Alarm Circuit

Abstract

An alarm system to detect combustion products such as smoke, gas and the like, has one or more alarm signal sending units and an alarm signal receiving unit interconnected by wiring to the sending units. Each alarm signal sending unit has a sensing or response element to detect the presence of vapors or smoke and the like and provide a signal; the sending unit also includes an amplifier non-continuously brought to its current-carrying state for amplifying an output signal of the response elements responding to a change of the surrounding physical condition. The alarm signal sending units are suitable for being connected in large numbers in parallel with a common alarm signal receiver unit, because the amplifier in each sending unit has a high impedance and dissipates low power. A smoke-sensing element of the ion type and necessitating no repeater is expediently included in the alarm signal sending device.

Parent Case Text

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation-in-part of my prior U.S. application Ser. No. 887,485 filed on Dec. 23, 1969, and now abandoned.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an alarm system for detecting smoke, vapor and the like, and more particularly to an alarm transmitter provided with an amplifier for intermittently amplifying a weak input signal given by a sensing element responsive to the presence of smoke and the like in a surrounding ambient medium.

2. Description of the Prior Art

In general, wired alarm systems, for example, fire alarm systems consist of one or more signal sending units a common signal-receiving unit and suitable wiring to interconnect these units. The receiver unit has a power source and a receiving member of alarm signals, the latter being connected with the power source in series. There are two kinds of signal sending units generally used, one being a manual type which emits a signal by manual operation and the other is an automatic type which emits a signal automatically in response to a change of the surrounding physical state. In the automatic type are included such signal sending units that use a semiconductor varying its resistance in response to the change of surrounding physical state, and those that use a sensing element having an ionization chamber changing its resistance in response to said change of surrounding physical state. The semiconductor and ionization chamber devices respond sensitively to said change, but however, their outputs are very weak. If the alarm system is so constructed that the weak signal is transmitted through alarm lines as it is, distinction of the weak signal from a disturbance or a spurious signal will become difficult, and disturbances do occur due to inductive interferences to which the alarm lines are apt to be subjected and to any slight defect of insulation which might possibly exist or develop in the lines. This makes it difficult to ensure an accurate and reliable operation of the receiver unit.

In such a case, it is expedient to amplify the weak output signal by a suitable amplifier means. However, the amplifiers in question have a small impedance, i.e., impedance seen from the source. If an amplifier means having a small impedance is connected to the transmitter, the resultant impedance of the signal sending or transmitting unit itself will naturally reduce. As it is customary to connect many signal transmitting units to a common receiving unit, the resulting impedance of transmitting units seen from the receiving unit in turn becomes very small. As a result, the current supplied from the source in the receiver becomes large, resulting in consumption of a large amount of power. In addition to the above, many disadvantages are brought about by the provision of an amplifier having a small impedance. For example, in the wired alarm circuit, discontinuity of the circuit is a serious problem; therefore, in order to detect the discontinuity, a resistor having a known resistance is connected to the ends of the alarm lines and the condition of the circuit, i.e., whether the circuit is normal or abnormal is ascertained by reading the resistance of the resistor by a suitable instrument upon circuit testing; however, when many signal transmitting units having a small impedance are connected in parallel, the detection of discontinuity becomes difficult as it is impossible to distinguish and isolate the impedance of the signal transmitting unit from the resistance of resistor. Moreover, when a great number of such signal transmitting units are connected in parallel, heavy current is drawn from the receiver and therefore there is a fear of causing malfunction of the alarm signal receiving means because of the heavy current. Thus, it will be required to limit the number of transmitters to be connected.

In order to solve these problems, the following means are considered to be effective.

1. making the impedance of the signal transmitting unit high,

2. providing a separate power source for the amplifying means. However, if a resistor having a high resistance is merely inserted in series into the signal transmitting unit in order to make the impedance of the unit high, it will result in the amplifying means not being supplied with enough power to operate. If, however, there is a suitable power source in the vicinity of the transmitter, the problem can be overcome, but such a power source will not generally be available and therefore it will be necessary to wire power supply lines from the power source in the receiver in addition to wiring the signal lines. This increases the cost of installation of the lines and makes the maintenance of the system involved and difficult.

Furthermore, it is desirable and necessary that alarm system must not respond to any spurious signals. Response elements responding to the change of physical state have, however, a tendency to respond also to physical states other than that to be supervised. Further, alarm systems, for example fire alarm systems, are generally provided with a great number of alarm signal transmitting units arranged in different spots, and it is therefore desired to fit up the alarm transmitting units simply and quickly.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide a novel alarm system having an improved alarm transmitter free from the above stated disadvantages.

Another object of the invention is to provide an alarm system which does not respond to any spurious signal.

It is a further object of the invention to provide an alarm system having signal transmitting units connectable to alarm lines without the necessity of paying attention to the polarity thereof, thereby enabling quick installation.

According to the invention, an amplifier non-continuously brought to the current-carrying state is used as an amplifier for the alarm transmitter. This amplifier consists of either an amplifier cyclically rendered operable by being cyclically energized only during a very short period of time, or an amplifier which is biased to "off" state though it is always energized and which is brought to an "on" state only when the response element responding to the change of surrounding physical staTe produces an output. In either case, there will not exist any current constantly flowing through the amplifier, and the current will flow through the amplifier only for a short period of time or only when the response element produces the output. The mean value of the current dissipated by the amplifier will therefore be rendered every small.

In fire alarm systems, the use of a smoke sensing element of the ion type responding to combustion products such as smoke or gas produced in the first stage of a fire makes it possible to detect fire at an early stage thereof. However, this kind of smoke sensor cannot be fitted readily within the ordinary alarm receiver and requires the addition of a repeater including a source with a voltage stabilizer because it uses an amplifier through which a current flows constantly and because the sensitivity of the sensing element varies according to the voltage applied across the element. This results in increased cost for installation and troublesome wiring work.

Accordingly, in order to obviate these disadvantages, the invention provides an alarm transmitter having an ion type smoke sensing element connectable directly to a signal transmitting unit without the necessity of any repeater.

According to the invention, an alarm system having an alarm transmitting unit, hardly responding to any spurious signal is provided by utilizing a signal amplifier non-continuously brought to the operable state.

Also, an alarm transmitting unit according to the invention is connectable independent of the polarity and this is rendered possible by the use of a diode bridge circuit.

Furthermore, according to the invention, an alarm transmitter having an ion type smoke sensor is provided making it unnecessary to use any repeater and this is rendered possible by utilizing an amplifier that is brought non-continuously to the current-carrying state and by utilizing a zener diode which limits voltage to a predetermined value.

Other objects, features and advantages of the invention will become apparent from the following description to be read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electric circuit diagram illustrating an embodiment of the invention;

FIG. 2 is a diagram of wave form of the output pulse of a switching element used in the circuit of FIG. 1;

FIG. 3 is an electrical circuit diagram illustrating another embodiment of the invention;

FIG. 4 is a circuit diagram of a diode bridge which may be used with an a.c. power source in the circuits of FIGS. 1 and 2; and

FIG. 5 is an electric circuit diagram illustrating still another embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, a receiver unit R includes therein a source E and an alarm signal receiving means, for example a relay L. One end of one line l.sub.1 of a pair of lines l.sub.2 for power supply and signal transmission is connected to one terminal of the source E, while one end of the other line l.sub.2 is connected to one terminal of the signal receiver means L. The other terminal of the receiver means L is connected to the other terminal of the source E, the other ends of lines l.sub.1 and l.sub.2 being connected to a transmitter unit P placed remote from the receiving unit R. The transmitter unit P comprises a switching element for short-circuiting the pair of alarm lines l.sub.1 and l.sub.2, for example a thyristor S, a RC circuit consisting of a resistor r and a capacitor C, a switching element FF and an amplifier A. The capacitor C is connected via resistor r between lines l.sub.1 and l.sub.2 to be charged by the current from source E limited by means of resistor r. The switching element FF has a high impedance and consists of for example a blocking oscilator circuit, a free-running multivibrator, or the like. The element FF will be "on" during a period t.sub.1, as shown in FIG. 2, and will be "off" during a period t.sub.2, the ratio t.sub.1 to t.sub.2 or the mark-space ratio being adequately changed for the purpose concerned. Preferably, the period t.sub.1 is chosen to 5 .times. 10.sup..sup.-5 seconds and the period t.sub.2 to 2 - 4 seconds, t.sub.1 being much smaller than t.sub.2. The amplifier A consists of for example a field-effect transistor amplifier, to the signal input terminals P.sub.3 and P.sub.4 of which are connected a response element N responding to a change of the surrounding physical condition. The element N responds for example to the changes of temperature, moisture, pressure, light, wind, smoke, weight and the like and emits an output signal.

In operation, a small current constantly flows from the source in receiver unit R through the pair of lines l.sub.1 and l.sub.2 and resistor r to capacitor C, the current being however extremely small in magnitude because it is limited by resistor r having a high value of resistance. As a result, the effective impedance of transmitter unit P seen from receiving unit R becomes very high. The capacitor is charged by the weak current, the voltage across the capacitor being increased gradually. When the voltage of capacitor C reaches a predetermined level, the switching element FF turns on impressing the voltage of the capacitor to amplifier A. As a result, the amplifier becomes operable and, if the response element N is emitting an output in response to the change of surrounding physical state, it will amplify the output and produce an output signal to turn short-circuiting element S on. Thus, the short-circuiting element S will short circuit the pair of lines l.sub.1 and l.sub.2, and a heavy current unrestricted by resistor r will flow from source E through lines l.sub.1 and l.sub.2 and receiving means L, actuating the latter to ring an alarm. The switching element FF turns off after the expiry of period t.sub.1 to halt the discharge of capacitor C through amplifier A. The output signal of amplifier A will therefore disappear, but the signalling of the alarm will not be interrupted since the short-circuiting element which in this case is thyristor S has self-holding ability. When no signal is emitted by response element N in response to the change of surrounding physical state, amplifier A will not of course emit the output signal even if switching element FF enters on-period t.sub.1, thus no alarm will be sounded. In this case, amplifier A will merely allow capacitor C to discharge at a rate corresponding to the conductivity of amplifier A determined by its bias value, and capacitor C will merely be re-charged when switching element FF enters the off-period t.sub.2.

According to the system above described, the current constantly supplied from the receiver unit to the transmitter unit is restricted by resistor r; thus, all of the aforementioned disadvantages which are caused by the otherwise constantly supplied heavy current such as in prior art apparatus, are eliminated. In addition, as the transmitter unit is provided with capacitor C as a charge storing means, amplifier A is satisfactorily operable with the help of the charge stored in that capacitor. Moreover, though in this case amplifier A becomes operable only for the short period of time t.sub.1 and inoperable for the long period of time t.sub.2, the operation of amplifier A for short period of time is satisfactory for the generation of an alarm; and the fact that the amplifier A is in an inoperable state for a long period of time t.sub.2 provides insensitivity of amplifier A for any spurious signal which would arise in that period. Thus, this alarm system is assured to be insensible to most of the spurious signals.

In FIG. 3 is shown another embodiment of the invention. Corresponding parts in FIG. 3 are designated by same reference numerals as in FIG. 1. The circuitry of FIG. 3 is different from the circuitry of FIG. 1 in that the former lacks current limiting means r and charge storing means C. The operation of the circuit of FIG. 3 is almost similar to that of the circuit of FIG. 1. Thus, switching element FF becomes "on" for period t.sub.1 and "off" for period t.sub.2 as shown in FIG. 2, connecting amplifier A between lines l.sub.1 and l.sub.2 for period t.sub.1 to bring the amplifier to the operable state. When the response element N is not detecting any change of the surrounding physical condition, the current flowing for period t.sub.1 from source E through switching element FF and amplifier A is so small in intensity that it cannot actuate receiving means L (the current is adjusted in that manner). During the period t.sub.2, the current from source E is directed to pass through a portion of the components of switching element FF and therefore is very weak in intensity. If response element N detects a change, in the ambient condition, amplifier A will emit an output signal when connected between lines l.sub.1 and l.sub.2 for period t.sub.1 and as a result an alarm will be sounded in the same manner as in FIG. 1.

A diode bridge circuit illustrated in FIG. 4 can be connected to the power input side of transmitter unit P. In order to assure normal functioning of unit P, it is normally necessary to take care of the polarity of voltage applied to the pair of power input terminals. For example, in the circuits of FIGS. 1 and 3, transmitter unit P should be connected to lines l.sub.1 and l.sub.2 so that a positive electric potential is applied to terminal P.sub.1 while a negative potential to terminal P.sub.2. This is a troublesome job in actual installation work. However, if a diode bridge circuit as in FIG. 4 is provided, it will be unnecessary to pay attention to the polarity of the lines during installation work because the bridge circuit always corrects the polarity so that the positive voltage be applied to the anode of thyristor S and the negative voltage to the cathode thereof irrespective of the polarity of voltage applied to terminals P.sub.1 and P.sub.2. Moreover, if the diode bridge circuit is provided, source E of the receiver unit may be an A.C. source instead of a D.C. source.

FIG. 5 illustrates a further embodiment of the invention. In this embodiment, an ion type smoke detector element is used as the response element responding to the change of surrounding physical state to emit a signal in response to the presence of smoke. The ion type smoke sensing element comprises an ionization chamber having a source of radiation for ionizing the surrounding air and a pair of electrodes provided in said chamber. In the embodiment shown, two such smoke sensing elements are used, in the outer ionization chamber OC of one of which is housed a pair of electrodes OC.sub.1 and OC.sub.2 and outside air can freely flow into the chamber, while in the inner ionization chamber IC of the other is housed a second pair of electrodes IC.sub.1 and IC.sub. 2 and the outside air cannot flow into the chamber. The two sensing elements are connected between supply lines l.sub.1 and l.sub.2 in series. That is, electrode OC.sub.1 is connected to line l.sub.1, electrode OC.sub.2 to electrode IC.sub.1 and electrode IC.sub.2 to line l.sub.2, respectively. The line l.sub.1 is connected via resistor R.sub.3 to line l.sub.2 and by virtue of a zener diode ZD connected between l.sub.1 and l.sub.2, the voltage across lines l.sub.2 and l.sub.1 is limited to a fixed value determined by diode ZD, even though the voltage between lines l.sub.1 and l.sub.2 may be higher, thus supplying the smoke sensing element with a constant voltage.

Between lines l.sub.1 and l.sub.2 are further connected a transistor amplifier A, a capacitor C having same function as that in FIGS. 1 and 3 and a free-running multivibrator FMV. The multivibrator FMV is a switching element corresponding to FF in FIGS. 1 and 3, one terminal P.sub.5 of output terminals P.sub.5 and P.sub.4 of which is connected to the source electrode of a field-effect transistor F.sub.1. The gate electrode of transistor F.sub.1 is connected to junction T of electrode OC.sub.2 with electrode IC.sub.1 , the drain electrode of which being connected to an input terminal of amplifier A, i.e. to the base of transistor.

The operation of multivibrator FMV is substantially the same as that of a wellknown multivibrator and can be briefly explained as follows: The FMV comprises two field-effect transistors F.sub.2 and F.sub.3, and it is firstly assumed that F.sub.3 is "on" while F.sub.2 is "off"; then capacitor C.sub.1 is gradually charged and the gate of F.sub.3 will be biased to positive as the charging considerably progresses, rendering the voltage between the gate and the source of F.sub.3 to be lower than the cut-off valve so that F.sub.3 turns off. As a result the gate of F.sub.2 is biased to negative and F.sub.2 turns on. Capacitor C.sub.1 is therefore charged at reversed polarity against the previous one, and as the charging progresses the gate of F.sub.3 will be biased to negative and when the voltage between the gate and the source of F.sub.3 increased above the cut-off voltage the transistor F.sub.3 will turn on, biasing the gate of F.sub.2 to positive to turn F.sub.2 off. In the manner described above, F.sub.2 and F.sub.3 are alternately in turn-on and turn-off stages, generating substantially rectangular pulses at the terminals P.sub.5 and P.sub.4. The pulses are set similar to the previous example so that the on-period t.sub.1 is a few 10 micro-seconds while the off-period t.sub.2 is a few seconds.

The circuitry of FIG. 5 thus constructed will function as follows: Since the air in the ionization chamber of ion type smoke sensing element is ionized as afore-mentioned, a weak current is normally flowing through ionization chambers OC and IC connected in series, and thus the junction point T has a certain voltage obtained from dividing the voltage between lines l.sub.1 and l.sub.2 by the ratio of impedances of two sensing elements. To the source electrode of transistor F.sub.1 is supplied the output voltage of FMV during period t.sub.1, but the transistor in amplifier A will remain in its off-state as the voltage of junction T at normal condition cannot turn F.sub.1 on and no base current is applied to the transistor. In a normal condition, therefore, the current to ion type smoke sensor P (which corresponds to the transmitting units in FIGS. 1 and 3) is only a current which is limited by resistor r having a high resistance and flows through elements ZD, C, FMV, OC and IC, the current being very weak in intensity. Upon occurrence of a fire, smoke or gas will flow into outer ionization chamber OC and prevent the air in the chamber from being ionized by absorbing air ions or energy of radiation material. As a result, the path between the electrodes of outer ionization chamber OC will have a higher impedance, thereby restricting the flow of current. The potential of the junction point T therefore will drop, and transistor F.sub.1 will turn on when period t.sub.1 is commenced. The output voltage of FMV therefore causes the base current of transistor of amplifier A to flow through F.sub. 1 to turn the transistor on, and the output voltage of amplifier A turns on thyristor SCR which short-circuits lines l.sub.1 and l.sub.2. As a result, an alarm will be caused in the same manner as in the circuits of FIGS. 1 and 3.

It is to be understood that this invention is not limited to the exact construction in the embodiments shown and described, and various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

What is claimed is:

1. An alarm system for detecting smoke, vapor and physical conditions in an ambient region comprising in combination, an alarm signal sending unit, an alarm signal receiving unit, connecting means interconnecting said sending and receiving units with a power source, said alarm signal sending unit comprising: a sensing means featuring a changed impedance in response to a change in said physical conditions in the ambient region of said sensing means, said changed impedance providing an output signal; and amplifier means connected to amplify said output signal; a pulse switching means producing a continuous series of conducting intermittent pulses each pulse of said series of pulses lasting for a first interval of time which is substantially shorter than a second interval of time between successive of said pulses; means connecting said power source to said amplifier means through said pulse switching means to place said amplifier means intermittently in conductive state for short durations of time each of which equals said first interval of time; and a power switching means connected to receive an amplified output signal through said amplified means and establish flow of current through said alarm signal receiving unit to actuate an alarm, whereby the alarm system is responsive to signals from said sensing means only during said first intervals of time.

2. An alarm system as claimed in claim 1 wherein said pulse switching means comprises a switching element cyclically opening and closing.

3. An alarm system as claimed in claim 1 wherein said pulse switching means comprises a blocking oscillator, generating a pulse train output having a short "on" period and a comparatively long "off" period, and wherein said power switching means comprises a thyristor.

4. An alarm system as claimed in claim 1 wherein said pulse switching means comprises a multivibrator, generating a pulse train output having a short "on" period and a comparatively long "off" period, and wherein said power switching means comprises a thyristor.

5. An alarm system as claimed in claim 1 wherein said connecting means interconnecting said sending and receiving units with the power source comprises first and second lines, and wherein a resistor is included in series with said first line and a capacitor is connected across said first and second lines.

6. An alarm system as claimed in claim 5 which includes a Zener diode connected in parallel with said capacitor so as to supply said sensing means with a constant voltage.

7. An alarm system as claimed in claim 6 wherein said sensing means comprises two ion-type smoke detectors connected in series.

8. An alarm system as claimed in claim 7, wherein said amplifier means comprises a first transistor, and wherein a field effect transistor is connected at the base of said first transistor so as to be able to adjust the conductivity of said first transistor.

9. An alarm system as claimed in claim 8 wherein a source-electrode of said field effect transistor is connected to an output terminal of said pulse switching means and a gate electrode of said field effect transistor is connected to a junction point between said two ion-type smoke detectors connected in series.