Arduino Breathalyzer

What is a breathalyzer you may ask? It is a device for estimating blood alcohol content (BAC) from a breath sample.In simple terms it is a device to test weather a person is drunk or not. As the title suggests it runs on the arduino.Our breathalyzer uses the MQ-3 alcohol sensor from sparkfun.It is a simple and fun to do project.

This instructable explains how to create your own breathalyzer , making a shield for the arduino , putting the breathalyzer in a box, and making some changes to your breathalyzer.

This Breathalyzer is not meant to be used as a means of breathalyzing. The MQ-3 is not accurate enough to register exact BAC and is sensitive to temperature and humidity.Never drink and drive and if you do call me =D .

Step 1: Parts & Tools


~ Arduino Uno –

~ MQ-3 Alcohol Sensor –

~ 100k Ohm Potentiometer –

~ 330 Ohm Resistor –

~ 5 x Green LED’s –

~ 3 x Yellow LED’s – Sparkfun.conm

~ 2 x Red LED’s –

~ 7805 Voltage Regulator – Local Electronics Store

~ 1000 uf Capacitor – Local Electronics Store

~ 2 x 9v Batteries – Local Electronics Store

Arduino Breathalyzer


~ Soldering Iron –

~ Solder Wire –

~ Jumper Wires –

~ Protoboard –

~ Project Box – Local Electronics Store

~ Inhaler Tube – I Found It In My House

Step 2: The Chemistry

When the user exhales into a breath analyzer, any ethanol present in their breath is oxidized to acetic acid at the anode:

CH3CH2OH(g) + H2O(l) → CH3CO2H(l) + 4H+(aq) + 4e-

At the cathode, atmospheric oxygen is reduced:

O2(g) + 4H+(aq) + 4e- → 2H2O(l)

The overall reaction is the oxidation of ethanol to acetic acid and water.

CH3CH2OH(l) + O2(g) → CH3COOH(l) + H2O(l)

The electrical current produced by this reaction is measured by a microprocessor, and displayed as an approximation of overall blood alcohol content (BAC) by the Alcosensor.

Step 3: The Code

Heres the code for our breathalyzer:

const int analogPin = 0;    // the pin that the potentiometer is attached to
const int ledCount = 10;    // the number of LEDs in the bar graph

int ledPins[] = {
10,9,8,7,6,5,4,3,2,1 // Here we have the number of LEDs to use in the BarGraph

void setup() {

for (int thisLed = 0; thisLed < ledCount; thisLed++) {
pinMode(ledPins[thisLed], OUTPUT);

void loop() {
//This is the code to light up LED’s
int sensorReading = analogRead(analogPin);

int ledLevel = map(sensorReading, 500, 1023, 0, ledCount);

for (int thisLed = 0; thisLed < ledCount; thisLed++) {

if (thisLed < ledLevel) {
digitalWrite(ledPins[thisLed], HIGH);

else {
digitalWrite(ledPins[thisLed], LOW);
}  }}

The code was taken from

Sorry for not mentioning it before

Step 4: Creating A 5V Supply

We will use a 7805 5v regulator to take in 9v from the battery and give out steady 5v to MQ-3 alcohol sensor.We are doing this so that the MQ-3 receives enough power(<750ma).If we connect the MQ-3 directly to the arduino , there is a danger of frying it.I used a single 1000 uf capacitor with the 7805 regulator to give out 5v.I created the 5v supply on a seperate circuit board you can use the same circuit board on which we are going to solder the LEDs.I also connected a switch so that the breathalyzer can be switched on or off.See the last image for the circuit diagram.In the circuit the capacitor is between the middle pin and the right pin (See the picture tag)

Arduino Breathalyzer circuit

Step 5: Build The Circuit

The Circuit is pretty straight forward.To connect the sensor, you have to connect one of the H pin to +5V Supply (use a external power supply) and the other one to Ground.Pin B (any of them) you connect to Ground.And pin A to arduino analog pin0 via 100k ohm potentiometer.To make the LEDs work, I have connected them in sequence using the Digital Pins 2 till 11 (ten LEDs total). Remember to use a resistor between 220Ω and 470Ω for each LED.I soldered the LEDs on a small protoboard and connected one wire to each LED which connected the LEDs to the arduino.To Power the arduino we are going to use a 9v battery and I also soldered a switch so that the breathalyzer can be switched on or off.I used a common switch for the 5v power supply and the arduino power supply.See the Last Image For The Circuit Diagram.

Step 6: Putting It All Togather

Now that we have built all our circuits and tested them lets put them in a nice case.I used a case from a torch.I drilled 10 holes in the box.I had to respace the solder LEDs to fit them in the holes.First I installed the switch and then the power supply circuit then the LED array and then the sensor circuit .For the sensor wire4s to come out of the box I also made a hole at the side.


For more detail: Arduino Breathalyzer

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  • arduino breathalyzer circuit
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