias or potential which exists when both the photocells are illuminated. These condensers are necessary due to the fact that the impedance of the photocells in their dark condition approaches infinity and hence if the grid were not stabilized at a given potential as described above it might assume such a potential that unstable operation of the amplifier would result. The importance of this bias adjustment will appear later in connection with trouble supervision. As a result of the bridge arrangement,'the smoke detector circuit is highly sensitive over a wide range of voltage fluctuations. age may actually vary as much as 20% without any perceptible change in the sensitivity of the system.
The relay 2 I is provided i'or transmitting trouble indications and is normally energized by a preset value of current during normal conditions condensers to a common point between said cells whereby each condenser shunts a cell, resistance means connected across said balanced circuit, a
source of alternating potential connected across said light source and said resistance means whereby the potential at said common points is normally constant regardless of fluctuations in The fluctuation of supply volt the voltage of said source, or said potentialassumes a certain value when the impedance of said detector cell changes or said potential assumes an opposite value when said light source fails, a vacuum tube switching device connected in series with said source of potential, a connection from said common points to the input oi. said switching tube for biasing it to decrease its output upon a decrease in light received by said detector cell or for biasing said tube to increase its output when said light source tails and for maintaining normal bias and output in said tube when the light is uninterrupted but fluctuating,
a control vacuum tube connected directly to the output of said switching tube and in series with said source of potential whereby said control tube is biased for increasing its'outputin response to decreased output in said switching tube and for decreasing its output in response to increased output insaid switching tube, alarm means and trouble indication meansconnected in series to 2,278,920 the output of said control tube, said alarm means ,"a balanced photoelectric circuit responsive to responding to increases of said control tube output resulting from detector cell actuation, and said trouble means responding to decreases in said control tube output resulting from failure of the light source or vacuum tube circuit.
2. In a photoelectric circuit, a source of light, a photoelectric circuit responsive to variations of light from said source comprising a compensator cell arranged to receive uninterrupted light from said source, a detector cell connected in series relation to balance the compensator cell and arranged to receive light from said source and detect the passage of foreign matter across a supervised area between said source and cell,
a pair of high impedances connected across said cells, and a connection from a common point between said impedances to a common point between said cells whereby each impedance shunts a cell for limiting the impedance increase in said balanced circuit when the light source fails, a source of potential connected across said light source and said photoelectric circuit whereby the potential at said common points is normally constant regardless of fluctuations in the voltage of said source, orsaid potential assumes a certain value when the impedance of said detector cell changes or said potential assumes an opposite value when said light source fails,-a vacuum tube control device connected in series with said source of potential, 9. connection from said common points to the input of said control-device for biasing it to increase its output upon a decrease in light received by said detector cell or for biasing said control device to decrease its output when said light source fails and for maintaining normal bias and output-insaid control device when the light is uninterrupted but fluctuating. alarm-means and trouble indication means connected in series to the output 01 said control device, said alarm means responding to increases of said control device output and said trouble means responding toldecreases in said control device output.
3. In a photoelectric circuit, a source of light,
variations of light from said source comprising a compensator cell arranged to receive uninterrupted light from said source, a detector cell connected in series with the compensator cell and arranged to receive light i'romsaid source and detect the passage of foreign matter across a supervised area between said source and cell,
high impedance means connected across said cells, and a common connection between each of said impedance means and said cells, a source of potential connected tosaid light source and said balanced circuit whereby the potential at said common connection is normally constant regardless of fluctuations in the voltage of said source, or said potential assumes a certain value when the impedance of said detector cell changes or said potential assumes an opposite value when said light source fails, alarm means and trouble indication means controlled by th potential of said common connection, said alarm means responding when change oi received light changes the impedance of the detector cell and said trouble means responding to voltage changes when the light source tails.
4. In a photoelectric circuit, a source of light, a plurality of photoelectric cells connected in series balanced relation, one of said cells arranged to receive uninterrupted light from said source and act as a compensator cell, the other of said cells arranged to detect variations in light received from said source across a supervised area whereby normal fluctuation of the light source has no eflect. a source of potential connected to the balanced circuit, high impedance means in parallel with each cell for limiting the impedance increase in said balanced circuit when said light source fails, alarm means responsive to said balanced circuit when light to said detector cell is obstructed and trouble indieating means responsive to voltage changes caused b the limited impedance increase when,