JPH11304745A - Gas detecting device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas detecting device which can prevent the disconnection of the built-in heater of a semiconductor gas detecting element even when the runaway of a microcomputer in charge of control and detection is generated. SOLUTION: When the runaway of a microcomputer 3 is generated due to an external noise, the output port 01 of the microcomputer 3 is fixed to an 'L' level. Then, the electric charge of a capacitor CO in a protective circuit 5 is discharged, and the voltage of the capacitor CO soon becomes less than the voltage of the connection point of resistances R6, R7. As a result, the output of a comparator CP is inverted from an 'H' level to the 'L' level, a transistor Q4 is turned on, the voltage of the base of a transistor Q1 is lifted up to a power-supply voltage, and the transistor Q1 is set forcibly to an OFF state.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a gas detection device for detecting the CO and CH ₄ gas by using a semiconductor gas sensing element.

[0002]

2. Description of the Related Art A heater is built in a semiconductor gas detecting element used in this type of gas detecting device, and the power supply to the heater is turned on and off at a predetermined cycle by a control circuit, so that the surface of the gas-sensitive body of the semiconductor gas detecting element is exposed. It is designed to be refreshed.

On the other hand, a gas detection device using a semiconductor gas detection element includes a control circuit for controlling the energization of the heater and a circuit for taking in the detection output of the semiconductor gas detection element and determining the presence or absence of a gas to be detected. The circuit is simplified by using a bit microcomputer.

FIG. 3 shows a circuit example of a heater energizing portion of a conventional gas detecting device. In this conventional example, a heater 2 in a semiconductor gas detecting element 1 is powered by a power supply (for example, a DC power supply of 5 V).
To the power supply via a transistor Q2 for controlling the detection voltage application and a load resistor R1, and to the microcomputer 3 via a PNP transistor Q1. Connected to the input port of The base of the transistor Q1 is connected to the positive pole of the power supply via a pull-up resistor R2 and to the output port O1 of the microcomputer 3 via a resistor R3. The base of the transistor Q2 is connected to the output port O2 of the microcomputer 3.

The microcomputer 3 performs duty control of energization of the heater 2 so that the temperature of the gas-sensitive body is set to a high temperature (for example, 5 sec) and a low temperature (15 sec.).
sec). For example, during the high temperature period, the heater 2 is energized for 300 μ
In the low temperature period, the energization of the heater 2 is set to 12 μsec in a cycle of 8 msec. The microcomputer 3 turns on the transistor Q1 to turn on the transistor Q1 by turning the output port O1 to "L", and turns on the transistor Q1 by turning it to "H".
A function of performing duty control of energization of the heater 2 for turning off the heater 1 and stopping the energization of the heater 2;
L ", the transistor Q2 is turned on to apply a detection voltage to the gas-sensitive body. At this time, the voltage across the gas-sensitive body is taken into the input port I1, and the resistance value of the gas-sensitive body is determined based on the voltage. And a detection function for judging the presence or absence of the gas to be detected based on the resistance value thereof. The sampling (sampling) of the voltage between both ends of the gas sensing element is performed when the gas to be detected is CO. When the detected gas is a flammable gas such as CH ₄ , the voltage between both ends of the gas sensitive body is taken in a high temperature period.

[0006]

When the microcomputer 3 is used as described above, the microcomputer 3 may fall into a so-called runaway state under the influence of external noise. In order to escape from this runaway state, if the power supply is once turned off and then turned on again, the microcomputer 3 is reset and returns to the normal operation, but the built-in heater 2 of the semiconductor gas detecting element 1 is energized as described above. Since the output time is extremely short, the output port O1 is fixed at "L" level due to runaway and the power supply to the heater 2 becomes long. Was.

As a measure against runaway of the microcomputer 3, it is conceivable to use a watchdog timer.
There was a limit to shortening the detection time required to perform runaway detection. For example, in the case of a 4-bit microcomputer operating at a clock of 4 MHz, about 20 msec is required, and the detection time is longer than the above-described energization period, so that the heater 2 is energized during the detection time and the wire is disconnected. However, there was no measure against disconnection of the heater.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and has as its object to prevent disconnection of a built-in heater of a semiconductor gas detection element even when a microcomputer which controls and detects control runs away. It is to provide a gas detection device which can be used.

[0009]

According to the first aspect of the present invention, there is provided a semiconductor gas detecting element having a built-in heater, a switching element inserted between the heater and a power supply, and the switching element being turned on at a predetermined cycle. A microcontroller having a control function of outputting a control signal voltage to be turned off from an output port to a control terminal of the switching element and a detection function of taking a detection output of a semiconductor gas detection element and determining the presence or absence of a gas to be detected based on the detection output. In a gas detection device provided with a computer, a voltage applied to a control terminal of the switching element is turned off so as to turn off the switching element when a predetermined time has elapsed since the control signal voltage became a voltage for turning on the switching element. A protection circuit for controlling the level is added.

According to a second aspect of the present invention, in the first aspect, the switching element is a PNP transistor, and the output port of the microcomputer connected to the base of the transistor is "H" in the protection circuit. And a switching element that is turned on when the voltage of the capacitor reaches a predetermined level or less and raises the base potential of the transistor to an “H” level. The discharge time constant is set so that the voltage of the capacitor does not fall below the predetermined level until the predetermined time has elapsed after the signal has reached the "level".

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to embodiments.

FIG. 1 shows an embodiment of the present invention.
In the circuit shown in this figure, the AC commercial power supply AC is stepped down by a step-down transformer Tr, the voltage is stepped down and full-wave rectified by diode bridges DB1 and DB2, respectively, and the voltage rectified by one diode bridge DB2 is smoothed by a smoothing capacitor C1. After that, the voltage is stabilized at a predetermined voltage by the three-terminal regulator IC1, and the voltage and the detection voltage of the heater 2 of the semiconductor gas detection element 1 and the power supply of the 4-bit microcomputer 3 for controlling and detecting are obtained.

The semiconductor gas detecting element 1 includes a heater 2 connected to a PN
It is connected to the output terminal of the three-terminal regulator IC1 via a P-type transistor Q1, and when the transistor Q1 is turned on, the heater 2 is energized and the heater 2 generates heat.

The detection output terminal of the gas sensing element of the semiconductor gas detection element 1 is connected to a detection voltage application control transistor Q2.
1 and a series circuit of a load resistor R11, a series circuit of a detection voltage application control transistor Q22 and a load resistor R12, and a diode D1 connected to an output terminal of a three-terminal regulator IC1.
Is connected to the input port I1.

The microcomputer 3 has an output port O1.
And the output port O2
The bases of the transistors Q21 and Q22 are connected to 1, O22. The output ports O3 to O5 are connected to cathodes of display light emitting diodes LED1 to LED3, respectively, and the output ports O6 and O7 are connected to photocouplers PH.
1, the cathodes of the light emitting diodes L1 and L2 of PH2 are connected. These light emitting diodes LED1 to LED3
And the anodes of L1 and L2 are three-terminal regulator IC1
Is connected via a current limiting resistor to the output terminal.

The photocouplers PH1 and PH2 are used as switching elements for outputting a gas detection signal corresponding to the concentration of the detection gas or the like as a voltage signal.

The series control type stabilizing circuit 4 is a circuit for outputting the voltage signal.
The reference voltage connected to both ends of the smoothing capacitor C2 for smoothing the rectified output of B1 and connected to the base of the series control transistor Q3 is switched by turning on and off the phototransistors PT1 and PT2 of the photocouplers PH1 and PH2. .

The reference voltage is obtained by dividing the voltage between both ends of a Zener diode ZD connected to both ends of a smoothing capacitor C2 via a resistor R4 by resistors R11 to R13. When the phototransistor PT1 is on, the resistors R11 to R11 are turned on. The voltage across the series circuit of R13, that is, the voltage across the Zener diode ZD is applied as a reference voltage to the base of the transistor Q3. When the phototransistor PT2 is on, the resistors R11, R12 and R13
Is applied as a reference voltage to the base of the transistor Q3, and a voltage signal corresponding to each reference voltage is output to the outside as a gas detection signal.

A pull-up resistor R2 is connected between the emitter and the base of the transistor Q1.
A transistor Q4 is connected to both ends of the pull-up resistor R2.

The transistor Q4 is connected to the comparator C
This constitutes a protection circuit 5 for the heater 2 together with P and the like.

That is, the protection circuit 5 comprises a series circuit of the resistors R6 and R7 connected to the three-terminal regulator IC through the diode D1.
1 to divide the output voltage, connect the voltage dividing point to the inverting input terminal of the comparator CP, and connect the resistor R8 and the capacitor C0 between the output port O1 of the microcomputer 3 and the ground. The series circuit is connected, the midpoint of the series circuit is connected to the non-inverted output terminal of the comparator CP, and the output terminal of the comparator CP is connected to the base of the transistor Q4 via the resistor R9.

A buzzer 6 is a piezoelectric buzzer, and has two comparators CP1 and CP2 which alternately output "L" and "H" in response to signals output alternately from output ports O8 and O9 of the microcomputer 3. , The polarity of the applied voltage is alternately inverted, and an alarm sound is oscillated and output. In the drawing, reference numeral 7 denotes a reference clock oscillation circuit for supplying a reference clock to the microcomputer 3, and IC2 denotes a reset IC for resetting the microcomputer 3 when power is turned on.

Normally, when the output port O1 of the microcomputer 3 is at "H" level, the capacitor C0 is charged via the resistor R8, and the charged voltage is the voltage at the connection point between the resistors R6 and R7. The output of the comparator CP becomes "H" level when the output voltage exceeds the threshold, and the transistor Q4 is turned off.

A predetermined period (8 msec) from the output port O1 of the microcomputer 3 for a predetermined period of time (for example, 300 μsec in a high temperature period, 12 μsec in a low temperature period)
c) An "L" level control signal is output, and during the predetermined time, the transistor Q1 is turned on and the heater 2 of the semiconductor gas detection element 1 is energized. On the other hand, when the output port O1 of the microcomputer 3 becomes "L" level, the charge of the capacitor C0 of the protection circuit 5 is discharged through the resistor R8, and the voltage across the capacitor C0 decreases. Until the predetermined time elapses, the voltage of the capacitor C0 does not become lower than the voltage of the connection point between the resistors R6 and R7. Therefore, even during the predetermined time, the output of the comparator CP does not become "L" level, and the transistor Q
4 is maintained in the off state.

In the normal state, when the predetermined time has elapsed, the microcomputer 3 sets the output port O1 to "".
The transistor Q1 is turned off by returning to the H "level. When the transistor Q1 is turned off, the energization of the heater 2 of the semiconductor gas detection element 1 is stopped.

By performing the duty control for energizing the heater 2 at a predetermined cycle for a predetermined time in this manner, a high-temperature period (for example, 5 sec) in which the temperature of the gas sensitive body is high and a low-temperature period (for example, 15 sec) in which the gas sensitive body is low are alternately performed. It is set to.

Here, when the gas to be detected is CO, a low temperature period
In the meantime, the microcomputer 3 is connected to the output port O2
1 (or O22 at "L" level)
By turning on 21 or Q22, the load resistance R2
1 (or R22) to sense the gas of the semiconductor gas detecting element 1
A detection voltage is applied to the semiconductor body, and the semiconductor
The voltage across the gas sensing element of the gas detection element 1 is input to the input port I1.
Sampling to determine the presence or absence of the detected gas
Operation, and detects the resistance value of the gas-sensitive material from the voltage between both ends.
The presence or absence of the gas to be detected is determined based on the resistance value.
The density determination is performed. If the concentration of the detected gas is
Then, the outputs of the output ports O8 and O9 are alternately inverted and the
The user 6 is sounded and an alarm is issued. At the same time,
Any of the output ports O3 to O5 to "L" level
And the light-emitting diodes LED1 to L connected thereto.
One of ED3 is turned on. Also, depending on the detected density,
Set the output port O6 or O7 to "L" level to respond
Turn on photocoupler PC1 or PC2 to stabilize
The circuit 4 outputs a predetermined voltage signal to the outside. CH
_FourIn the case of flammable gas such as
Do the pulling.

When the microcomputer 3 runs away due to external noise or the like and the output port O1 of the microcomputer 3 is fixed at the "L" level, the charge of the capacitor C0 is discharged in the protection circuit 5 and the capacitor C0 is eventually discharged. The voltage is lower than the voltage at the connection point between the resistors R6 and R7. As a result, the output of the comparator CP is inverted from the “H” level to the “L” level to turn on the transistor Q4 and reduce the base voltage of the transistor Q1. The lifting transistor Q1 is forcibly turned off at the power supply voltage.

Therefore, even if the microcomputer 3 runs out of control, the heater 2 of the semiconductor gas detecting element 1 is not energized more than necessary and disconnection of the heater 2 can be prevented.

As described above, in this embodiment, the capacitor C0
By appropriately setting the time constant of the resistor R8 and the resistor R8, the breakage of the heater due to the runaway of the microcomputer 3 can be easily prevented.

The setting of the load resistance connected in series to the gas sensing element of the semiconductor gas detecting element 1 is enabled by selectively turning on either the transistor Q21 or Q22. to is programmed load resistance connected in advance in accordance with the detected gas type, a load resistance R21 connected in series to the gas-sensitive body by turning the transistor Q21 at the time of sampling, for example, in the case of CO, the case of CH ₄ and The transistor Q22 is turned on at the time of sampling to connect the load resistor R22 to the gas-sensitive body in series. Therefore, in the present embodiment, it is possible to configure an apparatus for either gas by changing the operation program of the microcomputer 3, or to configure an apparatus for both gases. .

The circuit of the embodiment shown in FIG. 1 uses the comparator CP. However, if the threshold level of the emitter / base of the transistor Q4 is used as shown in FIG. 2, the comparator becomes unnecessary and the circuit becomes unnecessary. The structure can be simplified.

[0033]

According to the first aspect of the present invention, there is provided a semiconductor gas detecting element having a built-in heater, a switching element inserted between the heater and a power supply, and a control signal for turning on and off the switching element at a predetermined cycle. A microcomputer having a control function of outputting a voltage from an output port to a control terminal of the switching element, and a detection function of receiving a detection output of the semiconductor gas detection element and determining the presence or absence of a gas to be detected based on the detection output. In the gas detection device, a voltage applied to a control terminal of the switching element is controlled to an off level so as to turn off the switching element when a predetermined time has elapsed since the control signal voltage became a voltage for turning on the switching element. A protection circuit has been added to ensure that the heater is energized even if the microcomputer runs out of control. Can reliably be stopped after a predetermined time has elapsed, there is an effect that can prevent disconnection of the heater.

According to a second aspect of the present invention, in the first aspect, the switching element is a PNP transistor, and the output port of the microcomputer connected to the base of the transistor is "H" in the protection circuit. And a switching element that is turned on when the voltage of the capacitor reaches a predetermined level or less and raises the base potential of the transistor to an “H” level. Since the discharge time constant is set so that the voltage of the capacitor does not become lower than the predetermined level until the predetermined time has passed after the signal has reached the "level", the protection circuit can be realized with a simple configuration.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of one embodiment of the present invention.

FIG. 2 is a partially omitted circuit diagram showing a main part of another embodiment.

FIG. 3 is a circuit diagram in which a part of the conventional example can be omitted.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Semiconductor gas detection element 2 Heater 3 Microcomputer 5 Protection circuit CP Comparator C0 Capacitor R6-R8 Resistance Q1, Q4 Transistor O1 Output port

Claims (2)

[Claims] A semiconductor gas detection element having a built-in heater;
A switching element inserted between the heater and a power supply, a control function of outputting a control signal voltage for turning on and off the switching element at a predetermined cycle from an output port to a control terminal of the switching element, and a semiconductor gas detection element. In a gas detection device provided with a microcomputer having a detection function of capturing a detection output and determining the presence or absence of a gas to be detected based on the detection output, a predetermined value is set after the control signal voltage becomes a voltage for turning on the switching element. A gas detection device, further comprising a protection circuit that controls an applied voltage to a control terminal of the switching element to an off level so that the switching element is turned off when a time is exceeded. 2. The switching element is a PNP transistor, and the protection circuit has an output port of a microcomputer connected to a base of the transistor.
A capacitor that is charged during the period of the H level, and a switching element that is turned on when the voltage of the capacitor reaches a predetermined level or less and raises the base potential of the transistor to the “H” level; "L"
2. The gas detection device according to claim 1, wherein the discharge time constant is set so that the voltage of the capacitor does not become lower than the predetermined level until the predetermined time has elapsed after the level has been reached.