JP7287344B2 - gas sensor - Google Patents

Description

The present invention relates to a gas sensor that measures the concentration of a detection target gas contained in an atmosphere.

It is known that a gas sensor that measures the concentration of a detection target gas contained in an atmosphere causes a measurement error due to deterioration over time. Patent Literature 1 discloses a gas sensor capable of self-diagnosing deterioration over time.

Japanese Patent No. 4758145

However, the gas sensor described in Patent Document 1 causes an error in the self-diagnosis result itself if there is a gas to be detected in the atmosphere. There was a problem that it was not possible to

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a gas sensor capable of self-diagnosing deterioration over time even in an environment where a gas to be detected exists in the atmosphere.

A gas sensor according to the present invention includes first to fourth sensing elements whose resistance values change according to the concentration of a gas to be detected, heaters for heating the first to fourth sensing elements, and first to fourth sensing elements. a switch for switching between enabling and disabling the element; and a control circuit for generating an output signal indicating the concentration of the gas to be detected based on the potential of the connection node and for controlling the heater and the switch; The element and the second and third sensing elements have characteristics different from each other, the first and second sensing elements are connected in series or in parallel between the first power supply and the connection node, and the third and fourth are connected in series or parallel between the second power supply and the connection node, and the control circuit enables the second and fourth sensing elements during the first time period by controlling the switches. At the same time, the first sensing element is disabled, the first and third sensing elements are enabled and the fourth sensing element is disabled during the second period, and the second and third sensing elements are disabled during the third period. is enabled, the first and fourth detection elements are disabled, and the first and fourth detection elements are enabled during the fourth period.

According to the present invention, the concentration of the gas to be detected can be measured during the first and second periods, and self-diagnosis can be performed during the third and fourth periods. Moreover, since the presence of the gas to be detected does not affect the results of self-diagnosis, self-diagnosis can be performed at any timing.

In the present invention, the first and second sensing elements are connected in series between the first power supply and the connection node, and the third and fourth sensing elements are connected in series between the second power supply and the connection node. and the switches may include a first switch connected in parallel with the first sensing element and a second switch connected in parallel with the fourth sensing element. According to this, the first sensing element can be disabled by turning on the first switch, and the fourth sensing element can be disabled by turning on the second switch.

In the present invention, the control circuit disables the first sensing element by turning on the first switch during the first period, and turns on the second and fourth sensing elements by the heater. 2 temperature and turning on the second switch for a second period of time to disable the fourth sensing element, while the heater energizes the first and third sensing elements to the second and first respectively. and during the third period the first and second switches are turned on to disable the first and fourth sensing elements, and the heater either turns off the second or third sensing elements. may also be heated to the first temperature, the first and second switches may be turned off during the fourth period, and both the first and fourth sensing elements may be heated to the second temperature by the heater. do not have. According to this, it is possible to provide a thermal conduction type gas sensor capable of self-diagnosis.

In the present invention, the control circuit heats the third sensing element to the second temperature by the heater during the first period, and heats the second sensing element to the second temperature by the heater during the second period. However, both the second and third sensing elements may be heated to the second temperature by the heater during the fourth period. This makes it possible to simplify the circuit configuration.

In the present invention, the control circuit heats the first sensing element to the first temperature by the heater during the first period, and heats the fourth sensing element to the first temperature by the heater during the second period. I don't mind. According to this, since the thermal histories applied to the first to fourth sensing elements are the same, it is possible to reduce measurement errors due to aged deterioration.

In the present invention, the heaters include first to fourth heaters that heat the first to fourth sensing elements, respectively, and the control circuit controls the heating by the second, third, and fourth heaters during the first period. The second, third, and fourth sensing elements are heated to the first, second, and second temperatures, respectively, the heating by the first heater is stopped, and in the second period, the first, second, and fourth sensing elements are No. 3 heaters heat the first, second and third sensing elements to the second, second and first temperatures, respectively, while the fourth heater stops heating, and during the third period, the second and third sensing elements are The third heater may heat both the second and third sensing elements to the first temperature, and the heating by the first and fourth heaters may be stopped. According to this, power consumption can be reduced.

In the present invention, the switch further includes a third switch connected in parallel with the second sensing element and a fourth switch connected in parallel with the third sensing element, and the control circuit comprises the first During the period of , the first and third sensing elements are disabled by turning on the first and fourth switches, and during the second period of time, the second and third switches are turned on by turning on the second and third switches. 4 sensing elements may be disabled, and the second and third sensing elements may be disabled by turning on the third and fourth switches during a fourth period. This makes it easy to design the resistance values of the first to fourth sensing elements.

In the present invention, the heater includes first to fourth heaters that heat the first to fourth sensing elements, respectively, and the control circuit controls the second and fourth heaters to heat the second and fourth heaters during the first period. The fourth sensing element is heated to the first and second temperatures, respectively, the heating by the first and third heaters is stopped, and in the second period, the first and third heaters are heated to the first and third temperatures. are heated to the second and first temperatures, respectively, the heating by the second and fourth heaters is stopped, and in the third period, the second and third detections are performed by the second and third heaters Both the elements are heated to the first temperature, the heating by the first and fourth heaters is stopped, and in the fourth period, both the first and fourth sensing elements are heated by the first and fourth heaters. Heating by the second and third heaters may be stopped while heating to the second temperature. According to this, power consumption can be reduced.

In the present invention, the sensitivity to the detection target gas when the second and third sensing elements are heated to the first temperature and the sensitivity when the first and fourth sensing elements are heated to the second temperature may differ from each other in sensitivity to the gas to be detected. This makes it possible to detect a gas such as CO ₂ gas whose thermal conductivity changes with temperature.

In the present invention, the first and second sensing elements are connected in parallel between the first power supply and the connection node, and the third and fourth sensing elements are connected in parallel between the second power supply and the connection node. and the switches may include first through fourth switches connected in series with the first through fourth sensing elements, respectively. According to this, the first to fourth sensing elements can be disabled by turning off the first to fourth switches.

In the present invention, the control circuit may generate an error signal based on the potential of the connection node during the third and fourth periods. According to this, it becomes possible to alert|report generation|occurrence|production of aged deterioration.

Thus, according to the present invention, it is possible to provide a gas sensor capable of self-diagnosing deterioration over time even in an environment in which a gas to be detected exists in the atmosphere.

FIG. 1 is a circuit diagram showing the configuration of a gas sensor 1A according to the first embodiment of the invention. FIG. 2 is an operation waveform diagram of the gas sensor 1A. 3A is a circuit diagram for explaining the states of the switches Sw1 and Sw4 in operation 1, and FIG. 3B is a circuit diagram for explaining the states of the switches Sw1 and Sw4 in operation 2. FIG. 4A is a circuit diagram for explaining the states of the switches Sw1 and Sw4 in operation 3, and FIG. 4B is a circuit diagram for explaining the states of the switches Sw1 and Sw4 in operation 4. FIG. FIG. 5 is a waveform diagram showing changes in the detection signal Vout1 when thermistors Rd1 to Rd4 are normal. FIG. 6 is a waveform diagram showing changes in the detection signal Vout1 when the resistance values of the thermistors Rd3 and Rd4 increase from normal values. FIG. 7 is a waveform diagram showing changes in the detection signal Vout1 when the B constants of thermistors Rd3 and Rd4 decrease from their normal values. FIG. 8 is a waveform diagram showing changes in the detection signal Vout1 when the resistance values of the thermistors Rd3 and Rd4 increase from their normal values and the B constant decreases from their normal values. FIG. 9 is a circuit diagram showing the configuration of a gas sensor 1B according to a second embodiment of the invention. FIG. 10 is an operation waveform diagram of the gas sensor 1B. 11A is a circuit diagram for explaining the states of the switches Sw1 to Sw4 in operation 1, and FIG. 11B is a circuit diagram for explaining the states of the switches Sw1 to Sw4 in operation 2. FIG. 12(a) is a circuit diagram for explaining the states of the switches Sw1 to Sw4 in operation 3, and FIG. 12(b) is a circuit diagram for explaining the states of the switches Sw1 to Sw4 in operation 4. FIG. FIG. 13 is a circuit diagram showing the configuration of a gas sensor 1C according to the third embodiment of the invention. FIG. 14 is a circuit diagram showing the configuration of a gas sensor 2A according to the fourth embodiment of the invention. FIG. 15 is a circuit diagram showing the configuration of a gas sensor 2B according to the fifth embodiment of the invention. FIG. 16 is a circuit diagram showing the configuration of a gas sensor 3A according to the sixth embodiment of the invention. FIG. 17 is an operation waveform diagram of the gas sensor 3A. FIG. 18 is a circuit diagram showing the configuration of a gas sensor 3B according to the eighth embodiment of the invention. FIG. 19 is an operation waveform diagram of the gas sensor 3B. FIG. 20 is a circuit diagram showing the configuration of the gas sensor 4 according to the eighth embodiment of the invention. 21 is an operation waveform diagram of the gas sensor 4. FIG.

Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

<First embodiment>
FIG. 1 is a circuit diagram showing the configuration of a gas sensor 1A according to the first embodiment of the invention.

As shown in FIG. 1, the gas sensor 1A according to the first embodiment includes a sensor section S and a control circuit 20. As shown in FIG. Although not particularly limited, the gas sensor 1A according to this embodiment detects the concentration of CO ₂ gas in the atmosphere.

The sensor section S is a thermal conduction type gas sensor for detecting the concentration of CO ₂ gas, which is the gas to be detected, and has four sensor sections S1 to S4. The sensor units S1 to S4 are respectively composed of thermistors Rd1 to Rd4 as detection elements and heater resistors Mh1 to Mh4 for heating them. As shown in FIG. 1, the thermistors Rd1 and Rd2 are connected in series between a connection node N and a wiring supplied with a power supply potential Vcc, which is a first power supply. The thermistors Rd3 and Rd4 are connected in series between a connection node N and a wiring supplied with the ground potential Gnd, which is the second power supply. A detection signal Vout1 that is the potential of the connection node N is supplied to the control circuit 20 . Rd1 to Rd4 are made of a material having a negative temperature coefficient of resistance such as composite metal oxide, amorphous silicon, polysilicon, and germanium.

A control voltage Vmh1 is applied to the heater resistors Mh1 and Mh2, and a control voltage Vmh2 is applied to the heater resistors Mh3 and Mh4. The control voltages Vmh1 and Vmh2 are set to the first level or the second level during gas detection operation and self-diagnosis operation, which will be described later. When control voltages Vmh1 and Vmh2 are set to a first level, thermistors Rd1-Rd4 are heated to a first temperature by heater resistors Mh1-Mh4. When control voltages Vmh1 and Vmh2 are set to the second level, thermistors Rd1-Rd4 are heated to a second temperature by heater resistors Mh1-Mh4. The first temperature is for example 150°C and the second temperature is for example 300°C. The thermistors Rd2 and Rd3 are designed to have a predetermined resistance value when heated to 150°C, while the thermistors Rd1 and Rd4 are designed to have a predetermined resistance value when heated to 300°C. ing.

If CO ₂ gas exists in the measurement atmosphere while the thermistors Rd2 and Rd3 are heated to 150° C., the heat dissipation characteristics of the thermistors Rd2 and Rd3 change according to the concentration. Such changes appear as changes in the resistance values of the thermistors Rd2 and R3. On the other hand, even if CO ₂ gas exists in the measurement atmosphere while the thermistors Rd1 and Rd4 are heated to 300° C., the heat dissipation characteristics of the thermistors Rd1 and Rd4 hardly change according to the concentration. Therefore, the resistance values of the thermistors Rd1 and Rd4 hardly change due to the concentration of CO ₂ gas. In this embodiment, when there is (almost) no CO ₂ gas in the atmosphere, the resistance value of the thermistor Rd2 heated to 150° C. and the total resistance value of the thermistors Rd3 and Rd4 heated to 300° C. It is designed so that the resistance value of the thermistor Rd3 heated to 150.degree. C. and the total resistance value of the thermistors Rd1 and Rd2 heated to 300.degree. This can be realized by designing thermistors Rd1 and Rd4 to have the same characteristics, thermistors Rd2 and Rd3 to have the same characteristics, and thermistors Rd1 and Rd4 and thermistors Rd2 and Rd3 to have different characteristics.

The gas sensor 1A according to this embodiment further has a switch Sw1 connected in parallel with the thermistor Rd1 and a switch Sw4 connected in parallel with the thermistor Rd4. When the switch Sw1 is turned on, the thermistor Rd1 is disabled, and when the switch Sw4 is turned on, the thermistor Rd4 is disabled. In other words, when the switch Sw1 is turned on, the thermistor Rd1 does not contribute to the level change of the detection signal Vout1, and when the switch Sw4 is turned on, the thermistor Rd4 does not contribute to the level change of the detection signal Vout1.

The control circuit 20 includes differential amplifiers 21 to 23, an AD converter (ADC) 24, a DA converter (DAC) 25, a signal processor 26 and resistors R1 to R3. The differential amplifier 21 is a circuit that compares the detection signal Vout1 and the reference voltage Vref and amplifies the difference. The gain of the differential amplifier 21 is arbitrarily adjusted by resistors R1 to R3. An amplified signal Vamp output from the differential amplifier 21 is input to the AD converter 24 .

The AD converter 24 digitally converts the amplified signal Vamp and supplies the value to the signal processing section 26 . On the other hand, the DA converter 25 generates a reference voltage Vref by analog-converting the reference signal supplied from the signal processing unit 26, and plays a role of generating control voltages Vmh1 and Vmh2 to be supplied to the heater resistors Mh1 to Mh4. . A control voltage Vmh1 is applied to heater resistors Mh1 and Mh2 via a differential amplifier 22, which is a voltage follower. Similarly, the control voltage Vmh2 is applied to heater resistors Mh3 and Mh4 via a differential amplifier 23, which is a voltage follower. Further, the signal processing unit 26 generates switching signals Vsw1 and Vsw4 that control the switches Sw1 and Sw4.

FIG. 2 is an operation waveform diagram of the gas sensor 1A according to this embodiment.

As shown in FIG. 2, the operation of the gas sensor 1A according to the present embodiment includes operations 1-4. Operation 1 and operation 2 are gas detection operations, and are alternately performed at predetermined intervals (for example, once a minute). On the other hand, operations 3 and 4 are self-diagnostic operations, which are performed at longer intervals than the gas detection operation (for example, once a day). A period of one second or less is sufficient for each of the operations 1 to 4 to be executed.

During the period T1 during which operation 1 is performed, the control voltages Vmh1 and Vmh2 are set to the first and second levels, respectively, and the switching signals Vsw1 and Vsw4 are set to high and low levels, respectively. As a result, the switch Sw1 is turned on and the switch Sw4 is turned off as shown in FIG. connected to. Thermistors Rd1 and Rd2 are heated to a first temperature (150° C.) by heater resistors Mh1 and Mh2, and thermistors Rd3 and Rd4 are heated to a second temperature (300° C.) by heater resistors Mh3 and Mh4.

In this state, when there is (almost) no CO ₂ gas in the atmosphere, as described above, the resistance value of the thermistor Rd2 heated to 150° C. and the total resistance value of the thermistors Rd3 and Rd4 heated to 300° C. match. Therefore, the detection signal Vout1 is Vcc/2. On the other hand, if CO ₂ gas exists in the environment, the heat radiation characteristic of the thermistor Rd2 changes according to its concentration, and the resistance value of the thermistor Rd2 changes. On the other hand, even if CO ₂ gas exists in the measurement atmosphere while the thermistor Rd4 is heated to 300° C., the resistance hardly changes according to its concentration. As a result, the level of the detection signal Vout1 changes according to the concentration of CO ₂ gas. Since CO ₂ gas has lower heat dissipation than air, the higher the concentration of CO ₂ gas, the higher the temperature of the thermistor Rd2. As a result, the resistance value of the thermistor Rd2 decreases, so the level of the detection signal Vout1 increases do.

Even if the thermistor Rd3 designed for 150° C. is heated to 300° C. and CO ₂ gas is present in the measurement atmosphere, the resistance hardly changes according to its concentration. Moreover, the resistance value when the thermistor Rd3 is heated to 300° C. is significantly lower than the resistance value of the thermistor Rd2 heated to 150° C. and the resistance value of the thermistor Rd4 heated to 300° C. (for example, about 1/10). . Therefore, the thermistor Rd3 heated to 300° C. hardly contributes to the change in the detection signal Vout1.

The detection signal Vout1 obtained in this manner is supplied to the signal processing section 26 via the differential amplifier 21 and the AD converter 24, and the signal processing section 26 outputs an output signal OUT indicating the concentration of CO ₂ gas based thereon. to generate

During the period T2 during which operation 2 is performed, the control voltages Vmh1 and Vmh2 are set to the second and first levels, respectively, and the switching signals Vsw1 and Vsw4 are set to low and high levels, respectively. As a result, the switch Sw1 is turned off and the switch Sw4 is turned on as shown in FIG. connected to. Thermistors Rd1 and Rd2 are heated to a second temperature (300° C.) by heater resistors Mh1 and Mh2, and thermistors Rd3 and Rd4 are heated to a first temperature (150° C.) by heater resistors Mh3 and Mh4.

In this state, when there is (almost) no CO ₂ gas in the atmosphere, as described above, the resistance value of the thermistor Rd3 heated to 150° C. and the total resistance value of the thermistors Rd1 and Rd2 heated to 300° C. match. Therefore, the detection signal Vout1 is Vcc/2. On the other hand, if CO ₂ gas exists in the environment, the heat radiation characteristic of the thermistor Rd3 changes according to its concentration, and the resistance value of the thermistor Rd3 changes. On the other hand, even if CO ₂ gas exists in the measurement atmosphere while the thermistor Rd1 is heated to 300° C., the resistance value hardly changes according to its concentration. As a result, the level of the detection signal Vout1 changes according to the concentration of CO ₂ gas. Since CO ₂ gas has lower heat dissipation than air, the higher the concentration of CO ₂ gas, the higher the temperature of the thermistor Rd3. As a result, the resistance value of the thermistor Rd3 decreases, and the level of the detection signal Vout1 decreases do.

Even if the thermistor Rd2 designed for 150° C. is heated to 300° C. and CO ₂ gas is present in the measurement atmosphere, the resistance hardly changes according to its concentration. Moreover, the resistance value when the thermistor Rd2 is heated to 300° C. is significantly lower than the resistance value of the thermistor Rd3 heated to 150° C. and the resistance value of the thermistor Rd1 heated to 300° C. (for example, about 1/10). . Therefore, the thermistor Rd2 heated to 300° C. hardly contributes to the change in the detection signal Vout1.

The detection signal Vout1 obtained in this manner is supplied to the signal processing section 26 via the differential amplifier 21 and the AD converter 24, and the signal processing section 26 outputs an output signal OUT indicating the concentration of CO ₂ gas based thereon. to generate Since the change in the detection signal Vout1 in operation 2 is in the opposite direction to the change in the detection signal Vout1 in operation 1, the signal processing unit 26 calculates the output signal OUT in consideration of this.

As described above, in the gas sensor 1A according to the present embodiment, since the operation 1 and the operation 2 are alternately executed in the gas detection operation, the heat histories applied to the thermistors Rd1 to Rd4 are the same. This makes it possible to reduce measurement errors due to deterioration over time.

During the period T3 in which operation 3 is performed, both the control voltages Vmh1 and Vmh2 are set to the first level, and the switching signals Vsw1 and Vsw4 are both set to high level. As a result, as shown in FIG. 4A, the switches Sw1 and Sw4 are both turned on, so that the thermistors Rd1 and Rd4 are disabled, and the enabled thermistors Rd2 and Rd3 are connected between the power supply Vcc and the ground Gnd. They are connected in series. Thermistors Rd1 and Rd2 are heated to a first temperature (150° C.) by heater resistors Mh1 and Mh2, and thermistors Rd3 and Rd4 are heated to a first temperature (150° C.) by heater resistors Mh3 and Mh4.

As described above, the resistance of the thermistors Rd2 and Rd3 heated to 150° C. changes depending on the concentration of CO ₂ gas. changes occur equally for thermistors Rd2 and Rd3. Therefore, as long as there is no characteristic difference between thermistors Rd2 and Rd3, the detection signal Vout1 is maintained at the Vcc/2 level regardless of the concentration of CO ₂ gas in the environment.

The detection signal Vout1 thus obtained is supplied to the signal processing section 26 via the differential amplifier 21 and the AD converter 24, and the signal processing section 26 generates a characteristic difference between the thermistors Rd2 and Rd3 based on this. determine whether or not there is

During period T4 in which operation 4 is performed, control voltages Vmh1 and Vmh2 are both set to the second level, and switching signals Vsw1 and Vsw4 are both set to low level. As a result, as shown in FIG. 4B, the switches Sw1 and Sw4 are both turned off, so that the thermistors Rd1 to Rd4 are all enabled. Thermistors Rd1 and Rd2 are heated to a second temperature (300° C.) by heater resistors Mh1 and Mh2, and thermistors Rd3 and Rd4 are heated to a second temperature (300° C.) by heater resistors Mh3 and Mh4.

As described above, the resistance values of the thermistors Rd1 and Rd4 heated to 300° C. hardly change even in the presence of CO ₂ gas. /2 level. When the thermistors Rd2 and Rd3 designed for 150° C. are heated to 300° C., even if CO ₂ gas exists in the measurement atmosphere, the resistance hardly changes according to its concentration. Moreover, the resistance values when the thermistors Rd2 and Rd3 are heated to 300.degree. C. are significantly lower than the resistance values of the thermistors Rd1 and Rd4 heated to 300.degree. Therefore, thermistors Rd2 and Rd3 heated to 300° C. hardly contribute to the change in the detection signal Vout1.

The detection signal Vout1 thus obtained is supplied to the signal processing section 26 via the differential amplifier 21 and the AD converter 24, and the signal processing section 26 generates a characteristic difference between the thermistors Rd1 and Rd4 based on this. determine whether or not there is

In operations 3 and 4, which are self-diagnostic operations, the signal processing unit 26 generates an error signal ERR when the amount of offset of the detection signal Vout1 from the Vcc/2 level exceeds a predetermined value. This allows the user or administrator of the gas sensor 1A to perceive that the gas sensor 1A has measurement errors due to aged deterioration. When the error signal ERR is generated, for example, the gas sensor 1A is replaced.

5 to 8 are waveform diagrams specifically showing changes in the detection signal Vout1, FIG. 5 shows the case where thermistors Rd1 to Rd4 are normal, and FIGS. It shows the case where the constant changes from the normal value. 5 to 8, dashed lines indicate the absence of CO ₂ gas, and solid lines indicate the presence of CO ₂ gas. Also, the power supply Vcc is set to 3V.

As shown in FIG. 5, when the thermistors Rd1 to Rd4 are normal, the detection signal Vout1 is 1.5 V in all of operations 1 to 4 in the absence of _CO.sub.2 gas. Further, in a state where CO ₂ gas exists, the direction of change of the detection signal Vout1 in the operation 1 and the direction of change of the detection signal Vout1 in the operation 2 are opposite to each other, and the detection signal Vout1 in the operations 3 and 4 becomes 1.5V. .

On the other hand, when the resistance values of thermistors Rd3 and Rd4 increase from their normal values, as shown in FIG. The level of Vout1 and the level of the detection signal Vout1 in operation 4 match. Further, when the B constants of thermistors Rd3 and Rd4 decrease from the normal value, as shown in FIG. As a result, a level difference occurs in the detection signal Vout1 in the operations 3 and 4. Furthermore, when the resistance values of thermistors Rd3 and Rd4 increase from their normal values and the B constant decreases from their normal values, the center level is offset as shown in FIG. 54V), the direction of change of the detection signal Vout1 in operations 1 and 2 becomes the same, and a level difference occurs in the detection signal Vout1 in operations 3 and 4. FIG.

Therefore, in the signal processing unit 26, an offset exceeding a predetermined value occurs in the detection signal Vout1 in operations 3 and 4, or a difference exceeding a predetermined value occurs between the level of the detection signal Vout1 in operation 3 and the level of the detection signal Vout1 in operation 4. If this occurs, an error signal ERR should be generated.

As described above, according to the gas sensor 1A according to this embodiment, it is possible to perform the self-diagnostic operation even in an environment where the gas to be detected exists. Moreover, since there is no difference in the thermal histories applied to the thermistors Rd1 to Rd4, measurement errors due to aged deterioration are reduced.

<Second embodiment>
FIG. 9 is a circuit diagram showing the configuration of a gas sensor 1B according to a second embodiment of the invention.

As shown in FIG. 9, the gas sensor 1B according to the second embodiment has a switch Sw2 connected in parallel to the thermistor Rd2 and a switch Sw3 connected in parallel to the thermistor Rd3. This differs from the gas sensor 1A according to the first embodiment in that ON/OFF of the switches Sw2 and Sw3 are controlled by switching signals Vsw2 and Vsw3 output from . Since other basic configurations are the same as those of the gas sensor 1A according to the first embodiment, the same elements are denoted by the same reference numerals, and overlapping descriptions are omitted.

In this embodiment, when there is (almost) no CO ₂ gas in the atmosphere, the resistance value of the thermistor Rd2 heated to 150° C. is designed to match the resistance value of the thermistor Rd4 heated to 300° C. It is designed so that the resistance value of the thermistor Rd3 heated to 150.degree. C. and the resistance value of the thermistor Rd1 heated to 300.degree. This can be realized by designing thermistors Rd1 and Rd4 to have the same characteristics, thermistors Rd2 and Rd3 to have the same characteristics, and thermistors Rd1 and Rd4 and thermistors Rd2 and Rd3 to have different characteristics.

FIG. 10 is an operation waveform diagram of the gas sensor 1B according to this embodiment.

As shown in FIG. 10, in the gas sensor 1B according to the present embodiment as well, operations 1 and 2, which are gas detection operations, are alternately performed, and operations 3 and 4, which are self-diagnostic operations, are performed at predetermined timings. be.

During the period T1 during which operation 1 is performed, the control voltages Vmh1 and Vmh2 are set to the first and second levels, respectively, the switching signals Vsw1 and Vsw3 are set to high level, and the switching signals Vsw2 and Vsw4 are set to low level. set. As a result, as shown in FIG. 11A, the switches Sw1 and Sw3 are turned on and the switches Sw2 and Sw4 are turned off, so that the thermistors Rd1 and Rd3 are disabled and the enabled thermistors Rd2 and Rd4 are connected to the power supply Vcc. and the ground Gnd. Thermistors Rd1 and Rd2 are heated to a first temperature (150° C.) by heater resistors Mh1 and Mh2, and thermistors Rd3 and Rd4 are heated to a second temperature (300° C.) by heater resistors Mh3 and Mh4.

In this state, when there is (almost) no CO ₂ gas in the atmosphere, as described above, the resistance value of the thermistor Rd2 heated to 150° C. and the resistance value of the thermistor Rd4 heated to 300° C. are designed to match. Therefore, the detection signal Vout1 becomes Vcc/2. On the other hand, if CO ₂ gas exists in the environment, the heat radiation characteristic of the thermistor Rd2 changes according to its concentration, and the resistance value of the thermistor Rd2 changes. On the other hand, even if CO ₂ gas exists in the measurement atmosphere while the thermistor Rd4 is heated to 300° C., the resistance hardly changes according to its concentration. As a result, the higher the concentration of CO ₂ gas, the higher the level of the detection signal Vout1.

During the period T2 during which operation 2 is performed, the control voltages Vmh1 and Vmh2 are set to the second and first levels, respectively, the switching signals Vsw2 and Vsw4 are set to high level, and the switching signals Vsw1 and Vsw3 are set to low level. set. As a result, the switches Sw2 and Sw4 are turned on and the switches Sw1 and Sw3 are turned off as shown in FIG. and the ground Gnd. Thermistors Rd1 and Rd2 are heated to a second temperature (300° C.) by heater resistors Mh1 and Mh2, and thermistors Rd3 and Rd4 are heated to a first temperature (150° C.) by heater resistors Mh3 and Mh4.

In this state, when there is (almost) no CO ₂ gas in the atmosphere, as described above, the resistance value of the thermistor Rd3 heated to 150° C. and the resistance value of the thermistor Rd1 heated to 300° C. are designed to match. Therefore, the detection signal Vout1 becomes Vcc/2. On the other hand, if CO ₂ gas exists in the environment, the heat radiation characteristic of the thermistor Rd3 changes according to its concentration, and the resistance value of the thermistor Rd3 changes. On the other hand, even if CO ₂ gas exists in the measurement atmosphere while the thermistor Rd1 is heated to 300° C., the resistance value hardly changes according to its concentration. As a result, the higher the concentration of CO ₂ gas, the lower the level of the detection signal Vout1.

In the period T3 during which operation 3 is performed, both the control voltages Vmh1 and Vmh2 are set to the first level, the switching signals Vsw1 and Vsw4 are set to high level, and the switching signals Vsw2 and Vsw3 are set to low level. . As a result, the switches Sw1 and Sw4 are turned on and the switches Sw2 and Sw3 are turned off as shown in FIG. and the ground Gnd. Thermistors Rd1 and Rd2 are heated to a first temperature (150° C.) by heater resistors Mh1 and Mh2, and thermistors Rd3 and Rd4 are heated to a first temperature (150° C.) by heater resistors Mh3 and Mh4.

In the period T4 during which operation 4 is performed, both the control voltages Vmh1 and Vmh2 are set to the second level, the switching signals Vsw2 and Vsw3 are set to high level, and the switching signals Vsw1 and Vsw4 are set to low level. . As a result, as shown in FIG. 12B, the switches Sw2 and Sw3 are turned on and the switches Sw1 and Sw4 are turned off, so that the thermistors Rd2 and Rd3 are disabled and the enabled thermistors Rd1 and Rd4 are connected to the power supply Vcc. and the ground Gnd. Thermistors Rd1 and Rd2 are heated to a second temperature (300° C.) by heater resistors Mh1 and Mh2, and thermistors Rd3 and Rd4 are heated to a second temperature (300° C.) by heater resistors Mh3 and Mh4.

Thus, since the gas sensor 1B according to the present embodiment has the switches Sw2 and Sw3 added, it is possible to enable the two thermistors and disable the two thermistors in any of the operations 1 to 4. This facilitates the design of the resistance values of the thermistors Rd1 to Rd4 in addition to the effect of the gas sensor 1A according to the first embodiment. In addition, in the gas sensor 1B according to the present embodiment, the connection positions of thermistors Rd1 and Rd2 can be reversed, and the connection positions of thermistors Rd3 and Rd4 can be reversed, increasing design flexibility.

<Third Embodiment>
FIG. 13 is a circuit diagram showing the configuration of a gas sensor 1C according to the third embodiment of the invention.

As shown in FIG. 13, in the gas sensor 1C according to the third embodiment, the connecting positions of thermistors Rd1 and Rd2 are reversed, and the connecting positions of thermistors Rd3 and Rd4 are reversed. It differs from the gas sensor 1A. Since other basic configurations are the same as those of the gas sensor 1A according to the first embodiment, the same elements are denoted by the same reference numerals, and overlapping descriptions are omitted.

As illustrated in this embodiment, even if there are two switches, the connection positions of thermistors Rd1 and Rd2 and the connection positions of thermistors Rd3 and Rd4 can be exchanged.

<Fourth Embodiment>
FIG. 14 is a circuit diagram showing the configuration of a gas sensor 2A according to the fourth embodiment of the invention.

As shown in FIG. 14, in the gas sensor 2A according to the fourth embodiment, the heater resistors Mh1 and Mh2 are replaced with a heater resistor Mh5, and the heater resistors Mh3 and Mh4 are replaced with a heater resistor Mh6. It differs from the gas sensor 1A according to the embodiment. Since other basic configurations are the same as those of the gas sensor 1A according to the first embodiment, the same elements are denoted by the same reference numerals, and overlapping descriptions are omitted.

Heater resistor Mh5 is commonly assigned to thermistors Rd1 and Rd2, and is applied with control voltage Vmh1. Heater resistor Mh6 is commonly assigned to thermistors Rd3 and Rd4, and is applied with control voltage Vmh2.

<Fifth Embodiment>
FIG. 15 is a circuit diagram showing the configuration of a gas sensor 2B according to the fifth embodiment of the invention.

As shown in FIG. 15, the gas sensor 2B according to the fifth embodiment is second in that the heater resistors Mh1 and Mh2 are replaced with a heater resistor Mh5, and the heater resistors Mh3 and Mh4 are replaced with a heater resistor Mh6. It differs from the gas sensor 1B according to the embodiment. Since other basic configurations are the same as those of the gas sensor 1B according to the second embodiment, the same elements are denoted by the same reference numerals, and overlapping descriptions are omitted.

Heater resistor Mh5 is commonly assigned to thermistors Rd1 and Rd2, and is applied with control voltage Vmh1. Heater resistor Mh6 is commonly assigned to thermistors Rd3 and Rd4, and is applied with control voltage Vmh2.

As illustrated in the fourth and fifth embodiments, a common heater resistance may be assigned to thermistors Rd1 and Rd2 and thermistors Rd3 and Rd4.

<Sixth embodiment>
FIG. 16 is a circuit diagram showing the configuration of a gas sensor 3A according to the sixth embodiment of the invention.

As shown in FIG. 16, in the gas sensor 3A according to the sixth embodiment, the control circuit 20 is additionally provided with differential amplifiers 27 and 28 which are voltage followers. Control voltages Vmh1 to Vmh4 are supplied from the DA converter 25 to the differential amplifiers 22, 23, 27 and 28, respectively. It is applied to resistors Mh1 to Mh4. Since other basic configurations are the same as those of the gas sensor 1A according to the first embodiment, the same elements are denoted by the same reference numerals, and overlapping descriptions are omitted.

FIG. 17 is an operation waveform diagram of the gas sensor 3A according to this embodiment.

As shown in FIG. 17, in the gas sensor 3A according to the present embodiment as well, operations 1 and 2, which are gas detection operations, are alternately performed, and operations 3 and 4, which are self-diagnostic operations, are performed at predetermined timings. be.

In the period T1 during which operation 1 is performed, the control voltages Vmh2, Vmh3, and Vmh4 are set to the first, second, and second levels, respectively, the switching signal Vsw1 is set to high level, and the switching signal Vsw4 is set to low level. set to The control voltage Vmh1 is set to the ground Gnd level. As a result, thermistor Rd2 is heated to a first temperature (150° C.) by heater resistor Mh2, and thermistors Rd3 and Rd4 are heated to a second temperature (300° C.) by heater resistors Mh3 and Mh4. On the other hand, for thermistor Rd1, heating by heater resistor Mh1 is stopped.

In the period T2 during which operation 2 is performed, the control voltages Vmh1, Vmh2, and Vmh3 are set to the second, second, and first levels, respectively, the switching signal Vsw4 is set to high level, and the switching signal Vsw1 is set to low level. set to The control voltage Vmh4 is set to the ground Gnd level. As a result, thermistor Rd3 is heated to a first temperature (150° C.) by heater resistor Mh3, and thermistors Rd1 and Rd2 are heated to a second temperature (300° C.) by heater resistors Mh1 and Mh2. On the other hand, for thermistor Rd4, heating by heater resistor Mh4 is stopped.

In the period T3 in which the operation 3 is performed, both the control voltages Vmh2 and Vmh3 are set to the first level, and the switching signals Vsw1 and Vsw4 are set to high level. As a result, thermistors Rd2 and Rd3 are heated to a first temperature (150° C.) by heater resistors Mh2 and Mh3. Heating by heater resistors Mh1 and Mh4 is stopped for thermistors Rd1 and Rd4.

During the period T4 in which operation 4 is performed, all of the control voltages Vmh1 to Vmh4 are set to the second level, and the switching signals Vsw1 and Vsw4 are set to low level. As a result, thermistors Rd1-Rd4 are heated to the second temperature (300° C.) by heater resistors Mh1-Mh4.

As described above, the gas sensor 3A according to the present embodiment independently controls the heater resistors Mh1 to Mh4, so that the heating of the disabled thermistors Rd1 and Rd4 can be stopped. Thereby, power consumption can be reduced as compared with the gas sensor 1A according to the first embodiment.

<Seventh embodiment>
FIG. 18 is a circuit diagram showing the configuration of a gas sensor 3B according to the eighth embodiment of the invention.

As shown in FIG. 18, in the gas sensor 3B according to the seventh embodiment, differential amplifiers 27 and 28, which are voltage followers, are added to the control circuit 20, like the gas sensor 3A according to the sixth embodiment. Other basic configurations are the same as those of the gas sensor 1B according to the second embodiment or the gas sensor 3A according to the sixth embodiment.

FIG. 19 is an operation waveform diagram of the gas sensor 3B according to this embodiment.

As shown in FIG. 19, in the gas sensor 3B according to the present embodiment as well, operations 1 and 2, which are gas detection operations, are alternately performed, and operations 3 and 4, which are self-diagnostic operations, are performed at predetermined timings. be.

During the period T1 during which operation 1 is performed, the control voltages Vmh2 and Vmh4 are set to the first and second levels, respectively, the switching signals Vsw1 and Vsw3 are set to high level, and the switching signals Vsw2 and Vsw4 are set to low level. set. The control voltages Vmh1 and Vmh3 are set to the ground Gnd level. As a result, thermistor Rd2 is heated to a first temperature (150° C.) by heater resistor Mh2, and thermistor Rd4 is heated to a second temperature (300° C.) by heater resistor Mh4. On the other hand, for thermistors Rd1 and Rd3, heating by heater resistors Mh1 and Mh3 is stopped.

During the period T2 during which operation 2 is performed, the control voltages Vmh1 and Vmh3 are set to the second and first levels, respectively, the switching signals Vsw2 and Vsw4 are set to high level, and the switching signals Vsw1 and Vsw3 are set to low level. set. The control voltages Vmh2 and Vmh4 are set to the ground Gnd level. As a result, thermistor Rd3 is heated to a first temperature (150° C.) by heater resistor Mh3, and thermistor Rd1 is heated to a second temperature (300° C.) by heater resistor Mh1. On the other hand, for thermistors Rd2 and Rd4, heating by heater resistors Mh2 and Mh4 is stopped.

In the period T3 during which operation 3 is performed, both the control voltages Vmh2 and Vmh3 are set to the first level, the switching signals Vsw1 and Vsw4 are set to high level, and the switching signals Vsw2 and Vsw3 are set to low level. . As a result, thermistors Rd2 and Rd3 are heated to a first temperature (150° C.) by heater resistors Mh2 and Mh3. Heating by heater resistors Mh1 and Mh4 is stopped for thermistors Rd1 and Rd4.

In the period T4 during which operation 4 is performed, both the control voltages Vmh1 and Vmh4 are set to the second level, the switching signals Vsw2 and Vsw3 are set to high level, and the switching signals Vsw1 and Vsw4 are set to low level. . Thereby, the thermistors Rd1 and Rd4 are heated to the second temperature (300° C.) by the heater resistors Mh1 and Mh4. Heating by heater resistors Mh2 and Mh3 is stopped for thermistors Rd2 and Rd3.

As described above, the gas sensor 3B according to the present embodiment independently controls the heater resistors Mh1 to Mh4, so that the heating of the disabled thermistors Rd1 to Rd4 can be stopped. As a result, power consumption can be reduced more than the gas sensor 3A according to the sixth embodiment.

<Eighth embodiment>
FIG. 20 is a circuit diagram showing the configuration of the gas sensor 4 according to the eighth embodiment of the invention.

As shown in FIG. 20, in the gas sensor 4 according to the eighth embodiment, thermistors Rd1 and Rd2 are connected in parallel between the power supply Vcc as the first power supply and the connection node N, and the ground Gnd as the second power supply is connected. and the connection node N, and the switches Sw1 to Sw4 are connected in series to the thermistors Rd1 to Rd4. differ. Since other basic configurations are the same as those of the gas sensor 3B according to the seventh embodiment, the same elements are denoted by the same reference numerals, and overlapping descriptions are omitted.

In this embodiment, when thermistors Rd1 to Rd4 are to be enabled, the corresponding switches Sw1 to Sw4 are turned on, and when the thermistors Rd1 to Rd4 are to be disabled, the corresponding switches Sw1 to Sw4 are turned off.

FIG. 21 is an operation waveform diagram of the gas sensor 4 according to this embodiment.

As shown in FIG. 21, in the gas sensor 4 according to the present embodiment as well, operations 1 and 2, which are gas detection operations, are alternately performed, and operations 3 and 4, which are self-diagnostic operations, are performed at predetermined timings. be.

In the period T1 during which operation 1 is performed, the control voltages Vmh2 and Vmh4 are set to the first and second levels, respectively, the switching signals Vsw2 and Vsw4 are set to high level, and the switching signals Vsw1 and Vsw3 are set to low level. set. The control voltages Vmh1 and Vmh3 are set to the ground Gnd level. As a result, thermistor Rd2 is heated to a first temperature (150° C.) by heater resistor Mh2, and thermistor Rd4 is heated to a second temperature (300° C.) by heater resistor Mh4. On the other hand, for thermistors Rd1 and Rd3, heating by heater resistors Mh1 and Mh3 is stopped. Also, the thermistors Rd1 and Rd3 are disconnected from the connection node N because the switches Sw2 and Sw4 are turned on and the switches Sw1 and Sw3 are turned off.

During the period T2 during which operation 2 is performed, the control voltages Vmh1 and Vmh3 are set to the second and first levels, respectively, the switching signals Vsw1 and Vsw3 are set to high level, and the switching signals Vsw2 and Vsw4 are set to low level. set. The control voltages Vmh2 and Vmh4 are set to the ground Gnd level. As a result, thermistor Rd3 is heated to a first temperature (150° C.) by heater resistor Mh3, and thermistor Rd1 is heated to a second temperature (300° C.) by heater resistor Mh1. On the other hand, for thermistors Rd2 and Rd4, heating by heater resistors Mh2 and Mh4 is stopped. Also, the thermistors Rd2 and Rd4 are disconnected from the connection node N because the switches Sw1 and Sw3 are turned on and the switches Sw2 and Sw4 are turned off.

In the period T3 during which operation 3 is performed, both the control voltages Vmh2 and Vmh3 are set to the first level, the switching signals Vsw2 and Vsw3 are set to high level, and the switching signals Vsw1 and Vsw4 are set to low level. . As a result, thermistors Rd2 and Rd3 are heated to a first temperature (150° C.) by heater resistors Mh2 and Mh3. Heating by heater resistors Mh1 and Mh4 is stopped for thermistors Rd1 and Rd4. Also, the thermistors Rd1 and Rd4 are disconnected from the connection node N because the switches Sw2 and Sw3 are turned on and the switches Sw1 and Sw4 are turned off.

In the period T4 during which operation 4 is performed, both the control voltages Vmh1 and Vmh4 are set to the second level, the switching signals Vsw1 and Vsw4 are set to high level, and the switching signals Vsw2 and Vsw3 are set to low level. . Thereby, the thermistors Rd1 and Rd4 are heated to the second temperature (300° C.) by the heater resistors Mh1 and Mh4. Heating by heater resistors Mh2 and Mh3 is stopped for thermistors Rd2 and Rd3. Also, the thermistors Rd2 and Rd3 are disconnected from the connection node N because the switches Sw1 and Sw4 are turned on and the switches Sw2 and Sw3 are turned off.

As illustrated in this embodiment, thermistors Rd1 and Rd2 may be connected in parallel between the power supply Vcc and the connection node N, and the thermistors Rd3 and Rd4 may be connected in parallel between the ground Gnd and the connection node N. I do not care. Incidentally, the connecting positions of the thermistor and the switch may be reversed from those in FIG.

Although the preferred embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the gist of the present invention. Needless to say, it is included within the scope.

For example, in each of the above-described embodiments, the gas detection operation alternately performs the operations 1 and 2, but this is not essential in the present invention, and after performing the operation 1 multiple times, the operation 2 is performed multiple times. I don't mind.

Further, in each of the above embodiments, the gas sensor for detecting the concentration of CO ₂ gas in the atmosphere was described as an example, but the gas to be detected in the present invention is not limited to this. Further, the object of the present invention is not limited to the heat conduction type gas sensor, but can be applied to other types of gas sensors such as a catalytic combustion type gas sensor.

1A to 1C, 2A, 2B, 3A, 3B, 4 gas sensor 20 control circuit 21 to 23, 27, 28 differential amplifier 24 AD converter 25 DA converter 26 signal processing units Mh1 to Mh6 heater resistor N connection nodes R1 to R3 resistor Rd1 ~Rd4 Thermistors S, S1~S4 Sensors Sw1~Sw4 Switches Vmh1~Vmh4 Control voltages Vsw1~Vsw4 Switching signals

Claims (12)

first to fourth sensing elements whose resistance values change according to the concentration of the detection target gas;
a heater for heating the first to fourth sensing elements;
a switch that switches between enabling and disabling the first to fourth sensing elements;
a control circuit that generates an output signal indicating the concentration of the detection target gas based on the potential of the connection node and controls the heater and the switch;
the first and fourth sensing elements and the second and third sensing elements have different properties from each other;
the first and second sensing elements are connected in series or in parallel between a first power supply and the connection node;
the third and fourth sensing elements are connected in series or in parallel between a second power supply and the connection node;
The control circuit controls the switch to
enabling the second and fourth sensing elements and disabling the first sensing element during a first period;
enabling the first and third sensing elements and disabling the fourth sensing element during a second period;
enabling the second and third sensing elements and disabling the first and fourth sensing elements during a third period;
activating the first and fourth sensing elements during a fourth period ;
The control circuit generates the output signal based on the potential of the connection node in the first and second periods, and generates the first to the potential of the connection node in the third and fourth periods. 4. A gas sensor, characterized in that self-diagnosis of the sensing element is performed . the first and second sensing elements are connected in series between the first power supply and the connection node;
the third and fourth sensing elements are connected in series between the second power supply and the connection node;
2. The method according to claim 1, wherein said switch comprises a first switch connected in parallel with said first sensing element and a second switch connected in parallel with said fourth sensing element. Gas sensor as described. The control circuit is
In the first period, the first switch is turned on to disable the first sensing element, and the second and fourth sensing elements are turned on by the heater. heat to temperature,
In the second period, the second switch is turned on to disable the fourth sensing element, and the heater turns the first and third sensing elements to the second and first sensing elements, respectively. heated to a temperature of
In the third period, the first and the second switches are turned on to disable the first and the fourth sensing elements, and the heater either turns on the second or the third sensing elements. is also heated to the first temperature,
In the fourth period, the first and second switches are turned off, and the heater heats both the first and fourth sensing elements to the second temperature. Item 3. The gas sensor according to item 2. The control circuit is
in the first period, the heater heats the third sensing element to the second temperature;
during the second period, the heater heats the second sensing element to the second temperature;
4. The gas sensor according to claim 3, wherein both the second and third sensing elements are heated to the second temperature by the heater during the fourth period. The control circuit is
In the first period, the heater heats the first sensing element to the first temperature,
5. The gas sensor according to claim 4, wherein the heater heats the fourth sensing element to the first temperature during the second period. the heater includes first to fourth heaters for heating the first to fourth sensing elements, respectively;
The control circuit is
In the first period, the second, third and fourth heaters heat the second, third and fourth sensing elements to the first, second and second temperatures, respectively, and Stopping heating by the first heater,
In the second period, the first, second and third heaters heat the first, second and third sensing elements to the second, second and first temperatures, respectively, and Stopping heating by the fourth heater,
In the third period, both the second and third sensing elements are heated to the first temperature by the second and third heaters, and heating by the first and fourth heaters is performed. 5. The gas sensor according to claim 4, which stops. said switch further comprising a third switch connected in parallel with said second sensing element and a fourth switch connected in parallel with said third sensing element;
The control circuit is
during the first period, disabling the first and third sensing elements by turning on the first and fourth switches;
during the second period, disabling the second and fourth sensing elements by turning on the second and third switches;
4. The gas sensor according to claim 3, wherein during the fourth period, the second and third sensing elements are disabled by turning on the third and fourth switches. the heater includes first to fourth heaters for heating the first to fourth sensing elements, respectively;
The control circuit is
In the first period, the second and fourth heaters heat the second and fourth sensing elements to the first and second temperatures, respectively, and the first and third heaters stop heating,
In the second period, the first and third heaters heat the first and third sensing elements to the second and first temperatures, respectively, and the second and fourth heaters stop heating,
In the third period, both the second and third sensing elements are heated to the first temperature by the second and third heaters, and heating by the first and fourth heaters is performed. stop and
In the fourth period, both the first and fourth sensing elements are heated to the second temperature by the first and fourth heaters, and heating by the second and third heaters is performed. 8. The gas sensor according to claim 7, which stops. Sensitivity to the detection target gas when the second and third sensing elements are heated to the first temperature, and the first and fourth sensing elements are heated to the second temperature. 9. The gas sensor according to any one of claims 3 to 8, wherein sensitivities to the gas to be detected are different in different cases. the first and second sensing elements are connected in parallel between the first power supply and the connection node;
the third and fourth sensing elements are connected in parallel between the second power supply and the connection node;
2. The gas sensor according to claim 1, wherein said switch includes first to fourth switches connected in series with said first to fourth sensing elements, respectively. 11. The gas sensor according to any one of claims 1 to 10, wherein the control circuit generates an error signal based on the potential of the connection node during the third and fourth periods. first to fourth sensing elements whose resistance values change according to the concentration of the detection target gas;
a heater for heating the first to fourth sensing elements;
first and second switches;
a control circuit that generates an output signal indicating the concentration of the detection target gas based on the potential of the connection node and controls the heater and the switch;
the first and fourth sensing elements and the second and third sensing elements have different properties from each other;
the first and second sensing elements are connected in series between a first power supply and a connection node;
the third and fourth sensing elements are connected in series between a second power supply and the connection node;
the first switch is connected in parallel to the first sensing element;
the second switch is connected in parallel to the fourth sensing element ;
The control circuit generates the output signal based on the potential of the connection node during a period in which one of the first and second switches is on and the other is off, and controls the first and second switches. is on or off, and performs self-diagnosis of the first to fourth sensing elements based on the potential of the connection node.

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