The sample area can be decimated in tune with the sample volume without compromising the sensor reading accuracy. A sample volume in the range of 0.25 cm3 to 0.50 cm3 is sufficient for precise measurement. The smaller diameter of Solenoid air pump is preferable, however, it burdens the associated electronics as the signal amplitude reduces which in turn reduces SNR (signal-to-noise ratio).
Breath Analyzer Sensors
The sampling of breath is done by Breath Analyzer sensor. Various types of these sensors are available viz. fuel-cell sensor, semiconductor sensor and spectrophotometer sensor each having some advantages and limitations over the others.
Electrochemical fuel cell breath analyzers are devices in which an electrical current is produced as a result of a chemical reaction taking place on the surface of an electrode system. In Fuel cell sensors alcohol (ethanol), undergoes a chemical oxidation reaction at a catalytic electrode surface (platinum/gold) to generate a quantitative electrical response. These sensors are highly specific and sensitive to alcohol and measurement cannot be influenced by endogenous substances such as acetone (produced by diabetics), Carbon Monoxide or Toluene. They have high calibration stability. These have an average life span of 5 years. These sensors cannot detect if breath sample is alveolar (deep lung air). As a result it may produce a falsely high reading if a subject has recently drank and still has alcohol in his mouth (highly unlikely as mouth alcohol evaporates very quickly).
Semiconductor sensors offer an affordable means to measure their breath alcohol with some compromises on reliability and accuracy. Semiconductor technology uses an oxide sensor to measure the reactivity between the tin dioxide (SnO2) in the sensor and the ethanol molecules in the breath sample. When the ethanol molecules come in contact with the tin dioxide the reaction changes the electrical resistance of the sensor. The semiconductor measures this difference and calculates an estimate of the BAC of the sample. These sensors are of affordable cost because of lower cost of manufacturing and supports low power portable systems. On the flip side, these are unstable and highly sensitive to the atmosphere, altitude and elevation.
Spectrophotometer technology is used in large, table-top breath analyzers. Spectrophotometers work by identifying molecules based on the way they absorb infrared light. The level of ethanol in a sample is singled out and measured, and a subject’s alcohol level can then be determined. These devices are expensive and are normally available on request.
Considering the higher calibration stability, longer life span and higher accuracy, electrochemical fuel cell sensors are used in STMicroelectronics “Breath Analyzer” design.
ST Microelectronics Breath Analyzer Architecture
Breath Analyzer hardware architecture as shown in Figure 2 is a portable battery operated design based on 16 MHz proprietary STM8L core. STM8L includes an integrated debug module with a hardware interface (SWIM – Single wire interface module) which allows non-intrusive In-Application debugging and ultra-fast Flash programming. The ultralow power STM8L152R8T6 operates from a 1.8 V to 3.6 V power supply. A comprehensive set of power-saving modes allows the design of low-power applications.
Breath analyzer system reference design is shown in Figure 3. The front and back panels of the reference design PCB are shown in Figure 3(a) and Figure 3(b) while Figure 3(c) depicts the packaged product. It is powered by 3.7V Lithium ion battery which can be charged by a battery charging IC STC4054GR using a wall adapter and low battery is indicated using a voltage detector IC STM1061N31WX6F. LED backlight for LCD display is switched on, automatically by sensing the ambient light intensity using ALS (Ambient Light Sensor). A condenser microphone is used to detect if a person has blown into the mouthpiece in order to actuate the pump and collect a precise, fixed volume (0.25ml) of air sample. Air sample containing alcohol interacts with fuel cell sensor which produces current proportional to alcohol concentration.
The current produced by the sensor is converted into voltage using op-amp TS507ILT configured as a trans-impedance amplifier. Integrated 12-bit ADC of STM8L is used to sample this voltage proportional to the sensor current and integrated over time. The resulting output is converted into BAC using first order linear equation coefficients derived during calculation. This information is displayed on LCD and user can optionally save this reading into Dual interface EEPROM.