JPH0262816B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention uses a metal oxide semiconductor as a sensor.
This is a device that controls and alarms or monitors air pollution caused by CO, CH ₄ , etc. It automatically moves the operating point as closely as possible to the output level in clean air, keeps the sensitivity constant, and prevents the user from frequently The purpose is to eliminate readjustment and to provide a device that is suitable for the application.

A resistor R is connected to the sensor 1 using a metal oxide semiconductor such as SnO ₂ as shown in Figure 7a, and the voltage V _R across the resistor R with respect to the gas concentration has the characteristics as shown in Figure 7b. , changes between H characteristics and L characteristics due to changes in the environment such as temperature and humidity.
In addition, when used for air pollution control, it is required to operate in a state where the air contains a small amount of pollutant gas, and the input voltage Vin required for the relay circuit connected to the output of this sensor 1 to operate. must be set close to the output level of sensor 1 for Co in clean air. Therefore, when exhibiting H characteristics, it operates at a very high gas concentration _C1 , but
When exhibiting L characteristics, it becomes insensitive at C ₂ , so conventionally, the user finds the output of the sensor in clean air according to the state of the environment in which it is used, and sets the relay circuit operating point at a point slightly higher than that point. This is a relatively difficult operation that must be performed frequently. In order to explain this in more detail, FIG. 8 shows a typical characteristic of the sensor output over time in a kitchen where humidity is not controlled. Here, A, B, C...L
The mountain is the sensor output due to reducing gas generated during cooking, and the line connecting the bottom of the curve is considered to be the output in clean air. Now, if we capture the output point X in clean air on the first day and set the operating point to point O, then A, B, and C will operate on the first day, but D, E, and E on the second day will operate. The G of the eyes does not work,
H, I, J, K, and L work. Since it did not work on the second day, if I readjusted it on the second day and set it to point O', the problem was that it would continue to work on the third day.

The present invention has been made in view of these points, and the outline thereof is as follows.

The output voltage of the metal oxide semiconductor sensor is held by a sensor output holding circuit that has a large time constant when the sensor output rises and a relatively small time constant when it falls, and this holding voltage is compared with the sensor output voltage to determine outputs such as control. It is configured as a detection/alarm device such as a controller, an alarm device, a monitor device, etc. suitable for various uses. The present invention will be explained in detail below with reference to Examples.

FIG. 1 shows a circuit diagram of an embodiment, in which a power source 2 connects a metal oxide semiconductor sensor 1 and load resistors 3 and 4.
In addition to connecting the series circuit with the voltage dividing resistor 1
A series circuit of 0 and 11 is connected. The voltage comparator 5, which is an operational amplifier, has an inverting input terminal connected to the connection point between the sensor 1 and the load resistor 3, and a non-inverting input terminal connected to the output terminal _X1 . Further, the output of the voltage comparator 5 is connected via a backflow prevention diode 6, a charging resistor 7, and a resistor 12 to an inverting input terminal of a subtracter 14 consisting of an operational amplifier. Subtractor 1
The output of 4 is connected to the output terminal X ₁ , and
It is connected to the inverting input terminal of a subtracter 14 via a resistor 13. The resistor 13 and the resistor 12 constitute a voltage dividing resistor for the subtraction circuit. Further, a parallel circuit including a voltage holding capacitor 8 and a discharging resistor 9 is constructed between the connection point between the resistor 7 and the resistor 12 and the negative pole of the power source 2. In the figure, _X2 is an output terminal that outputs a voltage obtained by dividing the output voltage of the sensor 1 by resistors 3 and 4. Thus, a sensor output holding circuit 15 is composed of a capacitor 8, a resistor 9, a voltage comparator 5, a subtracter 14, an output terminal _X1, and the like.

Now, assuming that the sensor 1 made of a metal oxide semiconductor is kept at an appropriate temperature and in clean air, the output V _A of the sensor 1 is stable. Here, to simplify the explanation, assume that the resistors 12 and 13 are set to the same value. In the subtraction circuit consisting of the subtracter 14 etc., 2V _H −V _X1 (V _H , V _K , V _X1 are the first (The voltage at the relevant point in the figure is shown.) On _the other hand _{, the} voltage comparator 5 generates an output when _V ing. Therefore, the circuit including the voltage comparator ₅ and the subtracter ₁₄ has V _K
increases, V _X1 decreases, and V _X1 ≒ V _A.

Further, when V _X1 <V _A , V _K decreases and V _X1 increases, so that V _X1 ≈V _A. Therefore, the output V _X1 is V _X1 ≒ V _A
automatically controlled. Here, when V _X1 <V _A , the holding voltage V _X1 needs to rise as slowly as possible, and the discharge time constant of the capacitor 8 is made large so as to match the temperature and humidity fluctuations in the general atmosphere.

On the other hand, when V _X1 > V _A , the fall of V _X1 is caused by sensor 1.
The charging time constant of the capacitor 8 may be determined by taking into consideration the transient phenomenon time of the output voltage when it is returned from the gas to the atmosphere, and the charging time constant of the capacitor 8 may be small. Note that the capacitor 8 and the resistor 9 constitute a first time constant circuit that discharges the charge of the capacitor 8. Further, the capacitor 8 and the resistor 7 constitute a second time constant circuit that charges the capacitor 8. The output shown in FIG. 1 is obtained by comparing the divided voltage V _X2 of the sensor output voltage V _A and the holding voltage V _X1 by connecting them to a voltage comparator not shown. In other words, when the sensor output V _A is stable and V _A ≒ V _X1 ,
V _X2 > V _X1 . Here, V _X2 due to contaminated gas
If V _A quickly rises so that V _X1 ,
The voltage of V _X2 <V _X1 is reversed, and the output of the next connected voltage comparator (not shown) is reversed and becomes the output.
The operation will be explained with reference to FIG. In Fig. 2, V _A ' indicates the sensor output voltage at the operating point, and this sensor output voltage V _A ' means that the sensor output voltage V _A quickly rises from the state of V _A = V _X1 , V _X2 < V _X1 . , V _X2 = V _X1
, and the voltages at the output terminals X ₂ and X ₁ are reversed. The section in the figure shows a state where the temperature and humidity in clean air are stable, and at time t ₁ when the sensor output V _A is higher than the holding voltage V _X1 due to contaminated gas, V _K decreases and V _X1 increases, but since _the time constant for the rise of V _X1 is very large, V _X1 hardly changes and _the control output is obtained when _{V X2 > V} It will be done. The section indicates a state in which the sensor output V _A decreases as the temperature and humidity change,
Since V _A <V _X1 , V _K increases, V _X1 decreases, and V _X1 , which is the holding voltage, follows the sensor output V _A.
In the section, pollutant gas is superimposed on temperature and humidity changes,
Indicates that the sensor output V _A is trending upward,
The holding voltage _V In the section , the sensor output V _A continues to rise slowly for a long time due to temperature, humidity, or contaminated gas. The holding voltage V _X1 rises to V _X1 =2V _H and then maintains that voltage. Therefore, the control output is output at point ○A in Figure 2. The gas concentration at point ○a can be set according to the purpose, taking into consideration human danger, explosion risk, etc. As explained above, it is easy to operate when the sensor output V _A suddenly rises due to a sudden change in temperature or humidity, which can occur very rarely, and the operating point rises in response to a gradual rise in pollutant gas. In this state, it operates at an appropriate operating level, and there is no need to frequently adjust the sensitivity. By the way, the present invention is characterized by a method of reducing the voltage of the capacitor 8 when the sensor output voltage of the sensor 1 increases due to high humidity etc. by utilizing the discharge characteristics of the capacitor 8, which is described below. It's for a reason. In other words, large-capacity capacitors generally have a large leakage current, and it is extremely difficult to obtain a large time constant that matches a general humidity fluctuation curve when using charging characteristics. For example, the leakage current of a capacitor is 0.1 mA under the worst conditions (high temperature or after long-term storage, etc.) and under the best conditions (low temperature, etc.)
0.01mA, 0.02mA under normal conditions (appropriate temperature).
When utilizing the charging characteristics of a capacitor, the capacitor is generally charged with a current that is more than twice the leakage current of 0.1 mA under the worst-case conditions, taking into account factors such as aging of the capacitor. Therefore, if the charging current is 0.2mA, under normal conditions it will be 0.2mA-
Charged at 0.02mA = 0.18mA. Furthermore, when using the discharge characteristics of a capacitor, it is possible to discharge only by leakage current without using a discharge resistor, so under normal conditions the discharge is 0.02 mA. In other words, when comparing the above charging and discharging cases, 0.18mA/
0.02mA = 9 times the delay time.
In addition, the typical humidity fluctuation (between bottom and peak) takes about 6 hours, and it is currently extremely difficult to use charging characteristics to accommodate this fluctuation due to the delay time described above. On the other hand, if the discharge characteristics of a capacitor are used, a long delay time can be obtained. Figure 3 shows changes in temperature and humidity due to changes in usage locations, and artificial temperature and humidity changes such as air conditioning and heating.
Sensor output V _A in clean air due to humidity changes, etc.
In this embodiment, when the voltage increases and the capacitor operates for a long time until V _A ′ < V _A , an external signal is applied at an appropriate time to restore the capacitor. 8 is discharged for a certain period of time, and then charged promptly. That is, a voltage comparator 25 is separately provided, and the non-inverting input terminal of this voltage comparator 25 is connected to the sensor 1.
and a resistor 18 connected to the dividing point of the voltage dividing circuit of resistors 16 and 17 connected in parallel to the series circuit of resistors 3 and 4, and further connected in parallel to the voltage dividing circuit of resistors 16 and 17.
The inverting input terminal of the voltage comparator 25 is connected to the dividing point of the series circuit of the capacitor 19 and the capacitor 19, and the output of the voltage comparator 25 is connected to the transistor 23 via the resistor 22.
connected to the base of Transistor 23 is connected in parallel to capacitor 8 via resistor 24. On the other hand, the resistor 18 has a diode 2 in the opposite direction.
0 and a resistor 21 are connected in parallel.
A transistor 27 operated by an external signal is connected in parallel to the capacitor 19 via a resistor. V _M
represents the voltage of the capacitor 19, V _L represents the voltage of the resistor 16,
17 shows the divided voltage. Next, the operation of the circuit shown in FIG. 3 will be explained. But now, Switch 26
When the power supply 2 is turned off and the power supply 2 is cut off, or when the power supply 2 is turned on and an external signal is applied to the base of the transistor 27 to turn it on from off, the charge in the capacitor 19 is transferred to the resistor 21, the diode 20, and the resistors 16 and 17. or through transistor 27. Then switch 26
When the capacitor 19 is turned on and the power supply 2 is turned on, or when the external signal input to the base of the transistor 27 is stopped using the power supply 2 and turned off from on, the capacitor 19 is connected to the resistor 1.
8, the voltage V _M increases. Here, while V _M <V _L , the voltage comparator 25 produces an output and turns on the transistor 23, and the capacitor 8 discharges its charge. Further, when V _M >V _L , the output of the voltage comparator 25 becomes zero, the transistor 23 is turned off, and the capacitor 8 is charged until V _A ≈V _X1 . In such an embodiment, plugging and unplugging of the power outlet (turning the power supply 2 on and off) due to location relocation
The holding voltage V _X1 of the sensor output holding circuit 15 can be changed by an external signal from an external control device (air conditioner, heater), etc. Incidentally, the holding voltage V _X1 can be similarly changed by directly discharging the charge in the capacitor 8 using an external signal.

FIG. 4 shows a block diagram of an example circuit that does not require adjustment and is used as a ventilation fan control device that can be used in standard home kitchens, etc. The holding output _V output terminals X 1 and 1, respectively, so that the divided voltage V _X2 of the output of sensor 1 is input to the other input terminal _.
X ₂ are connected respectively. The output of this voltage comparator 28 becomes the driving power for a relay 29, and the relay 29 connects the ventilation fan 30 to an AC power source 31 via a contact r. Of course, the relay 29 and the contact r may be configured by a non-contact relay circuit such as a triac.

When the contaminated gas is detected at a certain level or higher, an output is generated in the voltage comparator 28 and the relay 2
9 is activated, the ventilation fan 30 is connected to the AC power source 31 through its contact r, and the ventilation fan 30 is activated. When the presence of polluted gas becomes below a certain level, the output of the voltage comparator 28 becomes zero, and the contact r is opened and the ventilation fan 30 is activated. The operation of the ventilation fan 30 is stopped by cutting off the power supply to the ventilation fan 30. The voltage comparator 28 and relay 29 constitute a control means. Note that a gas alarm can be constructed by driving an acoustic device such as a buzzer with the output of the voltage comparator 28 instead of the circuit such as the relay 29 or the ventilation fan 30.

5a and 5b show an embodiment corresponding to claim 2, which is a so-called sensitivity adjustable type. Each shows an embodiment for controlling and driving a ventilation fan 30 with fine adjustment aimed at people who are extremely sensitive to specific pollutant gases such as cigarette smoke. The voltage resistor 3 is used as a variable resistor to make it possible to variably adjust _the output _V
The voltage is divided by R ₁ , R ₂ , and R ₃ , and the output V _X1 can be selected using the changeover switch SW. Note that the voltage dividing resistor 3 in Figure 5a and the voltage dividing resistor 3 in Figure 5b
R ₁ , R ₂ , and R ₃ constitute a variable means. Also,
The voltage comparator 28 and the relay 29 constitute a control means.

FIG. 6a shows an embodiment corresponding to claims 3 and 4, which constitutes a ventilation fan control, trace gas alarm, and gas alarm device that can be used for various detection targets, and in which the detection sensitivity can be adjusted visually. An example circuit that can be used is shown below. That is, the holding output of the sensor output holding circuit 15 is connected to the voltage dividing resistor R ₁ ~
Each voltage is divided by _R4 , and _each voltage comparator ₂₈₁ to ₂₈₄ compares the voltage with the divided voltage output of the sensor _output . _-284 are arranged to light up light emitting diodes ₃₂₂ to ₃₂₄ provided at the output end when an output is generated. Further, the output of the voltage comparator 28 ₁ constitutes a control signal for the gas alarm means 33 .
Further, the outputs of the other voltage comparators 28 ₂ to 28 ₄ are arranged so that control signals of three stages of detection levels can be selected by a changeover switch 34. The control changeover switch 37 is a switch for selecting whether the control signal obtained by the above selection is used for controlling the gas trace alarm means 35 or for controlling the ventilation fan 31. Therefore, as the partial pressure V _X2 of the sensor output V _A increases, the voltage comparator 28 ₄ ,
Outputs are generated in the order of 28 ₃ , 28 ₂ , and 28 ₁ , and each output of the voltage comparators 28 ₄ , 28 ₃ , and 28 ₂ lights up the light emitting diodes 32 ₄ , 32 ₃ , and 32 ₂ in that order, and the voltage comparators The gas alarm means 33 is driven by the output of ₂₈₁ . Here, changeover switch 34 is connected to voltage comparator 28.
If you switch to the output side of ₂ , the light emitting diode 3
When 2 ₂ lights up, the control signal of the voltage comparator 28 ₂ is output to the switching side of the control changeover switch 37. The voltage comparator ₂₈₁ operates when the partial voltage output _VX1 of the sensor output _VA rises to a point that takes into consideration explosion and human danger, and outputs an alarm. In this embodiment, it is possible to select a plurality of displays that light up depending on the degree of contamination and a point from which an output is taken in relation to the display, so that the degree of contamination can be visually judged and the sensitivity can be easily adjusted. There are some features. The voltage comparators 28 ₁ to 28 ₄ and the like constitute a control means, the light emitting diodes 32 ₂ to 32 ₄ constitute a display means, and the changeover switch 34 and the control changeover switch 37 constitute a selection means. ing.

FIG. 6b shows an embodiment corresponding to claim 5. That is, a circuit of an embodiment used in a pollution monitoring meter is shown, in which the sensor output V _A and the held output _V and the differential amplifier 38
The output is used to drive a monitoring device 39 such as a digital meter, a movable ring type ammeter, or a recording device. Note that the differential amplifier 38 constitutes a control means.

As described above, the present invention provides a metal oxide semiconductor sensor that outputs an output voltage depending on the degree of air pollution, and compares the output voltage from this sensor with the voltage fed back from the output terminal to determine the sensor's output. If the voltage is higher, the voltage comparator outputs a signal, and the voltage comparator outputs a signal to measure the charge on the capacitor with a time constant that is approximately the same as the time fluctuations of the general atmosphere such as temperature and humidity. a first time constant circuit having a time constant large enough to cause discharge when the sensor is placed in a normal general atmosphere rather than a polluted atmosphere; a second time constant circuit that charges the capacitor with the signal when the output of the comparator is a signal of opposite polarity to the signal;
This is because a sensor output holding circuit is formed with a subtracter that subtracts the charging voltage of the capacitor from a preset voltage, feeds back the output voltage to the voltage comparator, and outputs it as an output voltage from the output terminal. , by utilizing the discharge characteristics of the capacitor in the sensor output holding circuit,
It is possible to obtain a long delay time that can be matched with the time of general atmospheric fluctuations, so it can absorb the fluctuations caused by normal gradual temperature and humidity fluctuations, and set the reference operating point to atmospheric pollution. It can be changed according to the situation, and
Gas contamination that has a time constant smaller than the discharging time constant of the capacitor in the sensor output retention circuit, which often occurs, can be detected by using the difference between the sensor output and the holding voltage due to gas contamination, and is effective against temperature and humidity fluctuations. This has the advantage that there is no need to constantly re-adjust the sensitivity.

Further, since a control means is provided to stop the operational output when a predetermined difference occurs between the sensor output voltage and the output voltage of the sensor output holding circuit,
It is easy to detect a specific pollutant gas, and after detecting a pollutant gas and activating an alarm device, for example, if the atmospheric condition returns to normal, the control means can stop the operational output. This has the effect that the alarm device can be reset and the entire device can automatically return to a state in which it can detect contaminated gas again.

Furthermore, in claim 2, a variable means is provided that can arbitrarily vary the difference between the sensor output voltage and the output voltage of the sensor output holding circuit, and the difference between the sensor output voltage and the output voltage of the sensor output holding circuit is provided. By making it possible to output an operating output when a difference occurs, and by providing a control means to stop the operating output when the above difference disappears, it is easy to detect a specific contaminant gas depending on the degree of contamination. It is possible.

Furthermore, in claim 3, when a difference between the sensor output voltage and the output voltage of the sensor output holding circuit occurs in stages, the operating output is output in stages, and when the difference disappears. a plurality of display means each displaying a display according to the order of the magnitude of the operation output of the control means that stops the operation output when the operation output is stopped; Since it is equipped with a removable selection means, the operating level can be selected so that the degree of contamination can be visually determined by the display means, and the operating level can also be set simply by looking at the display on the display means. is easy.

In claim 4, when a predetermined difference occurs stepwise between the sensor output voltage and the output voltage of the sensor output holding circuit, the ventilation fan control output and the gas alarm output are set according to the magnitude of the difference. In addition to producing control outputs such as It can be carried out.

In addition, in claim 5, by providing a control means for operating monitoring equipment such as an ammeter and a recording device based on the difference between the sensor output voltage and the output voltage of the sensor output holding circuit, it is possible to eliminate the need for adjustment. A contaminant gas monitoring device that is easy to use can be configured.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of one embodiment of the present invention, Fig. 2 is an explanatory diagram of the same operation as above, Fig. 3 is a circuit diagram of another embodiment of the invention, Fig. 4, Fig. 5 a, b, Figure 6 a, b
7 are circuit diagrams of other embodiments of the present invention, respectively.
8 shows a conventional example, in which 1 is a metal oxide semiconductor sensor, 5 is a voltage comparator, 8 is a capacitor, 14 is a subtracter, 15 is a sensor output holding circuit, 30 is a ventilation fan, 32 ₂ to 32 ₄ is a light emitting diode, 33 is a gas alarm means, 34 is a changeover switch,
37 is a control changeover switch, and 39 is a monitoring device.

Claims (1)

[Claims] 1. A metal oxide semiconductor sensor that outputs an output voltage depending on the degree of air pollution is provided, and the output voltage of the sensor is determined by comparing the output voltage from this sensor with the voltage fed back from the output terminal. is higher, the voltage comparator outputs a signal, and the voltage comparator outputs a signal that measures the charge on the capacitor with a time constant that is approximately the same as the time fluctuations of general atmospheric conditions such as temperature and humidity. Voltage comparison is performed using a first time constant circuit that has a large time constant to allow the discharge to occur, and a time constant that is similar to the time fluctuation of the output voltage of the sensor when the sensor is placed in normal general air rather than in polluted air. a second time constant circuit that charges the capacitor with the signal when the output of the device is a signal with a polarity opposite to that of the above signal, and a second time constant circuit that subtracts the charging voltage of the capacitor from a preset voltage and converts the output voltage to the above voltage. Along with feedback to the comparator,
A sensor output holding circuit is formed with a subtracter that outputs an output voltage from the output terminal, and when a predetermined difference occurs between the sensor output voltage and the output voltage of the sensor output holding circuit, an operating output is output, and the above-mentioned A contaminated gas detection control device comprising a control means for stopping an operation output when the difference disappears. 2 A metal oxide semiconductor sensor is provided that outputs an output voltage depending on the degree of air pollution, and the output voltage from this sensor is compared with the voltage fed back from the output terminal, and if the output voltage of the sensor is higher, is a voltage comparator that outputs a signal, and a time constant that is approximately the same as the time fluctuation of the general atmosphere such as temperature and humidity, and is large enough to discharge the charge in the capacitor when a signal is output from the voltage comparator. A first time constant circuit having a time constant, and a voltage comparator whose output voltage is set to the above-mentioned signal with a time constant comparable to the time fluctuation of the output voltage of the sensor when the sensor is placed in normal general air rather than in polluted air. a second time constant circuit that charges the capacitor with the signal when the signal is of opposite polarity; and a second time constant circuit that subtracts the charging voltage of the capacitor from a preset voltage and feeds back the output voltage to the voltage comparator. ,
A sensor output holding circuit is formed with a subtracter that outputs an output voltage from the output terminal, and a variable means is provided to arbitrarily vary the difference between the sensor output voltage and the output voltage of the sensor output holding circuit. Contaminant gas detection characterized by comprising a control means for outputting an operating output when the difference is greater than a value set by the variable means and for stopping the operating output when the difference disappears. Control device. 3 A metal oxide semiconductor sensor is provided that outputs an output voltage depending on the degree of air pollution, and the output voltage from this sensor is compared with the voltage fed back from the output terminal, and if the output voltage of the sensor is higher, is a voltage comparator that outputs a signal, and a time constant that is approximately the same as the time fluctuation of the general atmosphere such as temperature and humidity, and is large enough to discharge the charge in the capacitor when a signal is output from the voltage comparator. A first time constant circuit having a time constant, and a voltage comparator whose output voltage is set to the above-mentioned signal with a time constant comparable to the time fluctuation of the output voltage of the sensor when the sensor is placed in normal general air rather than in polluted air. a second time constant circuit that charges the capacitor with the signal when the signal is of opposite polarity; and a second time constant circuit that subtracts the charging voltage of the capacitor from a preset voltage and feeds back the output voltage to the voltage comparator. ,
A sensor output holding circuit is formed with a subtracter that outputs an output voltage from the above output terminal, and an operation output is generated when a predetermined difference occurs stepwise between the sensor output voltage and the output voltage of the sensor output holding circuit. A control means is provided to output the operation output in stages and to stop the operation output when the above-mentioned difference disappears, and a plurality of display means each display the operation output in accordance with the order of the magnitude of the operation output of the control means, and this arbitrary A contaminant gas detection control device characterized in that it is provided with selection means capable of extracting stepwise operation outputs of the control means when the display means is operated. 4 A metal oxide semiconductor sensor is provided that outputs an output voltage depending on the degree of air pollution, and the output voltage from this sensor is compared with the voltage fed back from the output terminal, and if the output voltage of the sensor is higher, is a voltage comparator that outputs a signal, and a time constant that is approximately the same as the time fluctuation of the general atmosphere such as temperature and humidity, and is large enough to discharge the charge in the capacitor when a signal is output from the voltage comparator. A first time constant circuit having a time constant, and a voltage comparator whose output voltage is set to the above-mentioned signal with a time constant comparable to the time fluctuation of the output voltage of the sensor when the sensor is placed in normal general air rather than polluted air. a second time constant circuit that charges the capacitor with the signal when the signal is of opposite polarity; and a second time constant circuit that subtracts the charging voltage of the capacitor from a preset voltage and feeds back the output voltage to the voltage comparator. ,
A sensor output holding circuit is formed with a subtracter that outputs an output voltage from the output terminal, and when a predetermined difference occurs stepwise between the sensor output voltage and the output voltage of the sensor output holding circuit, the difference is A contaminated gas detection and control device, characterized in that it is provided with a control means that outputs control outputs such as a ventilation fan control output and a gas alarm output according to the size, and stops the control output when the above-mentioned difference disappears. 5 A metal oxide semiconductor sensor is provided that outputs an output voltage depending on the degree of air pollution, and the output voltage from this sensor is compared with the voltage fed back from the output terminal, and if the output voltage of the sensor is higher, is a voltage comparator that outputs a signal, and a time constant that is approximately the same as the time fluctuation of the general atmosphere such as temperature and humidity, and is large enough to discharge the charge in the capacitor when a signal is output from the voltage comparator. A first time constant circuit having a time constant, and a voltage comparator whose output voltage is set to the above-mentioned signal with a time constant comparable to the time fluctuation of the output voltage of the sensor when the sensor is placed in normal general air rather than polluted air. a second time constant circuit that charges the capacitor with the signal when the signal is of opposite polarity; and a second time constant circuit that subtracts the charging voltage of the capacitor from a preset voltage and feeds back the output voltage to the voltage comparator. ,
A control means that forms a sensor output holding circuit with a subtracter that outputs an output voltage from the output terminal, and operates a monitoring device such as an ammeter or a recording device based on the difference between the sensor output voltage and the output voltage of the sensor output holding circuit. A contaminated gas detection control device comprising: