ATmega328P-PU + MIKROE-1586 Based Breathalyzer
Created: Jan 26, 2017
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This reference design features a Breathalyzer, which is based on ATmega328P-PU microcontroller and MIKROE-1586 board. The MIKROE-1586 is an evaluation board that utilizes the MQ-3 alcohol gas sensor. The MQ-3 sensor has a SnO2 gas-sensing layer, which has low conductivity in clean air. When an alcohol gas is present, MQ-3’s conductivity increases with respect to the concentration of the alcohol gas. The MQ-3 sensor operates under 5V power supply and consumes less than 900mW power. On the MIKROE-1586 board, the MQ-3 sensor is supplied through the +5V pin and the GND pin. The board also includes a potentiometer, which acts as a load resistance to the sensor’s output. In order for the MQ-3 sensor to perform efficiently, its load resistance must be calibrated by tweaking the on-board potentiometer.
The MIKROE-1586 on-board MQ-3 sensor has a layer created from an inorganic compound Tin dioxide (SnO2). As the concentration of the alcohol gas increases, the conductivity of the sensor’s layer also increases. The MQ-3 sensor requires two voltage inputs for its heater and circuit. The heater voltage is used to supply standard working temperature to the sensor while the circuit voltage is used to supply sensor’s varying resistance and the load resistance. The load resistance is in series with the sensor. When the level of alcohol gas rises, the sensor’s conductivity increases and the voltage drop across the load resistance also increases. The sensor can detect alcohol concentration from 25ppm up to 500ppm. But the sensor’s response is not linear, so it must be tested under different concentration to get the equivalent voltage drop across the load per specific concentration. The variation in voltage across the load is monitored by the ADC of the ATmega328P-PU microcontroller. After converting the analog voltage across the load, the microcontroller displays the equivalent concentration on the OLED display.
The whole circuit is designed to be powered by eight AA batteries since it has more energy density than a 9V battery and the MQ-3 sensor consumes a lot of power during heating. The switching regulator V7805-2000R regulates the voltage from the eight AA batteries connected in series to 5V in which all the components in the circuit operate. The MQ-3 sensor must not be exposed to hazardous or corrosive gas and elements in order not to damage it. This ATmega328P-PU + MIKROE-1586 based breathalyzer can be used to check whether a person is under the influence of alcohol, especially people who are driving.