CA1071774A - Smoke sensing fire alarm - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A battery-powered fire alarm including a smoke detector, a controllable horn circuit and a battery monitoring circuit. The smoke detector employs a pair of complementary field-effect transistor switches respectively connected to an ionization chamber and a potentiometer of a Wheatstone bridge circuit connected across the battery. Unlike known circuits of this type, the field-effect transistors are biased off to minimize standby power consumption and are connected such that the threshold voltages thereof are offsetting to minimize supply voltage sensitivity of the detector. When the voltage from the ionization chamber assumes a value approximately equal to the potentiometer voltage, both field-effect transistors turn on to energize an alarm circuit to sound an alarm. Hysteresis circuitry is provided to ensure that the complementary switches, once turned on, will not turn off and thereby terminate the alarm until after the alarm condition has terminated. The battery monitoring circuit employs a pair of complementary field-effect transistors connected with each other to establish a reference voltage to which the battery voltage is compared. An advantage over prior battery monitoring circuits of this type is that the reference voltage is a function of both the threshold characteristics of the field-effect transistors and the magnitude of a preselected control voltage. Both field-effect transistors remain off until a low battery voltage condition is sensed to minimize standby power consumption.

Description

107~7~

SMOKE SENSING FIRE ALARM

This application is a division of Canadian Serial No. 247,489, filed March 9, 1976.

BACKGROUND OF THE INVENTION
Smoke-sensing fire alarms which sense the change in impedance of an ionization chamber when smoke is introduced thereto are well known. Typically, an open ionization chamber -107~ 77~

is connected in a Yoltage diyidex circuit across a power source and a change in impedance is reflected in a voltage change thereacross. This sensing voltage is monitored by a detection circuit and when it exceeds a preselected alarm level, the detection circuit energizes a suitable alarm circuit.
In self-contained~ i.e., battery-powered, fire alarms of this type, it is known to provide a battery monitoring circuit which will cause a low battery signal to be generated when the battery has been depleted beneath a level at which successful operation of the alarm circuit is assured.
Known detection circuits have suffered from one or more disadvantageous characteristics. For example, in U.S.
Patent 3,688,119 of Kobayashi, issued August 29, 1972, to Nittan Company, Limited, a sensing voltage developed between an open and a closed ionization chamber connected in series across a power supply is applied through an impedance matching field-effect transistor amplifier across a Zener diode. An alarm is sounded when the sensing voltage exceeds the break-over voltage of the Zener diode. Disadvantageously, the field-effect transistor operates in its linear range, and a constant current drain is thereby imposed on the power supply.
Stricter tolerance on the Zener diode is required, for an alarm voltage is established solely by the Zener diode break-over characteristic. A further problem is that because the actual alarm voltage applied across the Zener diode is unaltered by supply voltage change while the sensing voltage does change with supply voltage, sensing level stability -requires regulation of the power supply.
In U.S. Patent 3,714,641 of Scheidweiler, issued January 30, 1973, to Cerberus AG, an alarm circuit is shown 1~7i~774 in ~hich the cuxrent d~a~n ~x~ble~ ~s oYerc~me ky employing a normally~off f~eld-ef~ect trans~t~r which turns on only when a sensing voltage developed across an ionization chamber exceeds its threshold voltage. However, the threshold voltage cannot be adjusted and th~s again requires strict selection of the field-effect transistors with respect to their threshold voltage characteristics, and means for varying sensitivity are lacking. For the same reasons as in the alarm circuit of Kobayashi noted above, the detection circuit requires a stable or regulated supply voltage for successful operation.
Other circuits wh~ch enable use of active elements in the detection circuits thereof of widely varying character-istics, or which are adjustable with respect to sensitivity, lack supply voltage stability and consume an unacceptable amount of standby power.
Known battery monitoring circuits also suffer from the problem that they substantially hasten the depletion of the battery which they monitor. This is due to the fact that most monitoring circuits compare the battery voltage to a reference voltage established by a reverse-biased or forward-biased PN junction which requires a significant amount of current. However, battery monitoring circuits are shown in an article entitled "The Lambda Diode: A Versatile Negative-Resistance Device" by Gota Kano et al, appearing at pages 105-109 in the June 26, 1975, issue of Electronics, which over-come the standby power drain problem. The circuits shown therein employ two junction field-effect transistors connected to form what is termed a lambda diode. The field-effect transistors of the lambda diode both remain off until the battery voltage imposed thereacross decreases below the sum :`
,.;

t of the respective pinch-off voltages thereof. Unfortunately, the reference voltage is thereby fixed, and may not be adjusted such that, again, strict tolerances on the characteristics of the device are required, which increases the cost of manufacturing a monitoring circuit employing the lambda diode.
It is desirable that once the alarm is sounded, it continues to sound until after the condition causing the alarm has terminated. The alarm condition is often represented by an electrical detection signal generated by the circuit exceeding a preselected detection level. One way of ensuring that the alarm remains on until after termination of the alarm condition is to require manual reset or turn-off of the alarm. This, however, may result in unnecessary alarm and depletion of the battery powering the unit due to relative inaccessibility of the alarm for manual reset. On the other hand, if the alarm is designed to automatically turn off at the same preselected detection level of the electrical detection signal which causes the alarm to turn on, electrical or physical transients may cause the alarm to turn off even though the alarm condition has not ended.
SUMMARY OF THE INVENTION
.. . . .
An important object of the present invention is to provide a detection circuit which is substantially independent of supply voltage fluctuations. Another object is to provide ~i such a detection circuit which has a standby power drain less than that of known fire alarms.
Another important objective of the present invention is to provide a battery monitoring circuit of general application and of particular use with such a detection circuit ~hich con-sumes virtually no power during standby, but which can be adjustedto sense a low battery condition at different voltage levels.
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An illustrative embodiment constructed to achieve these objectives employs a pair of complementary field-effect transis-tors connected across the output of a ~heatstone bridge circuit to compare a sensing voltage at an ionization chamber in one leg thereof with a preselected alarm voltage established by a poten-tiometer in another leg of the bridge. Both field-effect transistors remain off until the sensing voltage at the ionization chamber is substantially equal to the preselected alarm voltage.
Virtually zero power drain is imposed on the battery during standby while supply voltage independence is achieved. The ; field-effect transistors are connected such that the respective gate-to-source threshold voltages thereof offset one another so that they respond to a substantially zero voltage difference.
This occurs ~hen the impedance of the ionization chamber element assumes the alarm condition regardless of changes in supply volt-; age. When an alarm condition is sensed, both field-effect transistor switches turn on to energize an audible alarm circ-lit.
The output of the detection circuit is connected with its input - through one leg of the Wheatstone bridge to provide the switch - 20 with a hysteresis characteristic tending to keep the field-effect transistors on even after the sensing voltage has returned to a nonalarm condition. A circuit for monitoring the voltage source includes a pair of complementary voltage responsive amplifiers connected in series with one another across the voltage source and each having a turn-on voltage characteristic, means for developing a preselected reference voltage and means for applying the reference voltage to a control input of one of the amplifiers whereby the amplifier remains in a nonconductive state until the battery voltage applied across the semiconductor thereof exceeds the sum of the respective turn-on voltage and the preselected reference voltage. When the amplifier assumes - a conductive state, the alarm is sounded.
~6 ~ _ 5 _ Another important object of the present invention is to provide a detection circuit with a hysteresis characteristic to ensure that once the alarm is actuated in response to an ; alarm condition, it will not be turned off prematurely in response to physical or electrical transients. In the preferred embodiment of the detection circuit in which an alarm is caused to be sounded when a sensing voltage decreases below a preselected alarm voltage and causes the alarm to be terminated when the sensing voltage increases above the preselected alarm voltage, the desired hysteresis circuit comprises means responsive to the output signal for developing a feedback signal, and means connected with one of the alarm reference signal establishing means and the sensing signal generating means and responsive to the feedback signal for altering one of the sensing signal and the alarm reference signal to establish a hysteresis characteristic for the detect-ion circuit, with the detection circuit terminating generation of the output signal at a level of the phenomenon being sensed less than the level at which the output signal is initially generated.
Further features and advantages will be made apparent and the foregoing features and advantages will be described in more detail in the description of the preferred embodiment.

: ., ` BRIEF DESCRIPTION OF THE DRAWING
The following description of the preferred embodiment is given with reference to the drawing which is a schematic of one smoke-sensing fire alarm constructed in accordance .. . .
with the principles of the invention.

~, DESCRIPTION OF THE PREFERRED EMBODIMENT
~ 30 Referring to the drawing, one embodiment of the ; smoke-detecting fire alarm of the present invention is seen to include a detection circuit 10, a controllable horn circuit 12 for providing an audible alarm, a battery 14 providing ~ A
- 5.1 ---` 1071774 a power supply voltage Yb~ and a battery~monitoring circuit 16. The detection circu~t 10, upon sensing a fire alarm condition, causes horn circui~t 12 to generate a continuous ~i;
audible alarm. The battery monitoring circuit 16, upon sensing a fire alarm condition, causes horn circuit 12 to generate a continuous audible alarm. The battery monitoring circuit 16, upon sensing a selected low voltage condition of battery 14, causes the horn circuit 12 to generate an inter-,, .
mittent audible signal.
The detection circuit employs a pair of conventional ionization chambers 18 and 20 connected in series across battery 14 through a resistor 22. Ionization chambers 18 and 20 thus define a voltage divider which produces a sensing voltage Vc at a junction 24 between chambers 18 and 20 which is a function of the relative impedance values thereof. Cham-ber 20 is closed while chamber 18 is open to ambient air to permit the entry therein of smoke and other gases from a fire.
The entry of smoke into the open chamber 18 causes its impedance to increase while the impedance of the closed chamber 20 is unaffected. The positive terminal 25 of battery 14 is connected to chamber 18 and the negative ter-minal 27 is connected to chamber 20. Consequently, the fire-induced increase in impedance of open chamber 18 causes the sensing voltage Vc to decrease.
The chambers 18 and 20 are matched with respect to their other characteristics, such as their temperature coefficient, so that other changes in the enVirGnment affecting the chambers will produce offsetting changes in the chambers and will not alter the sensing voltage Vc. The impedances of both chambers are on the order of hundreds of thousands of 1~)7177~

~ megohms so that l~ttle dPa~n i~ ~mposed on the battery 14 ; thereBy. While d~fferent relatl~ve l~mpedance values for the ionization cha~bers will work, relative values which produce, in the absence of smoke, a sens~ng voltage Vc equal to 0.6 times the battery voltage Vb have been found suitable.
The sensing voltage Vc is compared by a voltage-responsive switching circuit 26 w~th a preselected alarm voltage V developed at the wiper blade output 28 of a potentiometer 30. When the sensing voltage Vc decreases to 10 a value substantially equal to the alarm voltage Va, the switching circuit energizes the horn circuit 12 to sound a continuous alarm. -~
Potentiometer 30 is connected in series with two fixed resistors 32 and 34 across battery 14 to form a variable voltage divider 36. The voltage divider has a resistance in the megohm range to minimize drain on the battery 14. The variable voltage divider 36, taken with the voltage divider formed by ionization chambers 18 and 20, defines a Wheatstone bridge circuit. Consequently, changes in the sensing voltage Vc due to fluctuations in battery voltage Vb are offset by like changes effected in the alarm voltage Va. This minimizes sensitivity of the switch circuit 26 to fluctuations of battery voltage Vb. The fact that the voltage-controlled switch 26 turns on when the bridge is approximately balanced further minimizes supply voltage sensitivity and imposes stability.
The wiper blade output 28 is positioned to establish a value for alarm voltage Va less than that of the quiescent value of the sensing voltage Vc. By varying the position of the wiper blade output 28, the sensitivity may be selectively varied. A value of 0.5 Yb for the alarm voltage Va has been found suitable when the quiescent value of the sensing voltage V is 0.6 V~ for a battery voltage of 9 volts.
. Switching circu~t 26 compr~ses an N-channel silicon junction field-effect transistor or J-FET 42 and a P-channel enhancement-type silicon MOS field-effect transistor or MOS
FET 44. MOS FET 44 has its gate 46 connected to junction 24.
The output of switch 26 is taken from the drain 48 of MOS
:~ FET 44 and connected to gate 51 of an SCR 53 of controllable horn circuit 12. The substrate 50 is connected to the positive battery terminal 25 through current limiting resistor 22. The source terminal 52 of MOS FET 44 is connected to the source terminal 54 of J-FET 42. J-FET 42 is in a source follower configuration with its gate 56 connected to the wiper output 28 of variable voltage divider 36 and its drain 58 connected to the positive battery terminal 25.
In keeping with an important aspect of the present invention, both MOS FET 44 and J-FET 42 are biased off to mini-mize current drain on battery 14, and turn on only when the sensing voltage Vc decreases to a value approximately equal to the alarm voltage Va. When this occurs, both MOS FET 44 and J-FET 42 turn on, and drain current from MOS FET 44 is applied to the gate 51 of SCR 53 which turns on in response thereto.
The MOS FET 44 functions both as a high impedance buffer between the SCR 53 and the sensing circuit of ioniza-tion chambers 18 and 20 and as a threshold switching device.
The MOS FET 44 turns on when the gate-to-source threshold voltage is exceeded. However, unlike known circuits, the sensing voltage at which this occurs may be selectively varied .:
. ~, through control of wiper blade output 28. The position o~ `
wiper blade output 2S establishes the voltage at source 52 and thereby the magnitude of the sensing voltage Vc at which the ~hreshold voltage of MOS FET 44 is exceeded. Consequently, '~ MOS FETS with differing threshold characteristics may be used , for alarms sounding at the same voltage and the cost of ~`~ manufacturing reduced.
The high input and low output impedance of the J-FET 42 provides impedance matching between the voltage .: , divider 36 and the MOS FET 44. In addition, the changes in -- the gate-to-source voltage or Vgs of the MOS FET 44 due to temperature changes is offset by changes of opposite sense in the Vgs of the J-FET and vice versa.
The complementary arrangement of MOS FET 44 and J-FET 42 also maximizes supply voltage independence of the detection circuit. This is because both MOS FET 44 and J-FET 42 turn on when the bridge is approximately balanced and the point of balance is established solely by the relative ~-impedance values of the legs of the bridge and is completely independent of the supply voltage.
In keeping with another aspect of the invention, the drain 48 of MOS FET 44 is connected with junction 49 of voltage divider 36 to provide hysteresis to switching cir-cuit 26. When MOS FET 44 and J-FET 42 turn on, a relatively low impedance shunt is applied across resistors 30 and 32 such that the alarm voltage Va at wiper output 28 increases slightly above its value when MOS FET 44 and J-FET 42 are off. Thus, the sensing voltage Vc at junction 24 must in-crease to a value greater than the value at which MOS FET 44 and J-FET 42 were turned on before they will turn off.

As stated, when drain current is applied to the gate 51 of SCR 53, SCR 53 turns on to energize the horn cir-cuit 12. A filter capacitor 70 is connected between ~he gate~51 of SCR 53 and its cathode and resistor 34 provides ` suitable gate bias for SCR 53. ~orn circuit 12 is conven-tional and includes a relay coil 60 connected in series ; between the anode of SCR 53 and a pair of relay contacts 62 operated thereby. Gne of contacts 62 is connected with a horn diaphragm (not shown) which vibrates to generate the audible alarm. The horn circuit 12 also includes a series connection of a resistor 64 and capacitor 66 connected across coil 60 and a starting capacitor 68 connected across battery 14.
SCR 53 turns off as soon as contacts 62 open after gate drive to SCR 53 has been removed. Gate drive is removed through closure of normally open switch 72 connected bet~een the gate 51 and the cathode of SCR 53. MOS FET 44 and J-FET 42 will also turn off and remove gate drive when the impedance of chamber 18 returns to alevel sufficiently low to decrease the voltages thereacross below pinch-off.
A test circuit is provided by a normally open switch 74 connected through a resistor 76 between negative battery terminal 27 and the junction between resistor 22 and chamber 18. Closure of switch 74 lowers the sensing voltage Vc to a level less than the alarm voltage to simulate an alarm condition.
The battery monitoring circuit 16 includes a con-ventional relaxation oscillator including a programmable unijunction transistor or PUT 80 which commences oscillating when the voltage across battery 14 decreases below a pre-selected value established by circuitry associated with a s 1071774 negative resistance switch circuit 96. PUT 80 has its anode connected to a junction between a timing capacitor 82 and resistor 84 connected across battery 14. The gate of PUT ~0 is connected through a resistor 86 to the positive said of battery 14 and is also connected through a resistor 88 to switch circuit 96. The output of the oscillator is taken off the cathode of PUT 80 and connected through a lead 94 to SCR gate 51. During oscillation, capacitor 82 is periodi-cally charged through resistor 84 and discharged through PUT 80 into gate 51 to turn on SCR 53 and momentarily energize the coil 60. The reduction of voltage across capacitor 82 at the end of the discharge cycle causes PUT 80 to turn off at the end of discharge.
The value of capacitor 82 is selected so that the coil is momentarily energized at freq~encies of approximately once per minute. The battery voltage level at which the relaxa-tion oscillator is enabled to oscillate is selected so that sufficient power is available to operate the oscillator for at least seven days.
An advantageous feature of the present invention is that if the battery should become disconnected, the starting capacitor will provide enough power to energize the horn circuit 12 for one cycle of the resonant oscillator when a low battery condition is sensed by circuit 96. Accordingly, both no battery and low battery indications are provided.
This feature can, of course, also be advantageously employed with a circuit powered by A.C.
The switch circuit 96 is connected between the nega-tive side of battery 14 and the junction between resistor 88 and capacitor 92. The oscillator circuit is enabled to oscillate only when switch circuit 96 assumes a conductive state. The switch circuit 96 comprises a pair of complementary, depletion mode, field-effect transistors: an N-channel J-FET 98 and a P-channel J-FET 100. The drain-source circuit of the J-FETS 98 and 100 are connected in series with each other between the negative battery terminal 27 and the gate input of the relaxation oscillator. The gate 102 of J-FET 100 is connected to the junction between resistor 88 and the drain 103 of J-FET 98. The gate 104 of J-FET 98 receives a control voltage VL at a wiper output 106 of a potentiometer 108 connected across battery 14 through a fixed resistor 110. Gate 104 of J-FET 98 is thus connected to a potential higher than that applied to its source terminal.
Both J-FETS 98 and 100 remain off until the voltage thereacross, i.e., the battery voltage Vb, decreases to a value less than a preselected low battery voltage VL estab-lished b~ the variable control voltage VL at the wiper blade output 106 and the threshold voltages of J-FETS 98 and 100.
This low battery voltage is approximately equal to the sum of the threshold voltages of J-FETS 98 and 100 and the control voltage VL. ~ntil this preselected low battery voltage is reached, the only current through circuit 96 is leakage cur-rent in the nanno ampere range. ~Jhen the battery voltage Vb across switch 96 decreases below the preselected low battery voltage VL, the current therethrough increases rapidly witl~
small decreases in the voltage thereacross until a peak cur-rent having a corresponding peak voltage is reached. For ' values of voltage less than the peak voltage, the switch circuit has a positive resistance current and decreases in voltage result in a decrease of current.
The switch 96 functions in a manner similar to that of a lambda diode, as described in the aforementioned article entitled "The Lambda Diode; A yer~atile Negati~ve-Resistance Device," and reference ma~ be had thereto for a schematic illustration of the current-voltage characteristic curve if desired. In the lambda diode, however, the voltage Vv corresponding to the low battery voltage of circuit 96 is solely dictated by the threshold characteristic of the field-effect transistors. The versatility of the switching circuit 96, ~n which the low battery voltage Vv is a function of the position of wiper blade 106, is not achieved.
When switchin~ circuit 96 turns on, the voltage thereacross drops to a level beneath its peak voltage. A
capacitor 92 connected between the anode of SCR 53 and the junction of the drain of J-FET 98 with resistor 88 provides a positive voltage feedback pulse from SCR 53 when it turns off to assist in resetting switch circuit 96 to its condition in which the voltage across switch circuit 96 is greater than the peak voltage.
The particular operating characteristics of a cir-cuit constructed in accordance with the present invention is of course dependent upon the particular characteristics and values of the various components which are used. Thus, while other components could be successfully employed, a circuit !, built in accordance with the schematic of the drawing and using the below-identified chart elements has been found to operate in a suitable manner:

Trade Ref.No. Description Characteristic Designation 14 battery Alkaline 9v 18 ionization chamber 300x10+15 ohms 20 ionization chamber 300x10+15 ohms 22 resistor 100 Kohm Trade Ref.No. Description Characteristic Designation potentiometer 1 Mohm 32 fixed resistor 1 Mohm 34 fixed resistor 5.1Kohm 42 J-FET Siliconix Inc.
~211 44 MOS FET General Instrument . Xnc. - 3N163 53 SCR General ~lectric : 60 coil . 9 volt Kobishi 64 resistor 100 ohms 66 capacitor .1 microfarad . ..
68 starting.capacitor 330 microfarad capacitor .. 01 microfarad 76 resistor 470Xohm 78 filter capacitor .01 microfarad .
PUT . N ippon Electric . N13T2 82 . timing capacitor 15 microfarad 84 resistor 4Mohm 86 resistor 100~'ohm 88 resistor 220Kohm 92 capacitor .01 microfarad 98 J-FET . 8 1 conix Inc.

100 J-FET - S liconix Inc.
., .
108 potentiometer lMohm 110 fixed resistor lMohm Because of the various advantageous features of the presen.t invention set forth above, the total battery current .

in standby is less than 10 microamperes and the circuit will successfully operate for one year on a single battery of the above-identified t~pe before a low battery indication is provided.

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Claims (7)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a detection circuit having means for generating a sensing signal which varies in accordance with a phenomenon being sensed, means for establishing an alarm reference signal, and means for generating an output signal in response to the sensing signal exceeding the alarm reference signal in one sense and terminating the output signal in response to the sensing signal exceeding the alarm reference signal in an opposite sense, a hysteresis circuit, comprising:
means responsive to the output signal for developing a feedback signal; and means connected with one of the alarm reference signal establishing means and the sensing signal generating means and responsive to the feedback signal for altering one of said sensing signal and said alarm reference signal to establish a hysteresis characteristic for said detection circuit, said detection circuit terminating generation of said output signal at a level of the phenomenon being sensed less than the level at which said output signal is initially generated.

2. The hysteresis circuit of claim 1, in which said phenomenon being sensed is the products of combustion.

3. The hysteresis circuit of claim 1, in which said altering means comprises means for changing the magnitude of the alarm reference signal in response to generation of said output signal.

4. The hysteresis circuit of claim 3, in which said magnitude changing means varies the magnitude of the alarm reference signal in said opposite sense.

5. The hysteresis circuit of claim 4, in which said magnitude changing means causes the magnitude of the alarm reference signal to increase to a value greater than its value when the output signal is generated in response to phenomenon detection.

6. The hysteresis circuit of claim 3, in which said alarm reference signal establishing means comprises a voltage divider having at least two resistors connected with one another at a junction, and said altering means includes means for connecting a relatively low impedance shunt across one of said resistors in response to generation of the output signal.

7. The hysteresis circuit of claim 6, in which said connecting means comprises means for connecting the output signal to said junction.