05/190050

04/16/1974

10/18/1971

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Inoue-Japax Research
Inc. (Midoriku, Yokohama, JA)

340/629

313/325

G08B17/117; G08B17/10; G08B17/10; G08B17/06

340/237S,237R,227.1,227 200/61.03 313/325 324/71PC

3611335	MULTIPLE COMBUSTION SENSING DEVICE WITH FALSE ALARM PREVENTION	October 1971	Ogden et al.
3657737	METHOD OF AND DEVICE FOR SMOKE DETECTION AND CIRCUITS THEREFOR	April 1972	Hamm et al.

Caldwell, John W.

Swann III, Glen R.

Ross, Karl Dubno Herbert F.

1. A system for warning of a smoke or fire condition comprising:

2. The system defined in claim 1 wherein said source produces pulses of a

3. The system defined in claim 1 wherein said source includes a dry-cell or battery, an oscillator operated thereby and a stepup transformer having a primary winding operatively connected with the output of said oscillator and a secondary winding connected in circuit with said gap for applying

4. The system defined in claim 1 wherein said gap has a width of 0.1 to 1

5. The system defined in claim 1 wherein said detector means includes a resistor in circuit with said gap and a transistor network bridged across said resistor for changing its state in response to the passage of current

6. The system defined in claim 1 further comprising a printed circuit board, said detector means, said source and said electrode means being

7. A system forwarning of a smoke or fire condition, comprising:

8. A system for warning of a smoke or fire condition, comprising:

9. The system defined in claim 8 further comprising energization means

10. The system defined in claim 8 further comprising a printed circuit board, said electrolet pile, said detector means and said electrode means

11. The system defined in claim 8 wherein said detector means includes an oscillator energizable upon breakdown of said gap, transducer means energizable by said oscillator, locking means for keeping said oscillator to oscillate once gap breakdown is detected to continue said transducer to be energized independently of gap potential, and switch means associated with said locking means for terminating the operation of said oscillator.

12. A system for warning of a smoke or fire condition, comprising:

13. The system defined in claim 12 wherein said source has an output frequency of 0.01 to 10 Hz and produces pulses of a pulse width of 1 to

14. The system defined in claim 13 wherein said source includes a dry-cell or battery, an oscillator operated thereby and a transformer having a primary winding operatively connected with the output of said oscillator and a secondary winding connected in circuit with said gap for applying

15. The system defined in claim 12 wherein said gap has a width of 0.1 to 1

16. The system defined in claim 12 wherein said electrode means includes a bimetal element deflectable by temperature rise to bridge said gap and

17. The system defined in claim 12 wherein said detector means includes a resistor in circuit with said gap and a transistor network bridged across said resistor for changing its state in response to the passage of current

18. The system defined in claim 12 wherein said detector means includes an oscillator energized upon breakdown of said gap and transducer means

19. The system defined in claim 12, furhter comprising a printed circuit board, said detector means, said source and said electrode means being

20. A system for warning of a smoke or fire condition, comprising:

21. The system defined in claim 20 wherein said source has an output frequency of 0.01 to 10 Hz and produces pulses of a pulse width of 1 to

22. The system defined in claim 20 wherein said source includes a low-voltage dry-cell or battery, an oscillator operated thereby and a stepup transformer having a primary winding operatively connected with the output of said oscillator and a secondary winding connected in circuit

23. The system defined in claim 20 wherein said gap has a width of 0.1 to 1

24. The system defined in claim 20 wherein said electrode means includes a bimetal element deflectable by temperature rise to bridge said gap and

25. The system defined in claim 20 wherein said detector means includes a resistor in circuit with said gap and a transistor network bridged across said resistor for changing its state in response to the passage of current

26. The system defined in claim 20 wherein said detector means includes an oscillator energized upon breakdown of said gap and transducer means

27. The system defined in claim 26 wherein said detector means further includes locking means for maintaining said oscillator in oscillation once gap breakdown is detected to continue energization of said transducer independently of gap voltage, and switch means associated with said

28. The system defined in claim 20, further comprising a printed circuit board, said detector means, said source and said electrode means being

29. A system for warning of a smoke or fire condition, comprising:

30. A system as defined in claim 29 wherein at least one of said electrodes is formed with bimetallic member for shifting said electrodes relatively upon a change in temperature.

FIELD OF THE INVENTION

My present invention relates to a system for detecting the presence of smoke and/or the existence of a fire at a protected site. The invention also relates to an alarm arrangement for warning of fire and smoke hazards at a protected site.

BACKGROUND OF THE INVENTION

Various alarm devices have been proposed heretofore in order to respond to the existence of a fire at a protected location or site and to trigger an alarm device capable of remote or proximal warning of personnel and other individuals in the vicinity. A typical fire alarm, for example, may include a temperature-sensitive mechanism responsive to a rise in temperature beyond a predetermined level (generally considered to be the maximum attributable to weather conditions and environmental considerations) to operate a switch mechanism which, in turn, is connected to an alarm circuit providing an audible, visible and graphic representation of the temperature rise and initiating, if desired, a suitable response.

Indirect methods of providing a warning of fire have also been suggested. A typical system of the latter type provides a thermally fusible element at the discharge side of a water-supply network, e.g., a sprinkler system, which is destroyed when elevated temperatures are detected to discharge a water spray over the region. An alarm may then respond to the operation of the sprinkler system to alert fire-fighting personnel or individuals who must, in turn, report the occurrence to the fire-fighting authority. Other systems have made use of the same or similar principles to provide direct or indirect warning of a fire condition. Associated with such systems may be remote-warning devices, triggered by an alarm system or operated independently of an acoustical or optical alarm network, for alerting fire-fighting authorities or other individuals remote from the site of the occurrence.

All of these systems are characterized by the fact that they respond to a temperature rise induced by combustion and generally must be monitored at a location remote from the actual site of combustion. Unless numerous sensors are provided, the response of a temperature-sensitive device may lag well behind the development of a fire, simply because the fire may begin at a location spaced from the sensor sufficiently to prevent detection of a significant temperature increase. Another disadvantage of existing fire-alarm systems is that their dependency upon a temperature rise to indicate the presence of a fire condition, prevents them from satisfactorily detecting smoldering fires or incipient combustion. It has long been known, for example, that spontaneous combustion may develop as a smoldering condition well before flames are of a sufficient intensity to cause a temperature-sensitive alarm remote from the smoldering site to respond. It is also known, in this connection, that one of the greatest hazards of the fire-fighting process is the occurrence of smoke and the danger is suffocation associated therewith. Furthermore, smoldering fires of low temperature may produce large quantities of smoke well before sufficient heat is developed to operate a temperature-sensitive alarm. Finally, I may point out that smoke is carried by diffusion, convection and other air currents at high rates through open spaces and may even be present in the air before its odor is detected by personnel in the vicinity of a fire.

OBJECTS OF THE INVENTION

It is the principal object of the present invention to provide an improved smoke and fire alarm system which avoids the disadvantages of earlier alarm systems as discussed above, which responds rapidly and effectively to a fire condition before high heat levels are developed, which has an adjustable threshold substantially independently of ambient temperature maxima, which is of low cost and high efficiency, and which has little danger of malfunction.

A more specific object of the invention is to provide a system for detecting the development of a combustion condition which affords improved sensitivity and better response than earlier systems for the same purpose.

It is also an object of the invention to provide an apparatus capable of generating a warning signal of a fire condition.

SUMMARY OF THE INVENTION

These objects and others which will become apparent hereinafter are attained, in accordance with the present invention, by a method which is based upon my discovery, in connection to research related with spark-type discharges, that a spark gap including normally dielectric air and across which certain potentials are applied, can be caused to break down simply by the presence of smoke particles in the gap. In other words, the invention provides a method of warning of a fire and/or smoke condition which comprises the steps of providing a gap between a pair of electrodes in ambient air traversed by smoke particles generated by the fire, applying a potential across this gap incapable of effecting dielectric breakdown in the absence of smoke particles in the gap but sufficient to effect a discharge between the electrodes in the presence of such smoke particles, and detecting a breakdown in the gap for generating the warning signal.

While I may provide a constant potential across the gap, at least of limited duration, it has been found to be advantageous to apply the potential periodically, thereby preventing cumulative influences, such as polarization at the gap from altering the selected response threshold and triggering an undesired alert when smoke particles are not present. Preferably, the energization period is relatively long and the energization frequency relatively low (e.g., of a frequency between 0.01 and 10 Hz), while the pulse width may have a duration ranging from 1 to 100 microseconds for suitable operation.

According to the apparatus aspects of the present invention, therefore, a system for detecting the development of fire or smoke comprises a spark gap exposed to ambient gases and connected in a circuit for periodically applying a potential sufficient to effect breakdown across the gap in the presence of smoke particles but insufficient to cause breakdown in the absence of such particles and during "normal" conditions. In addition, a circuit responsive to breakdown of the gap may be provided across the latter and may be connected to a warning, signaling or alerting device for generating a visible, acoustic or remotely sensed signal indicative of the breakdown of the gap. It has been found to be desirable to constitute the system with a spark-discharge gap upon a mounting body for the electronic components of the device, e.g., on a printed circuit board, upon which the two electrodes defining the gap are permanently fixed.

In some instances it has been found to be advantageous to provide means for adjusting the gap, e.g., for advancing one of the electrodes toward the other, although practical applications using a gap width of the order of a fraction of a millimeter, e.g., between 0.1 and 1 mm, have shown that a fixed positioning of the electrodes on a printed circuit board or like support is desirable.

The circuit of the present invention advantageously comprises a multivibrator circuit connected to and across the electrode of the spark gap to provide the potential pulses as noted earlier. In one particularly advantageous construction, the multivibrator circuit is operated at relatively low voltage and is connected across the primary winding of a stepup transformer whose secondary winding is bridged across the gap. It has been found that transformer turn ratios may range between 100:1 and 10,000:1 between the secondary winding and the primary winding.

Moreover, it has been found to be advantageous to provide the breakdown detector as a transistor circuit including a pulse-shaping network, e.g., a SCHMITT trigger, one or more amplifying transistors (preferably a plurality in cascade) and a multivibrator oscillator energized by the amplifier transistors for operating the signaling device. All of these components, including the transformer and the multi-vibrator energizing the gap, which is preferably adjustable both as frequency and pulse duration, can be mounted upon the printed circuit board forming the gap or can be formed as an integrated circuit upon which the gap is provided.

In place of the transformer or in addition thereto, it has been found to be desirable to derive at least part of the voltage required across the gap from an electrolet pile which may be provided in series with a source of lower voltage pulses derived from a multivibrator. In other words, the low-current drain electrolet pile and a pulse source can be connected in series to superimpose the pulses of this source upon the constant potential of the electrolet pile which normally maintains a potential somewhat below the breakdown potential of the gap in the presence of the smoke particles. A circuit may be provided using the electrolet pile as a voltage-storage device as well and I may apply a transformer across the electrolet pile to superimpose the pulses upon the normally static direct-current potential thereof. An electrolet pile, of course, generally is constituted as a stack of closely spaced elements with a gap between the electrolet material and a metal plate with a high capacity. Electrostatic potentials of the order of 400 volts per cell of the pile may be developed in this manner.

According to another feature of the invention, means is provided for activating the detecting circuit in the event a rise in temperature, indicative of a fire, is sensed by a suitable temperature-responsive device. This has the advantage of providing fail-safe operation of the alarm if, for some reason, the smoke-particle sensor has been contaminated or rendered inoperative for other reasons. Moreover, it guarantees early detection of a fire of smokeless or low-smoke type.

It this aspect of the invention, one or both of the smoke-detection electrodes defining the breakdown gap are provided on bimetallic members for mechanically contacting the other electrode member to form a switch which, when closed, operates the detecting circuitry.

DESCRIPTION OF THE DRAWING

The above and other objects, features and advantages of the present invention will become more readily apparent from the following description, reference being made to the accompanying drawing in which:

FIG. 1 is a circuit diagram of a system according to the present invention;

FIG. 2 is a plan view of a printed circuit board carrying the gap according to the present invention;

FIG. 3 is a graph illustrating the operation of the device of FIG. 1;

FIGS. 4, 5, 6 and 7 are circuits illustrating modifications of the invention;

FIGS. 8 and 9 illustrate embodiments of the present invention according to other principles; and

FIG. 10 shows in diagrammatic form a printed circuit board provided with another gap arrangement according to the invention.

SPECIFIC DESCRIPTION

In FIG. 1, I have shown a circuit illustrating principles of the present invention and constituting a preferred embodiment thereof which comprises a voltage source 1, conveniently in the form of a low-voltage, low-power dry cell or battery whose terminals are connected across a multivibrator circuit 2 for applying voltage pulses of predetermined duration and frequency to a spark gap 5 exposed to ambient gases adapted to carry smoke particles from a combustion site.

Between the multivibrator 2 and the spark gap 5, there is provided a stepup transformer 4 having a primary: secondary turn ratio of 1:400, for example, and applied through a resistor r ₁ which may be adjustable as shown.

Across the resistor r ₁ , is tapped an output through a resistor 11 and a rectifier diode 12 to the base of a transistor T ₁ forming with transistor T ₂ a SCHMITT trigger represented generally at 6, and having an output applied to the amplifier transistor T ₃ connected to a signal inverter transistor T ₄ . Another transistor stage T ₅ is applied between the SCHMITT trigger 6 and an oscillator 8 of the multivibrator type which is connected via transistor 13 to the warning device 9. The latter may be a bell or buzzer or may even be a transmitter signaling the response of the system to a remote location.

The multivibrator 2 comprises a pair of cross-coupled transistors 2a, 2b, the bases of which are tied to the junction between capacitors 2c and 2d and resistors 2e and 2f of respective RC networks in the collector circuits of the opposing transistor. Bias resistors 2g and 2h are bridged between the emitters and the positive terminal of the low-voltage battery 1. Resistors 2e, 2f, 2g and 2h may be adjustable to vary the time constants of the multivibrator circuit hence the pulse duration, interval and frequency. An output transistor 3 is connected via a base-bias resistor 3a to the collector of transistor 2b and has an emitter-collector circuit including the battery 1, a bias resistor 3b and a primary winding 4a of stepup transformer 4. The secondary winding 4b of this transformer, which has a turn ratio between secondary and primary of, say, 400:1 or 500:1, is connected in series with resistor r ₁ across the discharge gap 5.

In the detector stage 6, the transistor T ₁ is provided with a collector resistor 14 and an emitter resistor 15 in series with the emitter-collector electrodes across the battery 1 while the emitter of transistor T ₂ is tied to the emitter of transistor T ₁ which is brought to the negative terminal of battery 1 through resistor 15. A parallel coupling of resistor 16 and capacitor 17 connects the collector of transistor T ₁ to the base of emitter T ₂ and also to the negative terminal of battery 1 through resistor 15'. A resistor 18 is provided between the collector of transistor T ₂ and the positive terminal of battery 1.

The base of transistor T ₃ is energized from the collector of transistor T ₂ and is also tied via a diode D ₁ and a resistor 19 to the base of transistor T ₁ . A switch 20 in this feedback circuit can be closed to apply collector potential of transistor T ₂ to the base of transistor T ₁ and thus, as will be apparent, maintain operation of the alarm once a breakdown at gap 5 is detected and until it is set off.

The collector of transistor T ₃ , which is provided with a bias resistor 21 to the positive terminal of the battery 1, is connected via resistor 22 to the base of transistor T ₄ which is in cascade with a transistor T ₅ , whose base is connected by resistor 23 to the collector of transistor T ₄ and is brought to positive potential when the transistor T ₃ is conductive. The collector-base circuit of transistor T ₄ is completed by the resistor 24 the potential drop at which governs the conduction of transistor T ₅ . A load resistor 25 is connected in series with the collector of transistor T ₅ across the battery 1 and is tapped to the negative terminal of the multivibrator 8 which as illustrated is similar to the multivibrator 2 and provides an output in the

100 Hz to 10,000 Hz range for energizing the electroacoustic transducer 9 through the transistor 13 when transistor T ₅ is rendered nonconductive.

In operation, switch 20 is assumed to be closed and input transistor T ₁ nonconductive. In this state, the second or output transistor T ₂ of the Schmitt trigger is held conductive to maintain the nonconductivity of transistor T ₃ , and hence a positive potential is to be found at the latter's collector to maintain the conductivity of transistor T ₄ . Then, since transistor T ₅ is likewise conductive, a potential drop appears across the load resistor 25 to hold the multivibrator 8 ineffective.

Since, on the other hand, the multivibrator 2 is effective by virtue of its direct connection to the battery 1, it generates a substantially square wave output in the form of low-voltage pulses which are applied to the base of transistor 3. The latter is rendered periodically conductive for brief intervals and thus current pulses traverse the primary winding 4a of transformer 4 to generate high-voltage pulses in the secondary winding 4b. A potential is tereby applied across the gap 5 which, however, is not sufficient to break down the dielectric medium, e.g., normal air free from smoke particles and hence no change is detected by the sensing circuit 6. The potential is chosen to lie between about 600 and 1,500 volts with a spark gap of 0.1 to 0.5 mm and is established so as to be sufficient to break down the gap at any desired threshold level of smoke-particle concentration in the air.

Should a smoldering fire (or a fire of any other type) generate sufficient smoke in the area, the smoke particles entering the gap will cause a breakdown of the dielectric and phase-reversal triggering of the sensing circuit 6 to render transistor T ₃ conductive to trigger transistor T ₄ and in turn transistor T ₅ into non-conduction, and to eliminate the blocking bias potential across resistor 25. The multivibrator 8 is thereby energized to provide a sonic-frequency energization of transistor 13 and of the transducer 9. It will be apparent that an optical signal may be provided in place of an acoustical signal or in addition thereto by providing a filament of a lamp in the output circuit of multivibrator 8. Furthermore, the latter may represent any other oscillator including that of a radio frequency, transmittor, for remote signaling of the presence of fire and smoke. In addition, it is possible to constitute transistor 13 of Germanium and the like type transistor which increases the leakage conductivity at an elevated temperature and to dispose same in the detection zone so that when such an elevated temperature is experienced, the transducer 9 is energizable independently of the operation of the smoke detector 6.

In FIG. 2 of the drawing, I have shown a printed circuit board 10 in somewhat diagrammatic form. The spark gap 5 is provided between a pair of electrodes 5a and 5b permanently implanted in the board with a fixed spacing S. The other components of the system, namely, the transistors, resistors, capacitors, diode, transducer and even transformer 4 and battery 1 may be mounted upon the opposite side of the printed circuit board and interconnected by the printed circuit leads 26 in the usual manner. The printed circuit board may then be mounted in a housing with the surface 10a exposed to the ambient atmosphere at the protected location.

In FIG. 3, I have shown a graph of the relationship between the spark-discharge gap spacing (plotted along the abscissa) and the breakdown potential across the gap plotted along the ordinate using two different sets of electrode configuration for smoke present and absent in the gap area. Plots A and B were made with a needle electrode as cathode and 2 mm diameter planar disc electrode as anode while plots A' and B' were made when both anode and cathode had a needle shape. Plots B and B' represent breakdown potentials, when smoke is absent in the gap region while A and A' represent breakdown potential when smoke is present in the gap area. It can be seen that in each plot the relationship is substantially linear and it is preferably to employ a pointed electrode as cathode and a planar element as anode since this establishes a greater difference in breakdown potential level between the smoke present and absent states. The plots were made with a frequency of 10 to 0.01 Hz, a pulse width of 10 microseconds, a transformer having a turn ratio of 400:1, a potential across the multivibrator of 1.5 volts to 4.5 volts and a titanium-oxide lead transformer core.

FIG. 4 shows a circuit which represents a modification of that of FIG. 1. In this embodiment, the discharge gap 105 is subdivided by a conductive electrode 105a into a pair of gaps 105' and 105", each associated with an electrode 105a' and 105a". The multivibrator 102 is here constructed identically with the multivibrator 2 and has an output transistor 103 in series with the primary winding of the transformer 104 whose secondary is connected via resistors 105b and 105c across the series of discharge gaps 105' and 105". The detector circuit is here shown to include a monostable multivibrator 106 having transistors 106a and 106b of the cross-coupled type and a resistor 106c connecting the gap electrode 105a" across the emitter-collector network of transistor 106a. Conductivity of the gap system 105, resulting from a breakdown of the dielectric medium (e.g., air) in the gap, results in the development of a potential at resistor 106c which renders transistor 113 conductive and energizes the electro-acoustic transducer 109 in series therewith across the battery 101. Here again, the multivibrator 102 provides short-duration spaced pulses of low total current drain to provide, via the transformer, periodic high-potential pulses at the gap and allows the system to operate at low cost for long durations.

The circuit of FIG. 5 differs from those already described in that the titanium-oxide lead core transformer 204 has its secondary winding connected between the central electrode 205a and the outer electrodes 205a' and 205a " of the gap system represented generally at 205. A switch 220 is provided in the base circuit of transistor 213 and can be closed to terminate operation of the transducer 209 when the latter is energized by the cross-coupled coupled transistors of the network 206. This system differs from that already described in that a separate battery 201 is provided for the multivibrator 202 generating the energization pulses. The battery 201', of course, energizes the transducer 209 when transistor 213 is rendered conductive.

In FIG. 10, there is shown a printed circuit board 210 in which the central conductor 205a is shown permanently positioned between the electrodes 205a and 205a" (fixed gap), the remaining components of the system being mounted on the opposite face of the printed circuit board as already described.

In FIG. 6, I have shown another embodiment of the present invention wherein the voltage required across the gap 305 which is formed by a pair of electrodes 305a and 305b permanently mounted on a printed circuit board as represented at 305c, is applied by an electrolet pile 304a which has been polarized semi-permanently. The polarization source for the electrolet pile 304a is here constituted conveniently by a low-power pulser which includes a transformer 304 and a resistor 304b connected in series with the secondary winding of the transformer 304 and is connectable via a switch 304c across the pile 304a when the latter requires repolarization as a result of its spontaneous release of stored charges or discharge through the smoke-detected gap 305. An electrolet pile comprises an electrolet material spacedly juxtaposed with a plate and forming a capacitor-like voltage source. When the electrolet is a synthetic resin such as polydifluoroethylene, heated to 100° C and then while being cooled stretched under an applied voltage, and when, say, three layers of the electrolet material are used with intervening metal plates, the electrolet pile is able to deliver 100 nanocoulombs/cm ² at 100 volts/μ. A gap of 0.5 mm may then be used in the smoke detector. The electrolet pile can maintain such capacity semipermanently, say, for a 1-year period, without spontaneous discharge, and thus permit the smoke-detection gap to continue monitoring operation without consumption of power.

In the low-power electrolet polarization pulse supply which can be connected to the electrolet pile 304a when desired, the transformer 304 is energized by a voltage source consisting of a relaxation oscillator 302 having a unijunction transistor 302a and an RC network consisting of a variable resistor 302b and a capacitor 302c. The base-bias resistors are represented at 302d and 302e and are mounted, together with the rest of the relaxation oscillator as an integrated circuit chip or module 302f on the printed circuit board. The emitter of the unijunction transistor is tied to the junction between the resistor and capacitor of the RC network which, in turn, is bridged across a battery 301 in series with an switch 301a which is ganged with switch 304c. The output of the unijunction transistor is applied via a resistor 303a to the base of a transistor 303 in series with the primary winding of transformer 304 across the battery 301.

The detector stage connected across the gap is similar to that previously described and comprises the variable resistor r ₁ across which is connected a resistor 311 and a diode 312 which feed the base of a transistor T ₁ whose collector and emitter resistors are represented at 314 and 315, respectively. The detector is thus responsive to a surge in current across the gap and operates the transistor T ₂ through an RC network 316, 317 as previously described. The switch 320, which is operated to cut off the warning signal after it has been triggered, is connected to the base of transistor T ₁ from the collector of transistor T ₂ via a diode and the resistor 319. The collector resistor of transistor T ₂ is shown at 318. Transistor T ₃ of the cascade and transistor T ₄ function in the manner already described to render a transistor T ₅ non-conductive to operate the multivibrator oscillator 308 and, in turn, operate the transducer 309 via the transistor 313. In this embodiment, also, breakdown of the gap resulting from the detection of smoke particles or their presence in the region results in a breakdown of the dielectric (air) normally maintaining the gap in non-conductive condition. The transducer, of course, is thereby triggered and the potential applied across the gap derives from the electrostatic potential of the electrolet pile.

In the embodiment illustrated in FIG. 7, the electrolet pile 404a is shown to be connected in series with the secondary winding of transistor 404 across the gap 405. Thus, the square-wave pulses can be applied to the transformer 404 by relaxation oscillator as shown at 302 or a multivibrator circuit as shown at 202. In this embodiment, however, a portion of the potential applied across the gap derives from the electrostatic potential of the electrolet pile, while the additional voltage is supplied by the oscillator in the form of square-wave pulses stepped up at transformer 404. The output is detected across the resistor r ₁ , in series with the gap and is applied to the multivibrator oscillator 406 which operates the transducer 409 through a transistor 413 as originally described.

In FIGS. 8 and 9, I have shown still other embodiments of the present invention to which the system is applicable. In FIG. 8, the multivibrator square-wave generator 502 applies voltage pulses at the desired level across the detection gap 505 and its series connected breakdown-type capacitor 504a or 504b selectively, depending upon the setting of a switch 504g. The transformer 504 is thus provided with a stepup rectifier bridge 504c at its output, the bridge being formed with capacitor 504d to produce the desired d-c pulse level to be applied across the detection gap 505 which is formed between a pointed cathode 505a and a planar anode 505b. When the smoke enter the gap 505, the latter and capacitor 504b both break down and this capacitor provides a short-circuit path for the output current of battery 509a to energize acoustic transducer 509. After smoke detection is accomplished in this manner, the capacitor 504b broken down can be replaced in position by capacitor 504a as a spare by means of switch 504g. Either the planar anode element 505b or the support bar 505c for the pointed cathode element 505a is here comprised of bimetal designed to deflect with increasing temperature to shortcircuit the gap, even if the smoke detector has not been triggered into operation and to energize the acoustic transducer 509 in series with the battery 509a.

In FIG. 9, there is illustrated another embodiment of the invention operating on generally similar principles wherein a transformer 601 serves as the voltage source. This transformer energizes via a full-wave rectifier bridge a square-wave pulse generator 602 whose output may be applied through a stepup rectifier bridge 604 across the gap 605. The latter may have one or more bimetallic elements whose function has already been discussed. When low-power detector 609 is operated or as a result of entry of smoke or the temperature increase to a sufficient level, it may transmit a signal to a remote detector 609a which, in turn, may transmit its output to a relatively high power indicator 609b in circuit with a self-locking relay 609c operated by the detector 609a. In this manner, sufficient remote signaling may be provided even where the source connected across the gap may be of low power.