How to Understand Non-Invasive Breath Analyzers

6396
Advertisement

Key Features

  • STM8L low power MCU based reference design to measure the Blood Alcohol Concentration (BAC) non-invasively.
  • Fuel cell sensor is used because of its greater specificity as compared to semiconductor sensors. This is especially important in reducing false positives due to acetone, which can occur in substantial qualities in the breath of diabetics.
  • Current Integration Method of fuel cell output is used to determine the BAC. This method gives very stable short term and long term calibration stability.
  • A rechargeable 3.7V Lithium Ion battery is used to power the system.
  • Low battery detection is implemented using STM1061N31 voltage detector designed specifically for single cell Li-ion based solutions.
  • Glass LCD with 6 alphanumeric characters to display information.
  • Automatic LCD backlight is implemented using ALS to make the LCD screen visible in darkness.
  • Can save up to 396 BAC readings along with date and time stamp in an on-board dual interface EEPROM.
  • Android Application for data logging with RF EEPROM over NFC Interface.
  • On board battery charging circuit based on STC4054GR powered using external 5V wall adapter.
  • Ultra low standby power consumption in the order of 8uA including timekeeping operations by virtue of efficient hardware and firmware design.

Typical Applications

  • Breath Analyzers are being used by Law enforcement.
  • It is used as an Ignition Interlock device in vehicles.
  • It can also be used handy by individuals to ensure that they can drive legally and safely.

Response curves
Normally two methods viz. Peak voltage method and Current Integration methods are used to calculate the final BAC values.

In case of current integration method, the readings from the Breath Analyzer Sensor are captured and then converted to an equivalent voltage through some associated circuitry. The voltage so obtained is directly proportional to the electric current input from the sensor. The graph of the captured signal is shown in Figure 4 for normal as well as drunken person.

Figure 4: Response curve for (a) non-alcoholic person; (b) alcoholic person
Figure 4: Response curve for (a) non-alcoholic person; (b) alcoholic person
Advertisement

A current threshold is defined to fix the minimum resolution and the values above that threshold signify the alcohol content of a drunken person. A time window is defined to capture the breath through the Breath Analyzer sensor. All the values above the current threshold are integrated and averaged within that time interval to get the final BAC values.

Figure 5: Current to Voltage converter using operational amplifier
Figure 5: Current to Voltage converter using operational amplifier

On the other hand in case of peak voltage method, the current output from the Breath Analyzer sensor is fed to an associated circuitry to convert to an equivalent voltage as shown in Figure 5. The output voltage peaks are then processed by the associated microcontroller unit and then internal ADC inside microcontroller is actuated and finally ADC output is post-processed to quantify the BAC.

Chemical equations
In case of fuel cell sensor (used in the STMicroelectronics solution), two terminals viz. anode and cathode are available. When a drunken people breath is sampled, the alcohol content in its breath is sensed by anode terminal and converted to ethanoic acid a.k.a. acetic acid.

CH_3 CH_2 OH(Alcohol)+ H_2 O(Water)→CH_3 COOH(Ethanoic acid)+4H^++4e^-

Also at the cathode terminal, the oxygen present in the atmosphere is reduced to water.

<1--nextpage-->

O_2 (Atmospheric oxygen)+ 4H^++4e^-→ 2H_2 O(Water)

As a whole chemical system, the products are ethanoic acid and water as shown in the equation below:

CH_3 CH_2 OH(Alcohol)+O_2 (Atmospheric oxygen) →CH_3 COOH(Ethanoic acid)+H_2 O(Water)

Figure 6: Fuel Cell Based Breath Analyzer Sensor chemical reactions
Figure 6: Fuel Cell Based Breath Analyzer Sensor chemical reactions

Figure 6 shows the fuel cell based Breath Analyzer sensor chemical reactions and the current flow mechanism. The electric current is produced because of flow of electrons between anode and cathode as shown in the above equation. This current is proportional to the concentration of alcohol content at the anode terminal. It is captured by the microcontroller and post processed to get the BAC value on to the LCD display.

Device Calibration
Fuel cell based Breath Analyzer sensors are susceptible to errors due to anode/cathode corrosion/infection by the dirt and other chemical reactions in addition to ethanol. This requires the filth removal and calibration which is normally done once a year.

Breath Analyzer calibration process

Various methods of calibrating these sensors are available viz. dry and wet processes. The dry method involves passing the gaseous mixture of nitrogen and ethanol through the fuel cell filaments and then calibrating the sensor using hand-held device. This method requires less cost however accuracy available is also less. In case of wet method, the fuel cell filaments are soaked into ethanol and water solution and then system is simulated for calibration. This calibration strategy is expensive and requires bulky instruments; however, the higher calibration accuracy could be achieved.

The BAC value measured by the Breath Analyzer device is given by,

BAC %= (ADC value)/(Threshold)

Here ADC value is the value obtained after current integration and Threshold is the reference threshold to achieve the correct BAC%. During calibration, Threshold is normally changed to achieve the corrected results. Default Threshold in the demo board developed on STMicroelectronics’s STM8L device is 0x5000. If the Breath Analyzer sensor is exposed to some sample solution/mixture for which BAC% is previously known, the Threshold could be adjusted such that the output BAC% from the Breath Analyzer device matches exactly with the known BAC% value.

Figure 8: Calibrating Breath Analyzer device using STMicroelectronics Android App, Nfc-V-reader
Figure 8: Calibrating Breath Analyzer device using STMicroelectronics Android App, Nfc-V-reader

An Android based application, NfcV-reader, has been developed by STMicroelectronics to communicate with NFC enabled smartphones and change the Threshold value of Breath Analyzer device. This application can be directly downloaded from the Google Play Store.

To change the Threshold value, place a NFC enabled smartphone at defined location as shown in Figure 7 and Figure 8 and write the desired Threshold value (in hexadecimal) at 0x0000 location. After updating the value the device shall take the new Threshold value for BAC% calculations.

Conclusions
Convenient monitoring of Breath Alcohol content and in turn Blood Alcohol content is necessary for real-time applications viz. law enforcement, ignition interlock and self-assessment for safe driving. This can be made possible by portable machines designed specifically for such monitoring. The Breath Analyzer reference design developed by STMicroelectronics is low power (require rechargeable 3.7V Lithium Ion battery) and requires ultra-low standby power consumption. The sensor used in this design is of fuel cell type and STM8L microcontroller unit is used for data processing. In addition, STMicroelectronics free Android App ‘NfcV-reader’ provides quick calibration of the device. The present solution could be inchoatus for modern policing and law control.


Authors

· Salil Jain, Sr. Design Engineer, IPD System Lab and Technical Marketing, STMicroelectronics, India

· Alok Mittal, Group Manager, IPD System Lab and Technical Marketing, STMicroelectronics, India

· Saurabh Sona, Technical Leader, IPD System Lab and Technical Marketing, STMicroelectronics, India

Advertisement


1 COMMENT

SHARE YOUR THOUGHTS & COMMENTS

Please enter your comment!
Please enter your name here