GB2298285A - Carbon monoxide alarm device - Google Patents

Abstract

An alarm includes a carbon monoxide detector 6 such as a heated tin oxide sensor, a light emitting device 8 and a sound emitting device 9. The heating coils of the gas sensor are driven from an unregulated voltage supply, and means are provided for measuring the supply voltage and controlling the supply of current to the heating coils (e.g. by controlling pulse duration) in response to the measured voltage in order to ensure consistent heating of the sensor.

Description

"A carbon monoxide alarm device" The invention relates to a carbon monoxide (CO) gas alarm device for use in enclosed environments where carbon monoxide may be generated by such things as gas, oil or solid fuel fires or boilers.

Carbon monoxide is an invisible, odourless, tasteless and extremely toxic gas. It is produced by appliances burning fuels, such as gas, oil, wood, petrol or coal. Heavy doses of carbon monoxide will cause a person to collapse and die within minutes, whereas lesser doses can cause headaches, drowsiness, fatigue, nausea, vomiting or influenza symptoms. It affects the body by being absorbed in the lungs in preference to oxygen, thereby resulting in rapid damage to the heart and brain from oxygen starvation.

In recent years, there has been an increasing awareness of the dangers of carbon monoxide both in commercial and domestic environments and much work has been done in development of sensors for detection of this gas. For example, Japanese Patent Specification Nos. JP 01221649 (Osaka Gas) and JP 59105552 (Figaro Giken) describe carbon monoxide sensors which have been developed. In general, many of the sensors are based on a tin dioxide sensing element which is heated to a high temperature in a cyclic manner to avoid the effects of humidity and contamination.

Alarm devices incorporating such sensors have been produced, such as those described in US 5379026 (Whittle) and WO 93/25986 (Pearce Technology Group).

While there have been many developments in the field of sensor technology, problems have arisen in application of this technology to alarm devices for use in the home and in business premises.

One such problem is the expense involved in providing voltage step-down and regulation circuits which can provide a consistent and accurate potential across the sensor.

Another problem is variation of the response of sensors according to ambient conditions.

A further problem has been generation of false alarms.

The invention is directed towards providing a CO alarm device to overcome these problems.

According to the invention, there is provided a carbon monoxide alarm comprising:a housing; a DC power supply; a carbon monoxide sensor of the type in which resistance indicates CO level after being heated; a control circuit comprising means for supplying a drive current to the sensor, monitoring sensor resistance, and generating an alarm signal according to sensor resistance; and sound emitter and light output devices, wherein the control circuit comprises means for sensing actual voltage across the sensor, and for setting a drive current time duration in a cycle according to the sensed voltage.

In one embodiment, the drive current is supplied in pulses, the pulse duration being set according to the actual sensor voltage.

Preferably, the pulse duration is in the range of 100 gs to 200 zs.

In another embodiment, the control circuit comprises a capacitor, means for charging the capacitor with a known current and determining the time delay before the capacitor potential equals a set proportion of the sensor potential, a non-volatile memory storing calibration values for a known sensor voltage, and processor means for comparing the determined and calibrated times to determine the actual sensor voltage.

In a further embodiment, the control circuit comprises means for sensing ambient temperature and adjusting monitored sensor resistance accordingly.

Preferably, the temperature sensing means comprises means for charging a capacitor through a thermistor and determining temperature according to monitored charging time and a stored calibration time.

In one embodiment, the capacitor of the temperature sensing means is also used by the voltage sensing means.

Preferably, the capacitor is also used to determine sensor resistance.

In a further embodiment, the housing comprises louvres extending outwardly and downwardly in a step configuration to prevent water entry.

Preferably, the control circuit comprises means for operating the light output device only when relatively low CO levels are sensed.

Ideally, the light output device is operated intermittently when relatively low CO levels are sensed.

The invention will be more clearly understood from the following description of some embodiments thereof given by way of example only with reference to the following drawings in which: Fig. 1 is a front view of a CO alarm of the invention, and Fig. 2 is a side view; and Figs. 3(a) and 3(b) are together a circuit diagram of a control circuit of the alarm.

Referring to Figs. 1 and 2, a CO alarm 1 of the invention comprises a set of pins 2 for direct mounting of the alarm at a power point. The pins 2 are electrically connected to a control circuit 50 by means of spring-loaded terminals (not shown) to ensure reliable contact. The alarm 1 also comprises a housing base 3 and a cover 4 interconnected by screw fasteners 5 extending from the base 3.

The alarm 1 has a CO sensor 6 of the type having a pair of heating coils interconnected by a tin dioxide element. In this embodiment, the sensor 6 is of the type marketed under the code TGS 2032.

A test button 7 and a red LED 8 are mounted in the cover 4. The circuit 50 is also connected to a sound emitter horn 9. An important aspect of the alarm 1 is that there is excellent ventilation to allow gas to enter and access the sensor, and to allow heat and sound exit. At the same time, there is a barrier to entry of particles and water.

This is achieved in a simple manner by louvres 10 in the cover 4 which have an outwardly-directed part 11 and a downwardly-directed part 12 to form a step configuration.

The louvres 10 are incorporated during moulding of the cover 4 and are thus inexpensively provided.

The alarm 1 may be assembled in a simple manner. The horn 9 and circuit 50 are directly mounted in the cover 4 and this is easily connected to the base 3.

The circuit 50 operates in cycles of just over 2.5 mins, during which the sensor 6 is heated for 60 seconds at a high temperature to burn off impurities and is then heated for 90 seconds at an operating temperature of 100"C for CO sensing. Finally, the circuit 50 measures the resistance between the coils to determine if CO is present.

Referring to Figs. 3(a) and 3(b), the circuit 50 is now described in detail. The spring-loaded terminals are indicated by the numeral 51, and are connected to a capacitive step-down circuit 52(a) having parallel capacitors Cla and Clb which convert a received 240V supply to a level of 10-15V at the input of a bridge rectifier 53. The circuit 50 may alternatively have a transformer 52(b) on a relay connector 52(c) for receiving power from a transformer for voltage step-down.

The rectifier 53 supplies a DC voltage of 11V across a positive rail 54 and ground. This level is maintained with the help of a bulk capacitor C2 in parallel with a diode CR7 across the rails.

The circuit 50 also comprises an IC 57 which controls operation of the circuit. A constant current source 58 is connected to the positive terminal of a timing 1 F capacitor C3. Transistors Q1, Q2, and Q3 are connected for control by the IC 57 of sensor current. The IC 57 is also connected to a EEPROM 61 from which it retrieves data during operation.

During production, a known voltage signal is applied to the rail 54. A current source of approximately 1mA is applied to C3 by the IC 57. While Q2 is OFF, the current source 58 charges C3. The IC 57 monitors the time for the floating point 59 to reach a potential equalling that of a point 60 in a potential divider R8/R9 across the rails.

Because the level of 60 is known, the IC 57 stores in the EEPROM a calibrated rate for charging of C3.

Further, the alarm 1 is exposed to a known CO level at a known temperature. The IC 57 stores in the EEPROM 61 calibrated resistance values for the sensor 6 and for a thermistor TH1. For both devices, the resistance is quantified in terms of charging rates for C3 when the transistors are controlled.

In operation, at the start of a cycle the IC 57 has switched Q3 OFF, in turn causing Q1 and Q2 to be OFF. It also switches Q4 and Q5 ON to activate the current source 58, which in turn charges C3 at a constant rate. The IC 57 determines the time for point 59 to reach the same potential as the point 60. By reference to the voltage response calibration data of C3, the IC 57 knows the voltage of point 59 when it equals that of point 60, thus providing a measure of the level of point 60, and thus the rail voltage. The IC 57 then supplies a pulsed current drive for the high temperature 60s phase to eliminate contamination. For the sensing phase of 90s duration, the IC 57 controls Q3, Q1, and Q2 to cause a drive current pass through the sensor 6, Q1 and Q2 to ground.The duration of the pulses is set according to the sensed rail voltage level so that there is uniform and consistent heating of the sensor 6. The pulse duration range is 400 to 600 ws for the high temperature phase and 100 to 200 ps for the low temperature phase.

It will be appreciated that this level of consistency is achieved in a simple manner.

During supply of drive current, Q2 provides a path to ground. After 90s, the IC 57 switches Q3 OFF, which in turn switches Q1 and Q2 OFF. The sensor 6 then acts as a resistor in series with C3 across the rails. The rail 54 then charges C3 through the sensor 6, the time to charge to the level of point 60 indicating the resistance as the charging response of C3 is known. A/D RELEASE is then switched ON to discharge C3 and C3 is then charged through TH1 to sense ambient temperature. The response of TH1 has been calibrated in the EEPROM 61. The IC 57 modifies the sensed sensor resistance according to sensed ambient temperature. This adjusted resistance is used as an input to a table, the other coordinate of which indicates the alarm action according to initial calibration with a known CO concentration. The following table indicates the basis for the stored table in the EEPROM 61.

CO Level Red Light Time Until Horn Sounds 1. 0 ppm OFF ~~~~~~~~ 2. 100 ppm Flash at 2 sec. 30 min.

periods.

3. 200 ppm Flash at 0.5 sec 10 min.

periods.

4. 400 ppm On continuously. 2.5 min.

In another embodiment, the following table is used.

CO Level Red Light Time Until Horn Sounds 1. 0 ppm OFF ~~~~~~~~ 2. 100 ppm Flash at 0.5 10 min.

sec. periods.

3. 350 ppm On continuously. 2.5 min.

A horn cycle is a set of three 0.5 sec. pulses, with 0.5 sec. pauses in between, followed by a 1.5 sec. pause.

This alarm response is an effective way of avoiding nuisance alarms while at the same time providing a large amount of CO concentration information to the user and providing an effective warning when the concentration approaches a danger level.

The invention provides for very accurate CO sensing without use of expensive components. By storing simple sets of calibration data, the IC 57 can use its internal timing and comparator functions and a capacitor to achieve actual rail voltage and temperature sensing for high CO sensing accuracy. Circuit cost is also low because a single capacitor is used for all resistance measuring.

The invention is not limited to the embodiments described, but may be carried in construction and detail.

Claims (12)

1. A carbon monoxide alarm comprising: a housing; a DC power supply; a carbon monoxide sensor of the type in which resistance indicates CO level after being heated; a control circuit comprising means for supplying a drive current to the sensor, monitoring sensor resistance, and generating an alarm signal according to sensor resistance; and sound emitter and light output devices, wherein the control circuit comprises means for sensing actual voltage across the sensor, and for setting a drive current time duration in a cycle according to the sensed voltage.

2. An alarm as claimed in claim 1, wherein the drive current is supplied in pulses, the pulse duration being set according to the actual sensor voltage.

3. An alarm as claimed in claim 2 wherein the pulse duration is in the range of 100 gs to 200 gas.

4. An alarm as claimed in claims 1, 2 or 3, wherein the control circuit comprises a capacitor, means for charging the capacitor with a known current and determining the time delay before the capacitor potential equals a set proportion of the sensor potential, a non-volatile memory storing calibration values for a known sensor voltage, and processor means for comparing the determined and calibrated times to determine the actual sensor voltage.

5. An alarm as claimed in any preceding claim, wherein the control circuit comprises means for sensing ambient temperature and adjusting monitored sensor resistance accordingly.

6. An alarm as claimed in claim 5, wherein the temperature sensing means comprises means for charging a capacitor through a thermistor and determining temperature according to monitored charging time and a stored calibration time.

7. An alarm as claimed in claim 6, wherein the capacitor of the temperature sensing means is also used by the voltage sensing means.

8. An alarm as claimed in claim 7, wherein the capacitor is also used to determine sensor resistance.

9. An alarm as claimed in any preceding claim, wherein the housing comprises louvres extending outwardly and downwardly in a step configuration to prevent water entry.

10. An alarm as claimed in any preceding claim, wherein the control circuit comprises means for operating the light output device only when relatively low CO levels are sensed.

11. An alarm as claimed in claim 10, wherein the light output device is operated intermittently when relatively low CO levels are sensed.

12. An alarm substantially as hereinbefore described with reference to the drawings.