Fire Detecting System With Spurious Signal Rejection

Abstract

A fire detecting system having a plurality of fire detectors connected across conductors for applying a voltage thereto. The detectors each include switching means which is closed upon operation of a detector and opened upon reduction of the voltage on the conductors below a predetermined value. Voltage is supplied to the conductors by means which responds to the actuation of one or more detectors to automatically lower the conductor voltage below said predetermined value and then raise it again. After a predetermined number of cycles of operation an alarm is automatically actuated. In this way momentary actuation of a detector will not sound an alarm and thus false alarms are eliminated.

Description

This invention relates to a fire detecting system having a high degree of reliability.

Some of the fire detecting systems which have been widely used include fire detectors each having a controlled switch, such as a silicon controlled rectifier (hereinafter referred to as "SCR"), which is closed in response to actuation of the fire sensing means but is not opened again unless the voltage thereacross is reduced below a specific value. Such detectors are advantageous in that the alarm is continuous even though the fire may be sensed for a short time, but they also have an attendant disadvantage in that a continuous alarm is provided when the detector is actuated erroneously by a momentarily induced voltage, a short passage of smoke through the detector, a sudden gust of wind, or any other cause other than fire. Such an erroneous alarm often causes substantial confusion and, therefore, is undesirable.

Therefore, one object of this invention resides in the provision of a fire detecting system including means for ascertaining the truth of fire information and giving an alarm only when the presence of fire is ascertained.

Should the system be provided with means for immediately restoring an actuated detector to its original condition, the detector would be repeatedly actuated and restored until the cause of actuation is removed. As the above mentioned causes of erroneous actuation are generally momentary and disappear quickly, an erroneous alarm can be avoided in accordance with this invention by energizing the alarm device after a specific number of repetitions of actuation and restoration.

The fire detecting system in which this invention is embodied includes a plurality of fire detectors connected in parallel between a pair of conductors and a central unit connected to one end of said conductors and each of the detectors have a normally open switch which is connected between said conductors and closed in response to detection of fire to short both the conductors. According to this invention, the central unit includes a d.c. voltage source, a multivibrator having stable and unstable states, means for counting the number of times the multivibrator is operated and means for producing an output when a predetermined number of operations is detected. An alarm device is energized by said output, and a circuit connection is provided for supplying an operating voltage to said conductors through a path which is conductive during the stable state of said multivibrator and for driving said multivibrator into the unstable state to make said path non-conductive when the conductors are shorted through any of the detectors.

Other objects and features of this invention will be more clearly understood from the following description and accompanying drawings forming part of this application.

In the Drawings:

FIG. 1 is a schematic circuit diagram of one embodiment of a system in accordance with this invention,

FIG. 2 is a simplified circuit diagram of the voltage stabilizing circuit of FIG. 1, and

FIGS. 3 and 4 are waveform diagrams which will aid in understanding the operation of the system of FIG. 1.

Throughout the drawings like reference numerals are used to denote like structural components.

Referring to FIG. 1, the fire detecting system accordingly to this invention includes a central unit 1 having a pair of output terminals 3 and 4, and a plurality of fire detectors 2-1, 2-2, . . . connected in parallel between a pair of conductors 30 and 40 extending from the output terminals 3 and 4. The fire detector 2 includes a fire sensor 25 which senses a product of fire, such as smoke or temperature, to produce an output and a normally open switch, such as an SCR 26, which is closed by the output of the sensor to short the conductors 30 and 40 and is never restored to its non-conductive state unless the voltage thereacross is reduced below a specific value.

The central unit comprises a power source section 5, a multivibrator section 6, an integrating circuit section 7 and an alarm section 8.

The power source section 5 includes a d.c. voltage source 10 such as a battery and a normally closed pushbutton switch 11 connected in series with the voltage source 10 and used for restoration of the system when actuated.

The multivibrator section 6 includes resistors 12, 13, 14 and 15, capacitors 16 and 17, NPN transistors 18 and 19 and diodes 20 and 21. The collector of transistor 18 is connected through the resistor 12 to the positive terminal of the power source, the emitter is connected to the negative terminal of the power source and the negative output terminal 4 of the central unit 1, and the base is connected to the cathode electrode of the diode 21 which has its anode electrode connected through the resistor 14 to the positive terminal of the power source. The transistor 19 has its collector connected to the positive terminal of the power source through the resistor 15, its emitter connected to the anode electrode of diode 23, and its base connected to the cathode electrode of the diode 20 which has its anode electrode connected through the resistor 13 to the positive terminal of the power source. The capacitor 16 is connected between the junction A of the resistor 12 and the transistor 18 and the junction B of the resistor 13 and the diode 20, and the capacitor 17 is connected between the junction C of the resistor 14 and the diode 21 and the junction D of the resistor 15 and the transistor 19. The base electrode of the transistor 18 is also connected through a resistor 22 to the negative terminal of the power source and the base electrode of the transistor 19 is also connected to the cathode electrode of a Zener diode 24 which has its anode electrode connected to the negative terminal. The anode electrode of the diode 23 is connected to the positive output terminal 3 of the central unit. In this multivibrator section 6, the resistors 13 and 15, the transistor 19 and the diodes 23 and 24 enclosed by broken lines 9 constitute a known stabilized voltage source for the fire detectors in cooperation with the power source section 5 and is shown more clearly in FIG. 2.

The integrating circuit section 7 includes Zener diodes 27 and 31, capacitors 28 and 32 and a resistor 29. The Zener diode 27 has its cathode connected to the positive terminal of the power source and its anode to the junction D. One end of each capacitor 28 and 32 is connected to the positive terminal while the other end of the capacitor 28 is connected through the resistor 29 to the junction D and the other end of the capacitor 32 is connected to the cathode electrode of the Zener diode 31 which has its anode electrode connected to the other end of the capacitor 28.

The alarm section 8 includes a PNP transistor 33 having a base electrode connected to the cathode electrode of the Zener diode 31, an emitter electrode connected to the positive terminal of the power source and a collector electrode connected through a series connection of resistors 35 and 36 to the negative terminal of the source. An NPN transistor 37 has a base electrode connected to the junction between the resistors 35 and 36, a collector electrode connected through a parallel connection of a relay unit 41 and an audible alarm 42 to the positive terminal of the source, and an emitter electrode connected directly to the negative terminal thereof. This section further includes a resistor 34 connected between the emitter and base electrodes of the transistor 33 and a resistor 38 connected between the base electrode of the transistor 33 and the anode electrode of a diode 39 which has its cathode electrode connected to the collector electrode of the transistor 37. The relay unit 41 is arranged to energize and actuate various alarm equipment (not shown) other than the audible alarm 42.

In the ready condition, a predetermined constant d.c. voltage is supplied from the voltage source 10 through the voltage stabilizing circuit 9 included in the multivibrator section 6 to the respective fire detectors 2-1, 2-2, . . . connected in parallel between the conductors 30 and 40. The voltage stabilizing circuit 9 which is schematically shown in FIG. 2 is of a type well-known in the art and therefore further description is not deemed necessary. On the other hand, as the emitter electrode of the transistor 18 is directly connected to the negative terminal of the source, a current flows through the resistor 14 and the diode 21 into the base electrode of the transistor 18 and keeps the transistor 18 in the ON condition. Accordingly, currents flow through the resistors 12 and 14 respectively. However, as the emitter of the transistor 19 is connected to the negative terminal of the source through the very high impedance of the fire detectors, little current flows through the resistors 13 and 15. Therefore, the capacitor 16 is charged up to the Zener voltage of the Zener diode 24, making the junction B positive, and the capacitor 16 is charged up to the source voltage, making the junction D positive.

When a fire is sensed by the sensor 25 of one of the fire detectors and the corresponding SCR 26 is driven into conduction, the emitter electrode of the transistor 19 is shorted through the SCR 26 to the negative terminal of the source 10, Thus, the transistor 29 is driven into the ON state and the positive terminal of the charged capacitor 17 is connected to the negative terminal of the source 10. Consequently, the potential at the base electrode of the transistor 18 which is connected to the negative terminal of the capacitor 17 through the diode 21 is momentarily lowered to the potential of the emitter. Therefore, the transistor 18 is driven into the OFF state and exhibits a high impedance.

In this condition, the capacitors 16 and 17 are discharged through the ON transistor 19 and, thereafter, charged again in the opposite direction, that is, the junctions A and C become positive. When the base potential of the transistor 18 is raised up to a specific value during this charging process, the transistor 18 is driven again into the ON state and exhibits a low impedance. Therefore, the positive terminal A of the charged capacitor 16 is lowered to the negative terminal potential and, consequently, the base potential of the transistor 19 is also lowered momentarily to the potential of the emitter. Thus, the transistor 19 is driven again into the OFF state and exhibits a high impedance. At this time, the voltage across the Zener diode 24 is also reduced to about zero volts and, therefore, the output voltage applied between the conductors 30 and 40 is similarly reduced to restore the actuated SCR 26 of the fire detector concerned to its original open state. Thus, the system is restored to its original condition.

If the cause of the actuation of the detector remains after the completion of the above mentioned cycle of operation of the system, the same operation is repeated until the cause of actuation is removed. Every cycle of such operation, detected by the voltage across the resistor 15 which is produced in response to conduction of the transistor 19 is accumulated successively in the capacitor 28 with a delay determined by the resistor 29 and the capacitor 28 and, every time the voltage across the capacitor 28 exceeds the Zener voltage of the Zener diode 31, it is transferred to and accumulated in the capacitor 32. The Zener diode 27 connected across the resistor 15 serves the function of a voltage limiter to pass a voltage higher than its Zener voltage but to block undesirable low voltages.

FIG. 3 represents the voltage waveform appearing across the resistor 15. In the drawing, voltages V.sub.0, V.sub.1 and V.sub.2 are the voltages for restoring the detector, namely, the voltage at a ready condition and the voltage at an actuated condition. Time points t.sub.1, t.sub.2 and t.sub.3 correspond to actuation of the detector, conduction of the transistor 19 and restoration of the detector, respectively. During the repeated actuation of the detector, the voltage across the capacitor 32 is raised successively as shown in FIG. 4 and tends to let a significant amount of current flow through the resistor 34. When a sufficient voltage drop is obtained across the resistor 34, the transistor 33 is driven into the ON state and produces an output at the junction of the resistors 35 and 36. This output is amplified by the transistor 37 and supplied to the relay unit 41 and the audible alarm 42 to energize them. The series connection of the resistor 38 and the diode 39 serves the function of facilitating the voltage drop across the resistor 34 and maintaining the operation of the alarm section 8.

In order to stop the alarm and restore the system to the original condition, the pushbutton switch 11 in the power source section 5 is pushed to interrupt the source voltage supply for a moment.

When the cause of actuation of the detector is a momentarily induced voltage or sudden gust of wind which is not a product of fire, such cause does not continue very long and generaly disappears quickly. Therefore, the accumulation of an electric charge in the capacitor 32 does not become enough to drive the alarm section 8, and the alarm is not operated.

As described in the above, the system of this invention can distinguish the cause of actuation of the detector and give an alarm only when an actual fire occurs.

Claims

What is claimed is:

1. A fire detecting system comprising a plurality of fire detectors connected in parallel between a pair of conductors, a central unit connected to one end of said conductors, each of said detectors having a normally open switch connected between said conductors, said normally open switch being closed in response to actuation of said detectors and restored only by the reduction of a voltage between said conductors below a specific restoration voltage, said central unit including a voltage source for supplying an operating voltage to said detectors, first means responding to closure of said normally open switch to produce an output voltage and reducing said operating voltage below said restoration voltage, second means for accumulating said output voltage of said first means, third means for producing an alarm in response to a predetermined magnitude of said accumulated voltage of said second means.

2. A fire detecting system according to claim 1 wherein said first means includes a multivibrator having stable and unstable states and initiating operation in response to closure of said switch, and a circuit connection for supplying said operating voltage to said detectors through a path in said multivibrator which is conductive during said stable state but nonconductive during said unstable state.