How To Make Alcohol Breathalyzer Circuit?

Table of Contents

The effects of consumption of alcohol even in small amounts are noticeable. When consumed in large quantities, alcohol can act as a sedative and can depress the central nervous system.

Alcohol is very quickly absorbed in to blood and the amount of alcohol in blood reaches peak after an hour of drinking an alcoholic beverage. Hence, blood alcohol test is the easiest and fastest way to determine whether a person is in legal levels of consumption or intoxicated.

The main reasons for testing blood alcohol level are: to check the alcohol content in blood of legally drunk suspects, to test for alcohol content in blood when consumption is prohibited like in under aged people and other medical reasons like finding the cause of altered mind.

Breath Alcohol Tester is a device that is used to indicate or estimate the blood alcohol content of a person. The general term for this device is Breathalyzer or Breath Analyzer.

Law Enforcement Organizations often use breath alcohol testers to find consumption levels of alcohol by drivers.

Circuit Diagram

Breath Alcohol Tester

Components Required

  • MQ-3 Alcohol Sensor – 1
  • LM3914 – 1
  • 3.9 KΩ – 1
  • 2.2 KΩ – 1
  • 470 Ω – 11
  • 20 KΩ POT – 1
  • Green LEDs – 5
  • Yellow LEDs – 3
  • Red LEDs – 2

Component Description

MQ-3 Alcohol Sensor

MQ-3 is an alcohol sensor that can be used to measure or detect breath alcohol concentration. It is a highly sensitive sensor with fast response time.

MQ-3 can also detect Benzene, Methane, LPG, Carbon monoxide and Hexane. MQ-3 alcohol sensor is a 6 pin device out of which 4 pins are used to fetch signals and other 2 are used to provide current to heater coil.


LM3914 is a dot/bar display driver IC. It senses the analog voltage levels and drives 10 LEDs accordingly. It can be used to drive LEDs, LCDs and vacuum fluorescent displays.

It can also drive low current incandescent lamps. Several LM3914 ICs can be chained to form 20 to 100 segments.

Circuit Design of Alcohol Tester

MQ-3 has 6 pins and the data pins can be interchanged. The heater terminals of the sensor are connected to supply and ground respectively.

Two similar data pins (A or B) are connected to supply and other two similar data pins are given to one end of voltage divider network. The other end of voltage divider consists of a 470Ω resistor and 20 KΩ POT. The common point is given to pin 5 of LM3914.

Pins 2 and 3 of LM3914 are supply pins. They are connected to Vcc and Gnd respectively. Pin 9 is mode selection pin. For normal usage, it is connected to pin 3.

Pins 4 and 6 are divider low and high pins. Pin 4 is connected to ground and pin 6 is connected to pin 7. Pins 7 and 8 are reference voltage pins. Pin 7 is connected to one end of voltage divider network formed by 2.2 KΩ and 3.9 KΩ. The other end is connected to ground. The center of voltage divider is connected to pin 8.

Pins 1, 18, 17, 16, 15, 14, 13, 12, 11 and 10 are the output pins in sequence i.e. pin 1 is output 1 while pin 10 is output 10. They are connected to 10 different LEDs of different colors.

Working of Alcohol Tester or Analyzer

Breathalyzers are very useful devices in estimating the blood alcohol content of drunk drivers. Police officers often use more advanced and sophisticated testers with digital readings and with printer facility that prints results instantly.

In this project, a simple breath alcohol tester is designed which indicates the levels of alcohol through a set of 10 LEDs. The working of the circuit is explained below.

MQ-3 is the main module in the project. It is an alcohol sensor whose conductivity changes according to the concentration of the alcohol.

The output is given to the signal pin (pin 5) of LM3914, which is an analog voltage sensing IC. According to the value at pin 5, the outputs are activated. For voltages between 0 and 5V, the outputs are activated from 1 to 10.

When the concentration of alcohol sensed is very less, the analog value will be slightly greater than 0V and hence only first few LEDs will glow.

When the concentration increases, the number of LEDs turning on also increases. When the concentration of alcohol is at intoxicated level, all the LEDs will be turned on. The sensitivity of the module can be adjusted by the potentiometer.


  • Consumption of alcohol is injurious to health. The purpose of this project is to demonstrate the working of alcohol sensor.
  • A simple DIY blood alcohol tester that can be used as a breathalyzer is designed here.
  • The output of the project depends on the quality of the sensor.
  • The sensor can also be used for detecting other gases like methane, carbon monoxide etc.

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