When companies like SeeedStudio and Sparkfun began offer gas sensors, the alcohol sensor just looked like way to much fun to the beer drinking nerds out there. A few breathalyzer projects popped up as a result. However, those projects never seemed to capture this low quality alcohol sensor well enough. Well, I’m happy to report, I think I have found just the way to do it; An old fashioned analog panel meter.

Super organized breadboard prototype.

Now when I started this project, I knew I wasn’t going to make a police quality BAC tester, but I knew I could atleast make something fun. I wanted to lay down the project as I have it now for anyone who was interested in building it this way. However, in the future, I am planning an ATTiny based version that is ready to be kitted up into a nice, small, portable box. (Perhaps MintyBreathalyzer?)

Carrying on; Let’s get rolling. Here is a quick and easy parts list:

1. Arduino – Any model. (If you’re willing to re-write code, any uC with analog to digital will suffice.)
2. MQ-3 Alcohol Sensor: I picked mine up from Sparkfun, but there are other sources.
3. Panel Meter:  I am working with a 20mA current meter. However, it will be driven off of PWM so you can use anything as long as you are willing to experiment. I’ll cover this in detail later, but for reference, I picked up my meter from Sparkfun as well.

Step # 1: Wiring The Panel Meter:

Now, as I mentioned before, I’m working with a 20mA current meter. We will be using PWM to drive the meter, so if you’ve got any other meter will work, but you’ll need to determine the correct current limting resistor to attach. For myself, I worked mostly from guess and check. Using the example ‘Fade’ code in Arduino, I wired the panel meter with one pin going to ground and the second pin running through the current limiter, into a PWM enabled digital pin. (I used 6.)

As an example to determine your current limiting resistor;

From the Arduino website, I read that each I/O pint can pull 40mA. With that in mind, I knew I had to limit that current down to 20mA. Knowing the PWM will span from 0-5v, at the highest point, the pin will be writing 5v to the panel meter. So, using ohms law, lets determine our resistance:

R= V/I = 5v/0.02amp = 250.

To keep things simple and moving, I popped in a 220ohm resistor.

Step #2: Wiring The Alcohol Sensor:

The alcohol sensor is a bit more difficult. Essentially, it is composed of a heating element, and a voltage detector. This is the simplest way to wire the sensor as I found via Wiring.

Step #3: Code

The last and final step is the code. I’ve written a super simple little sketch to map the analog value to PWM values using the Map Function

/*
Super simple breathalyzer code.
Ryan Barnes - http://www.thelowtech.com/

To use:
1. Change pin definitions to match your setup.
2. Upload and run sketch.
3. View the serial monitor. The left number is the result your sensor
initially reading. Allow for ~2-3hours of burn in.
4. When the value has leveled off after burn in, plug value into the
map function as the low value (currently set to 300.)
5. Drink, blow, be marry!

*/
int sensorPin = A0;
int meterPin = 6;

void setup() {
Serial.begin(9600);
}

void loop() {
int sensorValue = analogRead(sensorPin);
Serial.print(sensorValue);
Serial.print("\t");
int level = map(sensorValue, 300, 1023, 0, 255);
Serial.println(level, DEC);
analogWrite(meterPin, level);
}

Final Wiring & Schematic

A little wiring diagram done in Fritzing. My first attempt with it, so bare with me! I made the lovely panel meter graphic though, and I think it is pretty awesome!

Their Schematic editor wasn’t easy to deal with…I can’t seem to find a way to lock nets to a grid…
However, their breadboard view is pretty great. Hope it is helpful, I spent WAY to long on it!

All the being said, let me know if anything is missing, or incorrect and I’d be happy to help fix it!
Cheers, drink up!