US3733595A - Condition responsive switching circuit - Google Patents

Condition responsive switching circuit Download PDF

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US3733595A
US3733595A US3733595DA US3733595A US 3733595 A US3733595 A US 3733595A US 3733595D A US3733595D A US 3733595DA US 3733595 A US3733595 A US 3733595A
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E Benedict
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E Benedict
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    • GPHYSICS
    • G08SIGNALLING
    • G08BSIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
    • G08B17/00Fire alarms; Alarms responsive to explosion
    • G08B17/10Actuation by presence of smoke or gases automatic alarm devices for analysing flowing fluid materials by the use of optical means
    • G08B17/117Actuation by presence of smoke or gases automatic alarm devices for analysing flowing fluid materials by the use of optical means by using a detection device for specific gases, e.g. combustion products, produced by the fire

Abstract

A circuit which actuates a visible or audible alarm in response to the reception of dangerous fluids such as smoke, fumes, or gas. The circuit includes a sensor which changes internal resistance when the fluid to be detected is present in the environment surrounding the sensor. The change of the sensor actuates a switching or trigger circuit which results in the sounding of an audible alarm or the lighting of a visual alarm or both. A delay circuit which maintains the sensor circuitry in an unenergized condition until the system stabilizes after power is first applied is also provided to prevent spurious alarms.

Description

ilnited States Patent 1191 1111 3,733,55 Benedict [4 1 May 15, 1973 54] CONDITION RESPONSIVE SWITCHING 3,264,634 8/1966 Voigt ..340 310 CIRCUIT Inventor: Elmer M. Benedict, 506 State Street, Schenectady, NY. 12305 Filed: Feb. 12, 1971 Appl. No.: 117,968

US. Cl. ..340/237 R, 340/3091 Int. Cl. ..G08b 21/00 Field of Search ..340/237 R, 413, 181, 340/309.1, 310, 258 C; 315/102, 103, 360; 307/193; 328/77, 129; 317/1415 References Cited UNITED STATES PATENTS Attorney-John B. Dickman III 7 [5 7] ABSTRACT A circuit which actuates a visible or audible alarm in response to the reception of dangerous fluids such as smoke, fumes, or gas. The circuit includes a sensor which changes internal resistance when the fluid to be detected is present in the environment surrounding the sensor. The change of the sensor actuates a switching or trigger circuit which results in the sounding of an audible alarm or the lighting of a visual alarm or both. A delay circuit which maintains the sensor circuitry in an unenergized condition until the system stabilizes after power is first applied is also provided to prevent spurious alarms.

9 Claims, 3 Drawing Figures Patented May 15, 1973 3 Sheets-Sheet 1 INVENTOR. [AMER M. BEA/LZYMT Patehted May 15, 1913 5 Sheets-Sheet 5 IN V EN TOR. [Mm M. flmm/ar BACKGROUND OF THE INVENTION I Detection devices are presently used in various environments for detecting several different types of dangerous conditions. Ordinarily these devices are responsive to condition changes and therefore frequently are temperature or light sensitive. As an example, fire detection devices respond to the increase in temperature caused by the fire. As another example, devices used in refrigeration environments are actuated when the temperature rises above a desired freezing level.

Temperature and other environmental responsive devices have many useful adaptations but nevertheless suffer some disadvantages. One major disadvantage stems from the fact that not all dangerous conditions result in temperature or light changes. For example, a dangerous level of lethal gas, such as carbon monoxide, does not necessarily result in an environmental condition change which is detectable by the prior art devices. Furthermore, fire detection devices which are temperature sensitive must be physically located such that the temperature sensor is subjected to the fire. In large areas, such as warehouses, this requires the employment of many sensors in order to more probably assure that the device responds before a fire spreads into an uncontrollable conflagration.

SUMMARY OF THE INVENTION The inventive system utilizes a sensor which is sensitive to atmospheric fluids such as smoke and gasses. Because smoke and dangerous gasses spread rapidly in air their presence can be detected before the condition which caused their presence would affect a temperature sensitive device.

This is readily evident when it is realized that smoke can fill a room through a ventilator. If no operating temperature sensitive sensor is present in the room where the smoke originated, no indication would be given by the prior art system. However, the inventive system will sense the smoke and give an alarm because of the smoke.

The inventive system is also useful in places like furnace rooms and sewage treatment plants where lethal gases can exist. Because many of the lethal gases are odorless and invisible their detection is difficult. Accordingly, the inventive system is quite useful for this purpose.

The inventive system is energized by connecting it to the standard 110 VAC supply present in all modern buildings. The AC current is rectified and then a delay circuit is actuated. The delay circuit maintains the firing circuit in an unenergized condition until the remainder of the circuit stabilizes. After stabilization of the sensor the firing circuit is energized, as is a small lamp which gives an immediate indication that the system is in operation. Upon reception of a dangerous fume the sensor changes condition and unbalances the device. The firing circuit is then fired to actuate an audible and/or a visible indicator to indicate that a dangerous condition exists.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a first preferred embodiment of the invention.

FIG. 2 is a second preferred embodiment of the invention.

FIG. 3 is a third preferred embodiment of the invention.

DETAILED DESCRIPTION As shown in FIG. 1 the inventive system is actuated by plugging the primary 11 of a transformer 10 into a standard VAC supply. The entire device is protected by a fuze 12 in known manner.

Serially connected across primary 11 is a switch 13, resistor 14 and neon bulb 15. As will be more fully described hereinafter, when switch 13 closes upon stabilization of the sensor element, bulb 15 is lit to indicate that the system is operative.

The first secondary 16 of transformer 10 is connected to a diode bridge rectifier 18. Rectifier 18 includes four diodes 19 to 22 and a capacitor 23 and operates in known manner so that an AC input across one diagonal results in a DC output across the other diagonal.

A second secondary 17 of transformer 10 is connected to place a reference voltage on sensor 24. Secondary 17 has only a few turns and therefore a small voltage is placed on the sensor 24. This voltage establishes a current flow through resistors 25 and 26. These resistors serve as a voltage divider and maintain transistor Q in a stabilized state. When sensor changes in internal resistance because of the impingement of detected gases, the current through resistors 25 and 26 changes and fires transistor Q A delay circuit 27 is connected across the DC output junctions of rectifier l8. Delay circuit 27 includes two transistors Q2 and Q3. The base of Q2 is connected to the junction of capacitor 28 and resistor 29. The collector of Q2 is connected to capacitor 28 by another capacitor 31. A coil 32 is connected in parallel with capacitor 31. Coil 32 is a relay coil and current passing through the coil causes the closing of normally open relay contacts 13 and 33.

The base of transistor Q3 is connected to the emitter of Q2. The collectoremitter junction of O3 is connected from the collector of Q2 to resistor 29.

Delay circuit 27 operates in well known manner. When the detection circuit is first energized capacitor 28 charges until Q2 fires. When Q2 fires Q3 can tire and current flows through coil 32 closing relay contacts 13 and 33. The delay circuit remains in this condition until the circuit is removed from the source, at which time capacitor 28 discharges deactivating the delay circuit. Capacitor 31 smoothes the voltage on coil 32 and prevents premature closing and chatter of the relay contacts 13 and 33 which are closed by coil 32.

The base of transistor Q1 is connected to the junction of resistors 25 and 26 via a diode 34;. Q1 is therefore prevented from firing until a voltage which exceeds the back voltage of diode 34 is realized. Resistor 36 is instrumental in establishing the breakdown voltage of diode 34 across the base-emitter junction of transistor Q1. The collector of transistor O1 is connected through relay contact 33 and the parallel combination of capacitor 38 and coil 37 to the junction of diodes 20 and 22 of rectifier 18. Coil 37 is a relay coil and closes normally open relay contact 39 when energized by a current flow.

In operation energization is initiated by connecting the primary of transformer 10 to a 110 VAC supply. This places a voltage on sensor 24 as well as a DC input to delay circuit 27. Relay contacts 13 and 33 remain 33 are closed.

Bulb 15 is now lit indicating the detection device is operative. Also, because relay contact 33 is closed the detection device is armed and ready to give an alann signal when a dangerous condition is detected. However, the current through resistors 25 and 26 is insufficient to cause a voltage drop in resistor 26 which will break down diode 34. When sensor 24 changes internal resistance in response to the impingement of a dangerous gas the current in resistors 25 and 26 increases and breaks down diode 34 thereby firing Q1. When Q1 fires current flows through coil 37 closing relay 39. Coil 40 is then actuated and is used to either ring a buzzer or actuate a lighting circuit, or both if desired.

Sensor 24 is a type known in the art and several suitable sensors are available. The detection circuit is readily formed by printed circuit techniques and major portions of it can be fabricated using integrated circuit techniques.

Another preferred embodiment of the invention is shown in FIG. 2. This embodiment is also actuated by connecting it to 1 l VAC supply. An indicator light 51 is energized through a resistor 52 to indicate that the circuit is energized. The primary of a transformer 53 is energized by the l 10 VAC supply. A tap 55 is provided on the primary so that a low voltage such as to volts can be taken from the primary.

A rectifying diode 58 and a resistor 59 are connected to the upper side of the tapped portion 54 to the tap 55. Connected in parallel with resistor 59 is a capacitor 61.

Diode 58 is poled so that the negative half-cycles of the input voltage are blocked. The positive half-cycles are coupled to the anode of a silicon-controlledrectifier 62 by a line 63. The cathode of SCR 62 is connected to tap 55 through a resistor 64.

A gas bulb 66, which can be neon, connects the control electrode of SCR 62 to the junction of a resistor 68 and capacitor 69. The gas sensor 71 is connected to the R-C network through a diode 67 which is poled so that only the positive half cycles are applied to the resistor 68. Diode 67, resistor 68, capacitor 69 and SCR 62 form a delay circuit which serves to provide time for sensor 71 to stabilize when the circuit is first plugged in.

Sensor 71 is connected across the secondary 57 which is energized by the lower tapped portion 56 of the transformer primary. Current from sensor 71 flows through resistors 72 and 73 which form a voltage dividing network. The base of transistor Q4 is connected to the junction 78 of resistors 72 and 73 through a rheostat 74 and a diode 76. Transistor Q4 remains off until a voltage sufficient to break down diode 76 is present at junction 78. This voltage can be adjusted by use of arm 75 of rheostat 74.

The collector of Q4 is connected to SCR 62 through a coil 79 having a capacitor 80 connected in parallel therewith. Coil 79 is a relay coil and current flow causes the closing of the normally open contacts of relay 81. When relay 81 is closed coil 82 is energized through diode 83 and a buzzer and/or a light circuit is actuated.

The operation of the circuit is quite simple. Upon energization of the detector circuit upper portion 54 of the transformer 53 is energized. This voltage is rectified by diode 58 and applied to the anode of SCR 62. At the same time the voltage at tap 55 is applied to the cathode of SCR 62. However, SCR 62 cannot fire until a voltage is applied to the control electrode.

The voltage for the control electrode of SCR 62 comes from secondary 57 via sensor 71. Because of diode 67 only positive half-cycles are passed to capacitor 69 which charges until the voltage is sufficient to fire neon bulb 66. SCR 62 then fires and is conductive. However, current cannot flow through coil 79 because transistor Q4 is non-conductive.

The voltage on sensor 71 causes current flow through resistors 72 and 73. However, this voltage is insufficient to turn on Q4 because of diode 76 and rheostat 74. Relay contact 81 therefore remains open but the detection circuit is in an armed or detection state.

When sensor 71 is subjected to a gas which is lethal or dangerous, the sensor changes resistance and increases the current flow through resistors 72 and 73. The increase is sufficient to render diode 76 conductive and therefore transistor Q4 fires. Current then flows through coil 79 and closes relay contact 81 thereby energizing the alarm.

F IG. 3 is another preferred embodiment of the invention. This embodiment also is energized by a standard 1 10 VAC supply and includes a serial connection of a resistor 92 and a neon bulb 91 which shows that circuit is activated. The primary 94 of a transformer 93 is connected to the 1 10 VAC supply.

Transformer 93 has two secondaries 96 and 97. Secondary 97 is connected across the fume sensor 98, which detects the presence of dangerous fumes or gases.

One end of secondary 96 is connected to the tap 100 while the other end is connected to SCR 101 through a diode 99. Diode 99 is poled to pass the positive half cycles from the supply voltage to the anode of SCR 101. It should be noted that secondary 96 is center tapped so that the voltages on the two ends are out of place.

A serial connection of a diode 102, normally open switch 103 and an indicator load 104 is connected from one end of secondary 96 to ground. Indicator load 104 can be a coil which closes another switch (not shown) to actuate a light and/or ring a buzzer. An R-C network composed of resistor 106 and capacitor 107 smoothes the input to SCR 101.

A voltage dividing network composed of resistors 108 and 109 is connected to sensor 98. Current through resistors 108 and 109 changes to actuate transistor Q5, the base of which is connected to the junction 111 of resistors 108 and 109, when sensor 98 indicates the presence of a dangerous gas or fumes.

The elements 74 to 77 connected to the base of transistor Q5 are similar to the correspondingly numbered elements of FIG. 2 and operate in the same manner.

The collector-emitter junction of transistor Q5 connects the cathode of SCR 101 to ground through a coil 112. Coil 112 serves to close switch 103 when transistor Q5 becomes conductive in response to condition changes in sensor 98.

A lamp 113 is connected from the top of coil 112 to ground through a resistor 114. When transistor OS becomes conductive current can flow through coil 112 and actuate switch 113. However, when transistor Q5 is nonconductive, current flows through lamp 113 to indicate that the circuit is operative but ambient conditions are safe. By properly selecting the parameters of coil 1 l2, lamp 113 and resistor 114 a conductive condition of transistor Q5 can result in an insufficient current flow through lamp 113 to light the lamp. Hence, an unsafe condition is also indicated by lamp 113 being extinguished.

The control electrode of SCR M)! is connected to sensor 98 through a neon lamp 116, resistor M7 and diode 118. The junction of neon lamp 116 is connected to ground through a capacitor 119. Resistor 11 .7 and capacitor 119 form a timing circuit to allow sensor 98 to stabilize when the circuit is first energized. Neon lamp 1 116 is not conductive until capacitor 119 charges to the firing voltage of lamp 116. After lamp 116 is conductive the control electrode of SCR is energized and the circuit is activated.

In operation the initial energization of the circuit places a potential across sensor 98. Also, as soon as ca pacitor 119 charges a sufficient amount lamp 116 fires and a control voltage is applied to SCR 101. At the same time, a rectified voltage is applied to the anode of SCR 101 through diode 99. This set of conditions causes SCR 101 to be conductive. However, current cannot flow through coil 112 because transistor Q5 is non-conductive. Current therefore flows through lamp 113 to indicate that the system is energized and properly operating.

When fumes impinge upon sensor 98 current flow through resistors 108 and 109 increases thereby increasing the voltage onto diode 75. When the voltage is sufficient to make the diode conductive transistor Q5 also becomes conductive. With transistor Q5 conductive current flows through coil 112 thereby closing switch 103 and actuating the alarm system.

I claim:

1. A system for giving an alarm in the presence of invisible and odorless gases comprising: a transformer including a primary winding, a low voltage winding and an intermediate voltage winding, a first resistor connected to said primary winding, a first lamp connected to said first resistor and said primary winding, said first resistor and said first lamp forming a series circuit in shunt with said primary winding for indicating the ap plication of electrical power to said system, a gas detector connected to said transformer, said gas detector including a resistance means which changes its electrical resistance when subjected to a predetermined gas, a voltage divider in series with said gas detector resistance means, a control transistor including collector, emitter, and base electrodes, said emitter electrode connected to said primary winding, a second resistor connected between said base electrode and said emitter electrode, a diode connected between said voltage divider and said base electrode and poled to render said transistor conductive when the voltage developed on said voltage divider reaches a predetermined value, an alarm activating means, a first switching means for controlling current flow through said alarm activating means, a voltage rectifier connected to said transformer, a current flow responsive means connected between said voltage rectifier and said collector electrode for controlling said first switching means, a second switching means connected between said voltage rectifier and said collector electrode for completing a series circuit comprised of said second switching means, said current flow responsive means and said control transistor, and a resistive and capacitive time delay network connected to said transformer for closing said second switching means when said capacitance reaches a predetermined charge level.

2. The system of claim ll wherein said voltage rectifier includes a full wave rectifier bridge network and said resistive and capacitive time delay network is connected across said full wave rectifier bridge network.

3. The system of claim It wherein said second switching means includes: a first transistor including collector, base and emitter electrodes, said first transistor base electrode connected to said resistive and capacitive time delay network, a second transistor including collector, base and emitter electrodes, said second transistor base electrode connected to said first transistor emitter electrode, said second transistor emitter electrode connected to said full wave rectifier bridge network, said second transistor collector electrode connected to said first transistor collector electrode, a first relay including a coil and first and second sets of normally open relay contacts, said first relay coil connected between said first and second transistor collector electrode and said full wave rectifier bridge network, said first relay first set of normally open contacts interposed between said first resistor and said primary winding connection, and said first relay second set of normally open contacts connected between said current flow responsive means and said control transistor collector electrode.

41. The system of claim 3 wherein said current flow responsive means includes: a second relay including a coil connected between said first relay second set of contacts and said full wave rectifier bridge network and a set of normally open contacts connected as said first switching means.

5. The system of claim It wherein said second switching means includes: a silicon controlled rectifier including a control electrode connected to said resistive and capacitive time delay network.

6. The system of claim 5 further including an indicator lamp interposed between said control electrode connection to said resistive and capacitive time delay network.

7. The system of claim 6 wherein said current flow responsive means includes: a relay including a coil connected between said silicon controlled rectifier and said control transistor collector electrode and a set of normally open contacts connected as said first switching means.

fi. The system of claim 7 further including: a variable resistance means interposed between said diode connection to said voltage divider for controlling the sensitivity of said system.

9. The system of claim 8 wherein said first lamp and said indicator lamp are neon bulbs.

I =5 =l =l

Claims (9)

1. A system for giving an alarm in the presence of invisible and odorless gases comprising: a transformer including a primary winding, a low voltage winding and an intermediate voltage winding, a first resistor connected to said primary winding, a first lamp connected to said first resistor and said primary winding, said first resistor and said first lamp forming a series circuit in shunt with said primary winding for indicating the application of electrical power to said system, a gas detector connected to said transformer, said gas detector including a resistance means which changes its electrical resistance when subjected to a predetermined gas, a voltage divider in series with said gas detector resistance means, a control transistor including collector, emitter, and base electrodes, said emitter electrode connected to said primary winding, a second resistor connected between said base electrode and said emitter electrode, a diode connected between said voltage divider and said base electrode and poled to render said transistor conductive when the voltage developed on said voltage divider reaches a predetermined value, an alarm activating means, a first switching means for controlling current flow through said alarm activating means, a voltage rectifier connected to said transformer, a current flow responsive means connected between said voltage rectifier and said collector electrode for controlling said first switching means, a second switching means connected between said voltage rectifier and said collector electrode for completing a series circuit comprised of said second switching means, said current flow responsive means and said control transistor, and a resistive and capacitive time delay network connected to said transformer for closing said second switching means when said capacitance reaches a predetermined charge level.
2. The system of claim 1 wherein said voltage rectifier includes a full wave rectifier bridge network and said resistive and capacitive time delay network is connected across said full wave rectifier bridge network.
3. The system of claim 1 wherein said second switching means includes: a first transistor including collector, base and emitter electrodes, said first transistor base electrode connected to said resistive and capacitive time delay network, a second transistor including collector, base and emitter electrodes, said second transistor base electrode connected to said first transistor emitter electrode, said second transistor emitter electrode connected to said full wave rectifier bridge network, said second transistor collector electrode connected to said first transistor collector electrode, a first relay including a coil and first and second sets of normally open relay contacts, said first relay coil connected between said first and second transistor collector electrode and said full wave rectifier bridge network, said first relay first set of normally open contacts interposed between said first resistor and said primary winding connection, and said first relay second set of normally open contacts connected between said current flow responsive means and said control transistor collector electrode.
4. The system of claim 3 wherein said current flow responsive means includes: a second relay including a coil connected between said first relay second set of contacts and said full wave rectifier bridge network and a set of normally open contacts connected as said first switching means.
5. The system of claim 1 wherein said second switching means includes: a siLicon controlled rectifier including a control electrode connected to said resistive and capacitive time delay network.
6. The system of claim 5 further including an indicator lamp interposed between said control electrode connection to said resistive and capacitive time delay network.
7. The system of claim 6 wherein said current flow responsive means includes: a relay including a coil connected between said silicon controlled rectifier and said control transistor collector electrode and a set of normally open contacts connected as said first switching means.
8. The system of claim 7 further including: a variable resistance means interposed between said diode connection to said voltage divider for controlling the sensitivity of said system.
9. The system of claim 8 wherein said first lamp and said indicator lamp are neon bulbs.
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Cited By (13)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3860919A (en) * 1973-12-26 1975-01-14 Funck Donald E Smoke and gas detection and alarm apparatus
US3879717A (en) * 1974-06-19 1975-04-22 K F Ind Inc Portable methane monitor and alarm system
US3895367A (en) * 1973-10-10 1975-07-15 Visser Johan Willem Gas indicator with semiconductive gas sensor
US3997837A (en) * 1974-02-21 1976-12-14 Fried. Krupp Gesellschaft Mit Beschrankter Haftung Gas analysis device
US4007456A (en) * 1975-12-01 1977-02-08 Craftor Inc. Gas detecting and warning system
US4335379A (en) * 1979-09-13 1982-06-15 Martin John R Method and system for providing an audible alarm responsive to sensed conditions
US4680576A (en) * 1982-11-29 1987-07-14 Gentex Corporation Photoelectric smoke detector and alarm system
US4686518A (en) * 1983-07-29 1987-08-11 Schweitzer Edmund O Jun Fault indicator having trip inhibit circuit
US4794332A (en) * 1986-03-28 1988-12-27 Schweitzer Edmund O Jun Fault indicator having improved trip inhibit circuit
US5419358A (en) * 1993-08-02 1995-05-30 Francis Myrtil Gas monitoring system for a boiler
US5576739A (en) * 1992-06-18 1996-11-19 Phy-Con. Inc. Carbon monoxide safety system
US20080284579A1 (en) * 2007-05-15 2008-11-20 David Contreras Carbon monoxide safety system
US20110025518A1 (en) * 2009-07-30 2011-02-03 Hong Fu Jin Precision Industry (Shenzhen) Co., Ltd. Under-voltage warning circuit

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2709251A (en) * 1951-08-02 1955-05-24 Kenneth H Schmidt Audio electromagnetic capacity alarm device
US3144591A (en) * 1962-05-11 1964-08-11 Hughes Aircraft Co Time delay relay
US3264634A (en) * 1964-03-02 1966-08-02 Aseco Inc Dual actuation signal alarm device
US3315245A (en) * 1964-01-10 1967-04-18 Johnson Williams Inc Gas detection apparatus

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2709251A (en) * 1951-08-02 1955-05-24 Kenneth H Schmidt Audio electromagnetic capacity alarm device
US3144591A (en) * 1962-05-11 1964-08-11 Hughes Aircraft Co Time delay relay
US3315245A (en) * 1964-01-10 1967-04-18 Johnson Williams Inc Gas detection apparatus
US3264634A (en) * 1964-03-02 1966-08-02 Aseco Inc Dual actuation signal alarm device

Cited By (15)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3895367A (en) * 1973-10-10 1975-07-15 Visser Johan Willem Gas indicator with semiconductive gas sensor
US3860919A (en) * 1973-12-26 1975-01-14 Funck Donald E Smoke and gas detection and alarm apparatus
US3997837A (en) * 1974-02-21 1976-12-14 Fried. Krupp Gesellschaft Mit Beschrankter Haftung Gas analysis device
US3879717A (en) * 1974-06-19 1975-04-22 K F Ind Inc Portable methane monitor and alarm system
US4007456A (en) * 1975-12-01 1977-02-08 Craftor Inc. Gas detecting and warning system
US4335379A (en) * 1979-09-13 1982-06-15 Martin John R Method and system for providing an audible alarm responsive to sensed conditions
US4680576A (en) * 1982-11-29 1987-07-14 Gentex Corporation Photoelectric smoke detector and alarm system
US4686518A (en) * 1983-07-29 1987-08-11 Schweitzer Edmund O Jun Fault indicator having trip inhibit circuit
US4794332A (en) * 1986-03-28 1988-12-27 Schweitzer Edmund O Jun Fault indicator having improved trip inhibit circuit
US5576739A (en) * 1992-06-18 1996-11-19 Phy-Con. Inc. Carbon monoxide safety system
US5419358A (en) * 1993-08-02 1995-05-30 Francis Myrtil Gas monitoring system for a boiler
US20080284579A1 (en) * 2007-05-15 2008-11-20 David Contreras Carbon monoxide safety system
US7683794B2 (en) 2007-05-15 2010-03-23 David Contreras Carbon monoxide safety system for preventing entry into a dwelling containing toxic gases
US20110025518A1 (en) * 2009-07-30 2011-02-03 Hong Fu Jin Precision Industry (Shenzhen) Co., Ltd. Under-voltage warning circuit
US8207862B2 (en) * 2009-07-30 2012-06-26 Hong Fu Jin Precision Industry (Shenzhen) Co., Ltd. Under-voltage warning circuit

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