Carbon monoxide concentration analyzer. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Carbon monoxide (carbon monoxide, CO) is one of the most toxic combustion products. It is part of the smoke and is released during smoldering and burning of all organic and carbon-containing substances. It is a highly poisonous gas. It has neither color nor smell, which makes it especially dangerous, making it difficult to detect in a timely manner. A person can feel the presence of carbon monoxide in the air only by the first symptoms of carbon monoxide poisoning, and this is very bad. Intoxication occurs imperceptibly and grows rapidly. Sometimes it is too late to make any attempts to save, as carbon monoxide is able to "turn off" a person's consciousness in a matter of seconds.

The main effect of carbon monoxide on the human body is to bind hemoglobin in red blood cells. By doing this, it blocks the path of oxygen to the cells, the body simply cannot continue to function. Unfortunately, one has to deal with this dangerous poison constantly in everyday life, and not only in a big city (on busy roads, near gas stoves and water heaters), but also in rural areas (near large highways, in baths and in houses with stove heating). ). Therefore, it is very desirable to have an indicator of the dangerous concentration of carbon monoxide in the air at home.

Figaro Engineering produces an inexpensive and reliable electrochemical sensor of this gas with a liquid electrolyte TGS5042 [1-3]. In terms of size and appearance, it is similar to a conventional AA cell and operates in the temperature range from -40 to 70 ^оC at CO concentration from 0 to 10000 ppm. Compared to other similar sensors, the TGS5042 has a number of advantages. It uses a weakly alkaline electrolyte that meets all the requirements of environmental safety, there are no electrolyte leaks from the body, wear of the electrodes and the consumption of chemical materials of the sensor during operation. It has low sensitivity to other gases, low cost, long service life and easy calibration.

(click to enlarge)

This sensor is used in the described device, the circuit of which is shown in the figure. The device detects the presence of carbon monoxide in the air and measures its concentration in the range of 1-999 parts per million (ppm). The device displays the result of the measurement on a three-digit seven-element LED indicator HG1, when the gas concentration exceeds 100 ppm, a signal is generated by the sound emitter HA1.

The maximum permissible concentrations of pollutants in the air are set in the Russian Federation in milligrams per cubic meter [4, 5]. According to these documents, the concentration of carbon monoxide in the open air should not exceed 3 mg/m³ (average daily) and 5 mg/m³ (peak), and in indoor air - 20 mg/m³ throughout the working day, 50 mg/m³ - within an hour, 100 mg/m³ - within 30 minutes or 200 mg/m³ within 15 min. For carbon monoxide, 1 mg/m3 is equivalent to 0,86 ppm.

The output current of sensor B1 is directly proportional to the concentration of carbon monoxide in the ambient air with a conversion factor

1,2...2,4 nA/ppm. With the help of the op-amp, which is part of the DA1 chip (MAX9001ESD), the sensor current is converted into voltage, which is measured by a digital voltmeter built on the DA2 and DD1 chips. With a sensor conversion factor of 2 nA/ppm and R1=500 kΩ, a carbon monoxide concentration of 1000 ppm corresponds to a voltage of 1 V at the output of the op amp.

In the DA1 chip, in addition to the op amp, there is a precision reference voltage source of 1,23 V and a voltage comparator with a hysteresis zone width of 2 mV. One of the inputs of the comparator in the device is supplied with voltage from the output of the op amp, and the second is supplied with a reference voltage of 2 mV obtained using a resistive divider R3R100, which corresponds to a concentration of carbon monoxide of 100 ppm. If the voltage from the output of the op-amp exceeds this value, the logic level at the output of the comparator will become low, the field-effect transistor VT2 will open and the supply voltage will be supplied to the sound emitter HA1 through it. The frequency of the oscillator built into the emitter is set by the capacitor C4.

Chip DA2 (CA3162E) is a digital voltmeter with a measurement interval of 0-999 mV, equipped with a dynamic indication of the result. To work with a three-digit seven-element LED indicator, you only need to add a DD1 code converter (CA3161E) and three VT3-VT5 transistor switches.

To prevent polarization of the sensor, when the power is off, it is necessary to connect its outputs to each other. For this, a p-channel field-effect transistor VT1 (J177) is designed, which is open in the absence of power, but closes when a voltage of +5 V is applied to its gate relative to the source. The power supply must be stabilized and designed for a load current of at least 200 mA.

The adjustment of the device begins with the calibration of the voltmeter. First, the input of the DA2 chip (pin 11) is temporarily disconnected from pins 3 and 10 of the DA1 chip and connected to a common wire (minus power). The trimming resistor R4 achieves zero readings of the indicator. Then served on the output. 11, a constant voltage of +999 mV and a trimming resistor R5 set the number 999 on the indicator. After that, the connection of pins 3 and 10 of the DA1 chip with pin 11 of the DA2 chip is restored.

Information about the individual conversion factor of the TGS5042 sensor is available on the body of each of its instances. If it differs from 2 nA / ppm, then the resistance of the resistor R1 must be changed inversely with this coefficient. The exemplary voltage at the input of the comparator (pin 11 DA1), corresponding to the required threshold for turning on the sound signal, is set by a selection of resistors R2 and R3. It is advisable to use resistors R1-R3 with a maximum resistance deviation from the nominal value of at least ± 1%.

Literature

1. Kraszewski R. A new series of carbon monoxide sensors TGS5042 from Figaro Engineering. - CHIP NEWS Ukraine, No. 4 (114), May, 2012, p. 44-46.
2. Romanova I. Highly sensitive gas sensors, novelties from FIGARO ENGINEERING. - "Electronics: NTB", 2011, No. 1 (00107), pp. 64-70.
3. TGS 5042 - for the detection of Carbon Monoxide. - URL: figarosensor.com/products/5042pdf.pdf.
4. "Maximum Permissible Concentrations (MPC) of Pollutants in the Atmospheric Air of Populated Areas". Hygienic standards GN 2.1.6.1338-03 (approved by the Chief State Sanitary Doctor of the Russian Federation on June 25, 2003). - URL: ohranatruda.ru/ot_biblio/normativ/data_normativ/42/42030/index.php.
5. "Maximum Permissible Concentrations (MPC) of harmful substances in the air of the working area". Hygienic standards GN 2.2.5.1313-03 (approved by the Chief State Sanitary Doctor of the Russian Federation on April 27, 2003). - URL: norm-load.ru/SNiP/Data1/42/42033/index.htm.

Author: A. Kornev

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