US3728706A

US3728706A - System for indicating aerosols in the atmosphere

Info

Publication number: US3728706A
Application number: US00076149A
Authority: US
Inventors: W Tipton; M Suchomel
Original assignee: General Signal Corp
Current assignee: SPX Corp
Priority date: 1970-09-28
Filing date: 1970-09-28
Publication date: 1973-04-17
Anticipated expiration: 1990-04-17
Also published as: GB1347306A; CA938385A

Abstract

A fire detector having an ionization sensing means which changes the level of an output signal in accordance with sensing the presence of smoke in the atmosphere has been provided. A detecting means selectively responsive to the output signal generates an alarm control signal and the improvement includes a filter means for rendering a detecting means operable if uninhibited to generate the alarm signal irrespective of an output signal from the sensing means. Inhibiting means normally operable to inhibit the filter is rendered ineffective in response to the output signal of the sensing means and the inhibiting means therefore checks the operability of the detecting means.

Description

United States Patent Tipton et al.

[ Apr. 17, 1973 [5 SYSTEM FOR INDICATING AEROSOLS 2,646,556 7/ I953 Allen ..340/237 SX IN THE ATMOSPHERE [75] Inventors: William C. Tipton, Newark; Michael gsx zf l glg g' fjfi j Mountamslde both of Attorney-Harold S. Wynn and John P. DeLuca [73] Assignee: General Signal Corporation, ABSTRACT RochesterNY' A fire detector having an ionization sensing means [22] Filed; se nzg, 1970 which changes the level of an output signal in ac- 2 cordance with sensing the presence of smoke in the at- I 1] Appl' 76149 mosphere has been provided. A detecting means selectively responsive to the output signal generates an 52 US. Cl. ..340/237 s, 250/83.6 FT alarm control Signal and the improvement includes a 511 lm. c1. ..G08b 17/10, H01 j 39/28 filter means for rendering a detecting means Perable 58 Field of Search ..340/237 sif uninhibited generate the alarm Signal irrespective 250/435 D 44 of an output signal from the sensing means. Inhibiting means normally operable to inhibit the filter is 56] References Cited rendered ineffective in response to the output signal of the sensing means and the inhibiting means therefore UNITED STATES PATENTS checks the operability of the detecting means.

3,462,752 8/1969 Stroh ..340/237 S 15 Claims, 4 Drawing Figures OTHER DETECTOR AMPL'F'ER INHIBIT FILTER 12 62 5 025 27 L 37 35 4| 44 59 u L G2 29 j 2% I 3 G y g 5 5 [I49 I |:)s u )aAl 33 39 .12 l 2 1 11 $53 I 2|l2'3 g" 32 N /43 Z @Q 26 G5\ l .J 38 46 4s 47 IP 28 1 46 %45 n- 5's I7 DETECTOR II L SYSTEM FOR INDICATING AEROSOLS IN THE ATMOSPHERE BACKGROUND OF INVENTION This invention relates to a tire detector system and in particular to the system for detecting the presence of combustion products, aerosols in the atmosphere.

Modern fire detection equipment generally utilizes an ionization chamber which uses a radio-active source to ionize the atmosphere within the chamber and a 1 and reduce the current through the chamber. Otherfactors, however, may provide a variation in voltage which would give false indication of a fire condition.' A draft, for example, caused by a slamming door might disturb the atmosphere within the chamber such that an alarm is sounded. Variations in atmospheric pressure also effect the characteristic of the chamber. As is well known in the art, two chambers can be used; one open to the atmosphere for receiving or detecting the presence of aerosols and another substantially closed to atmospheric aerosols. The closed chamber is a reference for compensating the system under changes in the atmospheric pressure. The system still may, however, be susceptible to false triggering and noise transients which may be received by the equipment.

Generally it is desirable to connect a plurality of fire detectors in a specific area and link them together to a receiver which would activate the alarm signal. However, noise, loading and supervisory problems have made installing such fire detectors on a system-wide basis with a central indication very difficult and costly.

In order to provide for a truly safety oriented system, supervision of the circuits joining multiple sensing units and individual trouble detection in each of the units is necessary. Trouble may be corrected as soon as it occurs. Discovery should not be delayed by the frequency of periodic checking, but rather continuous monitoring of the fire detection apparatus should be conducted.

It is therefore an object of the present invention to provide an arrangement which substantially obviates one or more of the difficulties of the described prior arrangements.

It is another object of the present invention to provide a. simplified fire detection system.

It is yet another object of the present invention to provide a safe fire detection system.

It is still another object of the present invention to provide an economically manufactured installed and maintained fire detection system.

SUMMARY OF INVENTION There has been provided a fire detection system including ionization sensing means for changing the level of an output signal in accordance with sensing the presence of smoke in the atmosphere. Detecting means selectively responsive to the output signal generates an alarm control signal and the improvement includes filter means for rendering the detecting means operable if uninhibited to generate the alarm signal irrespective of an output signal for the sensing means. Inhibiting means including a voltage level detector is also provided which is normally operable to inhibit the filtering means and it is rendered ineffective when a voltage level governed by the output signal is above or below a predetermined normal voltage range.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, while its scope will be pointed out in the appended claims.

FIG. 1 is a schematic diagram of the fire detector including the sensing means, detector circuitry and the improved apparatus of the present invention.

FIG. 2 is a diagram of the control unit which may be remotely located for central monitoring of a number of detectors coupled in parallel.

FIG. 3 is a diagram of the supervisory circuit coupled in parallel to the last unit in the multiple detector system.

FIG. 4 is a drawing showing the structure of the sensing unit.

DESCRIPTION OF THE PREFERRED EMBODIMENT The combustion products smoke detection system consists essentially of three units; a detector 11, control unit 15, and an end-of-line signal sender 16.

A detector unit 11 produces an output when combustion products are sensed by an ionization chamber 10.

A control unit 15 supplies regulated, short circuit protected, 16V D.C., to operate from one to 10 detectors connected in parallel. In addition, it indicates a trouble condition if the line continuity is broken, or if the 16V D.C. is lost for any reason. FIRE indication condition is provided when a detector 11 output, signals the presence of combustion products.

An end-of-line sender unit 1t; furnishes current pulses to a line L which are received by a trouble detection portion of the control unit 15 thus, monitoring continuity.

DETECTOR The detector 11 circuit operates on a relatively low input voltage in the order of 16V D.C., applied to terminals l7 and 18.

Numerals

20 and 21 indicate ionization chambers of sensing means 10 activated by a radio-active material 19 which emits alpha particles.

Chamber 21 is open to the atmosphere and exposed to combustion particles. Chamber 20 is the reference chamber closed to aerosols which is used to nullify the effects of atmospheric changes. The voltage across the

ionization chambers

20 and 21 connected in series is filtered by capacitor 22.

Particulate products of combustion entering the open sensing chamber 21 hinder the normal ion current flow through the chamber caused by alpha particles from the radio-active material 19, resulting in a rise of voltage across the chamber 21. The input stage of the detector 11 circuitry is a dual-gate MOSFET (Metal Oxide Semiconductor Field Effect Transistor) 23, connected as a source follower. The drain D electrode of F ET 23 is connected to the positive potential, the gate electrode G1, is connected to the junction of chambers and 21, the source electrode S is connected to resistor 24, and the remaining electrode G2, is connected to the source. In this connection the source S voltage follows the voltage at gate G1, which is the chamber 21 voltage. It should also be noted that a resistor 24 is selected to provide stable temperature characteristic to the response of PET 23 and is connected between electrode S and negative terminal 17. Capacitor 25 prevents damage to FET 23 should gate G1 suddenly go positive.

The next stage of circuitry functions as a voltage level detector. Transistor 26 is a Programmable Unijunction Transistor (PUT) and with resistor 27 and capacitor 28 forms an oscillator or inhibit means. Resistor 29, rectifier 30, and resistor 24 set a reference voltage level for PUT 26. Capacitor 28 charges through resistor 27 until its voltage exceeds the reference level which is the voltage at the gate electrode G3 of PUT 26 connected to

resistors

29, 31 and rectifier by about 0.3V. At this point, conduction begins between the anode A1 and the cathode Cl. Capacitor 28 then discharges through the base emitter junction of transistor 32 until its voltage drops to a cut off level at which conduction cannot be maintained. Cut off then restores the anode Al to cathode C1 blocking of PUT 26 and capacitor 28 may recharge through resistor 27. This charging and discharging of capacitor 28 continues, forming a free running oscillator. A voltage divider formed by

resistors

37, 38 and potentiometer 39 is a sensitivity adjustment for the detector. The voltage at the slider of potentiometer 39 sets the maximum voltage that capacitor 28 can charge to. Rectifier 33 permits discharge of capacitor 28 if it exceeds the voltage set at potentiometer 39. if the voltage that capacitor 28 charges to is not greater than the gate voltage by 0.3V, conduction of PUT 26 does not take place, and oscillations cease. This can occur if the gate G3 voltage of PUT 26 is raised high enough as is the case when the sensing chamber 21 voltage rises from the entrance of particulate matter created by combustion. This, therefore, inhibits the filter means 12 described further in the discussion. A fire condition, therefore, causes the oscillations of PUT 26, resistor 27 and capacitor 28 to cease.

Each time capacitor 28 discharges through PUT 26, it biases transistor 32 into conduction. Filter 12 comprises capacitor 40, resistor 41 and transistor 32. When transistor 32 conducts, capacitor 40 which is charged through resistor 41 discharges through rectifier 42. Since transistor 32 conducts periodically, capacitor 40 is discharged periodically. During the non-conduction periods, capacitor 40 recharges through resistor 41. The discharge of capacitor 40 depends on the repetition rate of the pulses gating transistor 32 from capacitor 28, and the charge of capacitor 40 depends on the time constant of resistor-capacitor 41-40 combination. It should be noted that resistor also provides bias voltage for transistor 32. The voltage that appears on capacitor depends on the relation of this repetition rate to the resistor'capacitor 41-40 time constant. The average value of voltage on transistor 32 is in the order of 2V. The resistor-capacitor 41-40 combination in cooperation with transistor 32 therefore filters out signals ofless than a predetermined rate.

Transistor 43 is a Programmable Unijunction Transistor (PUT) with

resistors

44 and 45 setting the voltage reference level on gate G4. If the pulses which make transistor 32 conduct, stop, capacitor 40 will charge to a higher voltage than the average. When this voltage exceeds the reference set by

resistors

44 and 45 by approximately 0.3V, conduction takes place from anode A2 to cathode C2 and capacitor 40 discharges through PUT 43 and resistor 46. The voltage across resistor 46 is coupled to the gate GS of Silicon Controlled Rectifier SCR 47 through resistor 48 and triggers conduction from its anode A3 to cathode C3. This then effectively grounds resistor 49 and lights a fire indication lamp 50, and also any external light (not shown) that may be connected across

terminals

51 and 52.

Resistors

49 and 53 are dropping resistors for low voltage lamps such as lamp 50. Resistor 54 and capacitor 56 form a filter network to prevent firing of (SCR) 47 from spurious transients. Rectifier 57 allows the fire indication lamp to light immediately if the line should be connected in reverse polarity. Capacitor 58 is a filter capacitor to filter R.F. transients from the unit. Diode 62 coupled between capacitor 58 and resistor 44 is provided for polarity protection for the semi-conductors if the apparatus of FIG. 1 is inadvertently connected in reverse.

Resistors

59 and 60 assure closure of the Fire Sense Relay 61 in the control unit 15 should the fire indication lamp 50 be open circuited.

The control unit 15 contains the voltage regulated supply, fire sensing circuitry, trouble circuitry, power failure sensing and emergency power transfer circuitry.

POWER INPUT Normal power is supplied to the control unit through a suitable fuse 156 and transformer 67 which is full wave rectified by bridge 68 and filtered by capacitor 69. Emergency battery power is applied to terminal 70 and externally switched, full-wave rectified DC. power may be applied to terminal 71. Resistor 77 limits the surge current due to the charging of the filter capacitor 69. When AC power is applied, relay 73 is energized thus opening normally closed contacts 74 and closing normally open contacts 75. This opens the battery input and connects the AC power to the control unit. In addition, if A.C. power is lost for any reason, relay 73 deenergizes and closes contacts 152 for producing an A.C. power loss signal. Diode 76 blocks battery voltage from re-energizing relay 76 through resistor 77 if AC power is lost.

VOLTAGE REGULATOR Zener diode 78 and resistor 79 form a voltage divider. The zener voltage of 78 is about 10V.

Resistors

80 and 81 in parallel with zener diode 78 furnish a reference voltage for the base of transistor 82. Since the base voltage is fixed, the drop across resistor 83 is also fixed producing constant emitter current. Neglecting base current the emitter and collector currents are equal and fixed and transistor 82 forms a constant current generator. The collector current of transistor 82 supplies the base of transistor 84 and the collector of transistor 85.

Transistors

84 and 86 are Darlington connected current amplifiers. Zener diode 87 and resistor 88 are in series across the output. The junction of zener diode 87 and resistor 88 is at approximately 10V, with respect to ground, and is a reference for the emitter of transistor 85.

Resistors

89, 90 and 91 form a voltage divider across the output of transistor 86. Resistor 90, which is adjustable, is used to vary the output. The output is varied in the same manner that the output is regulated. If the slider on resistor 90 is made more positive, or the output voltage increases, the base current of transistor 85 is increased, which causes 85 to conduct more. Since the collector current of transistor 82 is constant, the additional current must come from a decrease in base current of transistor 84 and in turn a decrease in the collector current of transistor 86. This decrease in current is accomplished by an increase in drop across 86. The output voltage will therefore decrease, since transistor 86 is in series with the supply and load. For decrease in the slider voltage of resistor 90 or a decrease in output voltage, the opposite sequence occurs with a resulting increase in the output voltage. Capacitor 92 is used as a filter capacitor.

OVER-CURRENT PROTECTION Over-Current Protection is provided to prevent damage due to an accidental short circuit of the output terminals. Output current passes through resistor 93 which is connected to the emitter of transistor 94 and base of transistor 94 through current limit resistor 95. An increase in output current of transistor 86 increases the drop across resistor 93 and increases the current through transistor 94 until saturation occurs. The increased current draws base current from transistor 84 thus decreasing the drive of transistor 86 and limits its output current.

FIRE SENSING A parallel combination of relay 61, resistors 96 and 97 is placed in series with the detectors 11. When a detector 11 is activated due to the presence of smoke, there is an increase in current due to the added load of the tire indicating lights 50 and the

resistors

59 and 60 in the detector 11. This increase of current actuates the sensing relay 61. The current drawn per detector ranges from a quiescentcurrent of about 2.5 ma to a fire condition current of 30 ma minimum. The relay 61 is calibrated to close with an increase of ma by closing switch 100 (which connects the resistor 98 and 99 across the supply) and increasing the resistance with resistor 96 until relay 61 pulls in. This step is performed with all the detectors 11 connected. When switch 101 controlled by relay 61 closes, voltage is impressed across

resistors

102 and 103 applying voltage to the gate G6 of silicon control rectifier (SCR) 104 causing anode-cathode conduction. This applies voltage across relay 105 energizing it. One set of contacts 109 of relay 105 is available for fire indication, and the other set 110 is used to short out the fire sensing relay, 61 to protect it from over current. Diode 106 prevents inductive voltage spikes when relay 105 deenergizes. Resistor 107 and capacitor 108 limit the rate of rise of voltage applied to SCR 104 to prevent accidental triggering. Diode 111 is a Stabistor" diode used to bypass the surge currents of the detectors from relay 61. For a 0.5V pickup voltage of relay 61, diode 111 conducts less than I ma.

TROUBLE CIRCUIT The trouble circuit detects the pulses placed on the line L connecting all detectors 11 by the End of Line Sender 16 unit located at the last detector. The primary of transformer 112 is placed in series with the control unit 15 output terminal 113. The pulses which appear across the secondary of transformer 112 are rectified by diode 115, filtered by capacitor 116, and impressed across

resistors

117 and 118. A portion of this voltage is applied to the base of transistor 119. Transistor l 19 which is non-conducting is turned on by the incoming pulses. When 119 is on, capacitor 120 charges through resistor 121 and when transistor 119 is off, capacitor 120 discharges through resistor 122. The discharge time constant is chosen to be much longer than the repetition rate of the input pulses from the sender 16. The average value of the voltage at the junction of

resistors

121, 122 and capacitor 120 which is tied to the gate G7 of field effect transistor FET 124 is about 8-9V. This reduces the gate voltage from the no signal condition. Since the source terminal S2 follows the gate G7 except for a small voltage difference, the source voltage is reduced also. Connected to the source S2 is a series combination of

resistors

125, 126 and a 10V zener diode 127. The source voltage under these conditions is not sufficient to overcome the zener voltage and cause conduction through this path. The junction between diode 127 and resistor 126 is at a low voltage, so that transistor 128 is cut off with a resulting high voltage on the collector. Transistors 129 and 130 are in series with the Trouble Relay 131. In a normal ready condition, both transistors 129 and 130 will be conducting and 131 will be energized. Rectifier 139 coupled across the coil of relay 131 acts as an electrical surge suppressing device. Transistor 129 will be conducting when transistor 128 is cut off since resistor 132 is supplying base current. Should the pulses fail to be received for any reason, transistor 129 would cut off since transistor 128 would divert its base current. This would cause the Trouble Relay 131 to deenergize, indicating trouble. Transistor 130 is maintained conducting by the current through the fire lamp 133, wire 133a and

resistors

134, 135 and 136. A portion of this current supplies the base of transistor 130 to maintain it conducting. This supervisory current through the lamp 133 is not sufficient to light it. If the lamp 133 is removed or opens, transistor 130 will cut off and trouble relay 131 will become deenergized. In summary,

Trouble" will be indicated when relay 131 deenergizes opening normally closed contacts 151 for providing a trouble signal, this occurring from loss of pulses received from the line L, the loss of the lamp 133, or the loss of the regulated positive voltage for the system.

The

diodes

137 and 138 are connected to the anode of SCR 104 which is caused to conduct when a fire signal occurs. This places the

diodes

137 and 138 at near ground level as the anode of SCR 104 would then be at a low voltage. The lamp 133 with its protective fuse circuit 140 will then light as almost full supply voltage is across it and resistor 134, its dropping resistor. Also, Trouble Relay 131 will be locked out giving priority to the fire signal.

END-OF-LINE SENDER The End-of-Line Sender 16 generates pulses and places them on the lines L connecting the detector 11 to the control unit 15. The unit is connected to the line at the most remote detector. The operation is as follows: diode 141 provides protection against connecting the leads in reverse polarity. Transistor 142 is a U- nijunction Transistor which, with resistor 143 and capacitor 144 forms an oscillator. The pulses are developed across resistor 145 and the divider formed by

resistors

146 and 147. The pulse voltage across resistor 147 is applied to the base of transistor 148. The emitter current of 148 supplies enough drive to the base of transistor 149 to saturate it. Thus, almost the full l6V supply is placed across resistor 150. The supply will be loaded by about 100 ma for the duration of the pulse. The pulse duration is set for about 500 microseconds and it occurs periodically every 500 milliseconds.

P16. 4 shows a preferred structure of a typical detector 11 having sensing means which is used to sense the presence of aerosols in the atmosphere and produce an output signal indicative of a fire condition. The sensing means includes a base number 160 which has an adjustable reference electrode 161 appropriately coupled or mounted thereto. Cylindrical housing 162 is fitted over the reference electrode 161. A common electrode 163, which is a bobbin shaped body of conductor having upper and lower surfaces A and B respectively, is fit into the housing 162. On each of the surfaces of the cylindrical electrode 163 is deposited a small amount of radio-active material, which in the preferred embodiment, is Americium 241 and is designated by the reference 164.

The reference chamber is that volume disposed between the reference electrode 161 and the lower surface B of the common electrode 163. The adjustment of the distance between reference electrode 161 and surface B sets up a reference potential. This is substantially isolated from the atmosphere such that no aerosols enter the chamber 20 and interfere with the current produced by the radio-active source 19.

The sensing chamber 21 is formed between the upper surface A of the common electrode 163 and sensing electrode 168 mounted above the common electrode 163. This chamber is substantially opened to the atmosphere and may receive aerosol particles present in the atmospheric medium.

The upper surface A of the common electrode 163 is recessed in the cylindrical housing 162 in order to deflect radiation emitted by the source 19 towards the sensing electrode 168. The cylindrical housing 162 is composed of teflon material and serves as an absorber of alpha particles which may be emitted at angles away from the sensing electrode 168. This cylindrical housing 162, therefore, serves to columnate or direct the radio-active emissions toward the sensing electrode 168. Another reason for the use of teflon in the construction of the cylindrical 162 is that the teflon has a very high insulative quality and facilitates the use of PET 23 to detect extremely low level changes in ionization potential due to presence of smoke aerosols.

A printed circuit board 165 is mounted about the housing 162 to the base 160 and includes the components of the electronic circuitry shown in FIG. 1. The

field effect transistor 23 is mounted to the board 165 and coupled to the common electrode 163 through a hole in the cylinder 162.

To the base is mounted a cover 166 for protecting the circuitry and internal components from dust and the like. Warning lamp 50 is coupled to the output of the circuitry on the printed circuit board 165. The warning lamp 50 is mounted in the cover 166 and gives visual indication of a fire condition. This is convenient for checking where a fire or trouble condition exists in a specific chain of detectors. Over the cover 166 is mounted the sensing electrode 168 and between the electrode 168 and wind shield 170 is a screen 169 used to protect the sensing electrode and to permit the free flow of atmosphere into the sensing chamber 21.

It can be seen from the drawing that the construction of the sensing unit 10 is simplified and that the reference electrode 161, sensing electrode 168 and common electrode 163 may be coupled to the input of the detector circuit mounted on the printed circuit board very readily. The

chambers

21 and 20 are also designed for ease of manufacture because they are one next to the other rather than one inside of the other as in other detectors in the art. It is apparent from the construction of the chambers in the present invention that assembly time of the unit is greatly reduced and the efficiency of the device is unaffected.

The system shown in the present disclosure therefore provides for accurate sensing of a fire condition as well as warning of a malfunction in any of the units or a break in the transmission line between the central indication unit and any one of the sensing units remotely located. The system is provided with a safety feature for eliminating the condition whereby a false alarm is produced, and the construction of the sensing means has beengreatly simplified in order to reduce the cost of manufacture and reduce maintenance cost by making the unit reasonably inexpensive and readily replaceable.

While there has been shown what is considered to be the preferred embodiment of the present invention, it will be obvious to those skilled in the art that certain modifications and changes may be made without departing from the true spirit and scope of the invention, and it is intended in the appended claims to cover all such modifications and changes within the scope of the appended claims.

What is claimed is:

1. A fire detector including; ionization sensing means for increasing the level of an output signal relative to a normal threshold value in accordance with sensing the presence of smoke in the atmosphere, and detecting means selectively responsive to the output signal for generating an alarm control signal wherein the improvement comprises:

filter means effective if uninhibited for generating the alarm signal; and inhibiting means including a voltage level detector governed by the output signal, operable to inhibit the filter means only if the output signal is within a normal voltage range,

whereby an alarm signal is generated'when the output signal is above the voltage range as when smoke is present in the chamber or when the output signal is below the voltage range as when there is a malfunction in the ionization sensing means.

2. The fire detector of claim 1 wherein said detecting means comprises: a field effect transistor amplifier having its input governed by the output signal and switching means including a plurality of electronic gates energized in accordance with an output of the amplifier for generating said alarm output signal.

3. The fire detector of claim 2 wherein said inhibiting means includes: charging means coupled to a first of said gates, said first gate being responsive to the level of the output of the amplifier and said charging means to periodically trigger said gate upon each accumulation of energy beyond the level of said output of the amplifi- 4. The fire detector of claim 3 wherein said charging means includes a resistor and capacitor combination coupled to said first gate.

5. The fire detector of claim 4 wherein said filter means comprises: timing means responsive to the periodic triggering of said first gate producing a pulse for each occurrence.

6. The fire detector of claim 5 wherein said timing means includes a charging circuit coupled between a second and third of said gates, said charging circuit being discharged through said second gate when said second gate is in a conductance state, said second gate conductance state being governed by said triggering of said first gate; and the third gate being turned on to produce a signal when said charging rate is less than said triggering rate of said first gate.

7. The fire detector of claim 6 wherein said detecting means includes: a fourth of said gates, operative to produce said output alarm signal in response to the conductance state of said third gate.

8. A system as defined in claim 1 further comprising: centrally located control means including line wires coupled to each of a plurality of said fire detectors for indicating said alarm condition.

9. A system as defined in claim 8 wherein said indicating means comprises: current sensitive means responsive to a change in current occasioned by the occurrence of said alarm signal for providing said indication including a relay energized in accordance with said current increase.

10. A system as defined in claim 8 further comprising: an end of line code sender connected across the line wires for terminating a line circuit connecting the detectors to the centrally located control means including:

an oscillator powered by energy applied to the line wires for generating pulses for transmission over the line wires to the centrally located control means, and

centrally located receiver means responsive to said pulses for indicating a trouble condition upon the cessation of pulses.

11. A system as defined in claim 10 wherein said receiver means comprises:

switching means having a conductance state in accordance with said pulsed energy signals,

timing means including a resistor and capacitor charging circuit discharged each time said switching means is in its conductance state, said charging circuit having a discharge time greater than the normal frequency of said pulsed energy;

and means responsive to a charged condition of said charging means for indicating a trouble condition upon the cessation of pulses.

12. The fire detector of claim 1 wherein said ionization sensing means comprises: two ionization chambers coupled serially across a source of low voltage including a reference chamber and a measuring chamber, said reference chamber being substantially isolated from smoke particles establishing a reference potential for said detector and said measuring chamber serially coupled adjacent to said reference chamber for producing a change in the level of said reference potential thereby providing said output signal.

13. The fire detector of claim 1 comprising:

a base member;

a cylindrical tube of insulative material mounted thereto;

a common electrode plate secured coaxially within said tube, said electrode coupled to said detector means;

a reference electrode substantially parallel with said common electrode movably mounted to said base for establishing the reference potential in accordance with the spacing of said common electrode and said reference electrode;

a measuring electrode mounted to said base at the opposite end of said tube in spaced relation with the opposite side of said common electrode, said measuring electrode having openings therein for admitting smoke particles.

14. The fire detector of claim 13 wherein said ionization chamber includes a source of alpha particle emitting material disposed on each side of said common electrode for ionizing the atmosphere with said chambers and producing an ionization current in accordance with a potential imposed across the reference and measuring electrodes.

15. The fire detector of claim 14 wherein said measuring electrode and the associated side of said common electrode are in a spaced relation such that the walls of the tube tend to columnate the alpha particles towards said measuring electrode for uniform ionization of the atmosphere.

Claims

1. A fire detector including; ionization sensing means for increasing the level of an output signal relative to a normal threshold value in accordance with sensing the presence of smoke in the atmosphere, and detecting means selectively responsive to the output signal for generating an alarm control signal wherein the improvement comprises: filter means effective if uninhibited for generating the alarm signal; and inhibiting means including a voltage level detector governed by the output signal, operable to inhibit the filter means only if the output signal is within a normal voltage range, whereby an alarm signal is generated when the output signal is above the voltage range as when smoke is present in the chamber or when the output signal is below the voltage range as when there is a malfunction in the ionization sensing means.

3. The fire detector of claim 2 wherein said inhibiting means includes: charging means coupled to a first of said gates, said first gate being responsive to the level of the output of the amplifier and said charging means to periodically trigger said gate upon each accumulation of energy beyond the level of said output of the amplifier.

4. The fire detector of claim 3 wherein said charging means includes a resistor and capacitor combination coupled to said first gate.

10. A system as defined in claim 8 further comprising: an end of line code sender connected across the line wires for terminating a line circuit connecting the detectors to the centrally located control means including: an oscillator powered by energy applied to the line wires for generating pulses for transmission over the line wires to the centrally located control means, and centrally located receiver means responsive to said pulses for indicating a trouble condition upon the cessation of pulses.

11. A system as defined in claim 10 wherein said receiver means comprises: switching means having a conductance state in accordance with said pulsed energy signals, timing means including a resistor and capacitor charging circuit discharged each time said switching means is in its conductance state, said charging circuit having a discharge time greater than the normal frequency of said pulsed energy; and means responsive to a charged condition of said charging means for indicating a trouble condition upon the cessation of pulses.

13. The fire detector of claim 1 comprising: a base member; a cylindrical tube of insulative material mounted thereto; a common electrode plate secured coaxially within said tube, said electrode coupled to said detector means; a reference electrode substantially parallel with said common electrode movably mounted to said base for establishing the reference potential in accordance with the spacing of said common electrode and said reference electrode; a measuring electrode mounted to said base at the opposite end of said tube in spaced relation with the oppositE side of said common electrode, said measuring electrode having openings therein for admitting smoke particles.