Alcohol vapor sensor. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Electronics in medicine

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The amount of alcohol consumed "per capita" (more precisely, per body) of alcohol is in some cases very critical (for example, for drivers). In many European countries (Germany, Finland, Poland, etc.), a few years ago, alcohol vapor detectors, or the so-called "alcotesters" (Roadtest), appeared on the free market. Of course, these are not professional devices, but they also allow you to control the "smell" and evaluate your condition after taking something "warming". Breathalyzer options. manufactured by different companies, there are many, but there are no similar devices of domestic production on free sale yet.

"Alcotester" (Fig. 1) is an analyzer of lars of alcohol, as well as toluene, xylene and other volatile organic substances. In the upper part of the device there is a replaceable plastic tube, which is designed for blowing air through the mouth (into the device). When the power is turned on, a short sound signal (peak-peak) is heard, after 1 ... 2 s - the second (similar to the first), and the word "Wait" flashes on the indicator (below, under the numbers). During this period of 10...12 s, the sensor enters the measurement mode. After that, the third beep indicates that. that the device is ready for operation (for the analysis of the received air). At the same time on the indicator "Wait" is replaced by "Ready".

If after the third signal "do not blow into the tube *", the device perceives the same air that it has already analyzed, and, not finding differences, within 10 ... 12 s issues a negative verdict (in medicine, a negative result is considered good, not confirming the diagnosis ).This state is indicated on the indicator by the inscription "OFF" (without any signals).The auto-off system turns off the power of the device after about 1.5 minutes (to save batteries).When the presence of alcohol vapor is detected in the breath, the device gives digital readings on the indicator (maximum 4000 - you just need to forget about the car) and confirms its study with endless signals, which can be turned off either with the "Reset" button (testing first) or "Power".

A special sensor for the presence of impurities in the air of the TGS-2620 type (Taguchi Gas Sensor - Fig. 2) from Figaro Engineering Inc (Japan) is installed in the device. The pioneer of these sensors in 1962 was the Japanese inventor N. Taguchi. Most TGS sensors (TGS-822, TGS-2620) are based on tin oxide. The resistance of these sensors to direct current in ordinary air is high, and in the presence of impurities (vapors of organic origin) in the air, the resistance decreases sharply.

The dependence of the relative resistance of the sensor on the gas concentration is shown in Fig.3. The sensors are not universal, for example, the alcohol vapor sensor does not respond to the presence of freon. A typical sensor switching circuit is shown in Fig.4.

(click to enlarge)

If you connect such a sensor to a comparator (comparison device), the latter will react to a change in the resistance of the sensor and turn on the alarm. For effective operation of the sensors, a constant voltage of about 5 V is required. Therefore, such a device can be successfully used with autonomous power supply, for example, from 3-4 miniature AAA batteries. Only the cost of sensors upsets - almost 50 USD. The proposed device operates on a similar principle, with the only difference that it does not have intermediate sound signals and digital indication, but shows only two states: drunk (the sound lasts until the power is turned off) or not drunk (no sound).

The scheme of the "Alcotester" using the TGS-2620 sensor is shown in Fig.5.

(click to enlarge)

The supply voltage for the sensor heater B1 is supplied to its output 1, the output of the heater 4 is connected to the common wire. To process the output signal of the sensor, the comparator DA2 (K554SAZ) is used, which compares the voltages at its inputs. The non-inverting input of the comparator is connected to output 2 of the sensor, and the inverting input is connected to the R2-R3-R4-R5 chain, which creates a temperature-compensated (due to the thermistor R2) reference voltage U0.

Operational amplifier DA1 with elements VD1, R6, C2, R7, R9 provides a delay of 1 ... 1,5 minutes, necessary to eliminate false positives when the power is turned on and measurements. Without this delay, the device may sound an alarm regardless of the presence of alcohol vapor.

When a change in the output voltage of B1 under the influence of alcohol vapor with a concentration exceeding the set limit triggers the comparator DA1, a high level at its output ensures that the sound capsule with a built-in generator HA1 or other alarm device is turned on.

Without a temperature-compensating circuit, the sensor signal could vary in the range of 600 ... 3400 ppm at a given value of the alcohol vapor concentration of 1500 ppm (at an ambient temperature of 20°C and a humidity of 65%). The dependence of the relative resistance of the sensor on the ambient temperature is shown in Fig.6.

Thermistor R1 is used for temperature compensation. The voltage U0 can vary in the range of 2,5...3,2 V at an ambient temperature of +40°C and a relative humidity of 65% and, accordingly, in the range of 1,9...3,1 V at a temperature of -10° WITH.

The results of using a temperature-compensating resistor are presented in the table.

Capacitor C1 smooths out voltage ripples when the device is powered from a mains power source. Diode VD1 prevents the leakage current of the oxide capacitor C2, details. Thermistor R1 - type NTC. MMT or similar. The VT1 transistor is replaced by KT601, KT603, KT940 with any letter index, the DA1 chip is replaced by KR140UD1208, KR140UD6. Diode VD1 - KD521, KD522 with any letter index. Oxide capacitors CI, C2 - type K50-29 or similar.

Sound emitter HA1 - any with a built-in AF generator for a voltage of 5 ... 12 V. For additional light signaling, in parallel with the HA1 capsule, you can connect an LED (AL307BM type or similar with a current of up to 10 mA) with an anode to the "+" power supply with a resistor connected in series 470...750 Ohm. The power supply of the device (if you do not use a battery) is stabilized, with a voltage of 5 V (± 5%). Current consumption does not exceed 70 mA (excluding LED indicator). Except TGS-2620 sensor. In this circuit, Murata TGS-880, TGS-2181 or TGS-822 sensors (supply voltage - 12...15 V) can be used.

Author: A.Kashkarov, G.S. -Petersburg

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