Design And Construction Of An Alcohol Breathalyzer Circuit Using 8051 Microcontroller (At89S51)

Electrical Electronics Engineering

The design and construction of an alcohol breathalyzer circuit employing the 8051 microcontroller, specifically the At89S51 model, involves several key components and stages. Primarily, it integrates a gas sensor to detect alcohol vapor levels from a breath sample, which then triggers the microcontroller to process the data. The At89S51 microcontroller manages the sensor’s output and implements algorithms to determine the alcohol concentration. Additionally, the circuit includes an analog-to-digital converter (ADC) to convert the sensor’s analog signal into digital data for the microcontroller to analyze. LED displays or LCD screens are often incorporated to visually present the results of the breathalyzer test to the user. Furthermore, the circuit typically features calibration mechanisms to ensure accurate readings over time. Overall, this project combines sensor technology, microcontroller programming, and hardware integration to create a functional and reliable alcohol detection system for various applications, such as automotive safety or law enforcement.

ABSTRACT

This project work is on a breathalyzer circuit using 8051 microcontroller. This device outputs the blood alcohol content (BAC) from the breath. The BAC is displayed in percentage on a 3 digit seven segment display. The microcontroller used if AT89S51 which belongs to the 8051 family and the alcohol sensor is MQ135 gas sensor from Futurelec.

TABLE OF CONTENTS

TITLE PAGE

APPROVAL PAGE

DEDICATION

ACKNOWLEDGEMENT

ABSTRACT

TABLE OF CONTENT

CHAPTER ONE

INTRODUCTION
OBJECTIVE OF THE PROJECT
SIGNIFICANCE OF THE PROJECT
PURPOSE OF THE PROJECT
LIMITATION OF THE PROJECT
APPLICATION OF THE PROJECT
FUTURE DEVELOPMENT OF THE PROJECT
PROJECT ORGANIZATION

CHAPTER TWO

LITERATURE REVIEW

2.0 LITERATURE REVIEW
2.1 REVIEW OF ALCOHOL SENSING

2.2 REVIEW OF ALCOHOL USAGE

2.3 FACTS ABOUT ALCOHOL IN THE FACTORY/WORKPLACE

2.4 BLOOD ALCOHOL CONTENT

2.5 REVIEW OF DIFFERENT ALCOHOL TESTING METHODS

CHAPTER THREE

3.0 CONSTRUCTION METHODOLOGY

3.1 BLOCK DIAGRAM OF THE SYSTEM

3.2 CIRCUIT DIAGRAM

3.3 CIRCUIT DESCRIPTION

3.4 SYSTEM OPERATION

3.5 COMPONENTS LIST

3.6 POWER SUPPLY UNIT

CHAPTER FOUR

4.0 RESULT ANALYSIS

4.1 CONSTRUCTION PROCEDURE AND TESTING

4.2 ASSEMBLING OF SECTIONS

4.3 CONSRUCTION OF THE CASING

4.4 TESTING

CHAPTER FIVE

CONCLUSIONS
RECOMMENDATION

5.3 REFERENCES

MQ135 gas sensor.

MQ135 is a stable and sensitive gas sensor which can detect ammonia, carbon dioxide, alcohol, smoke, nitrogen dioxide etc. The sensor consists of a tin dioxide sensitive layer inside aluminium oxide micro tubes, measuring electrode and a heating element inside a tubular aluminium casing. The front end of the sensor is covered using a stainless steel net and the rear side holds the connection terminals.

The ethyl alcohol present in the breath is oxidized into acetic acid while passing over the heating element. This ethyl alcohol falls on the tin dioxide sensing layer and as a result its resistance decreases. This resistance variation is converted into a suitable voltage variation using an external load resistor. The typical connection arrangement of an MQ135 alcohol sensor is shown below.

MQ135 alcohol sensor

MQ135 has different resistance values at different temperature and different concentration of gases. The manufacturer recommends to calibrate the sensor at 100ppm of ammonia or 50ppm of alcohol. The recommended value of the load resistor is between 10K to 47K.

Circuit diagram.

The voltage output of the alcohol sensor is converted into a digital format using the ADC0804 (IC1). The Vref/2 pin of the ADC is held at 1.28V using the voltage divider network made of R14 and R15. Vref/2 =1.28V means the step size of the ADC will be 10mV and the output of the ADC will increment by one bit for every 10mV increment in the analog input. Refer the datasheet of ADC0804 for a better grasp. Digital out of the ADC (D0 to D7) is interfaced to Port1 of the microcontroller. Control signals CS, RD, WR, INTR are obtained from the microcontrollers P3.7, P3.6, P3.5, P3.4 pins respectively. R9 and C1 are associated with the clock circuitry of the ADC0804.

Capacitor C3 connected between Vin+ and Vin- of the ADC0804 filters of noise (if any) in the sensor output. If C3 is not used the digital output of the ADC will not be stable. This filter capacitor will surely induce some lag in the ADC response but it is not very relevant in this entry level application. The microcontroller performs required manipulations on the ADC digital output in order to convert it into BAC % and displays it on the three digit seven segment display. Port0 of the microcontroller is interfaced to the multiplexed three digit seven segment display. The drive signals for the threes digits are obtained from the microcontroller’s P3.0, P3.1, P3.2 pins respectively.

Program.

ORG 00H

MOV P1,#11111111B

MOV P0,#00000000B

MOV P3,#00000000B

MOV DPTR,#LUT

MAIN: MOV R4,#250D

CLR P3.7

SETB P3.6

CLR P3.5

SETB P3.5

WAIT: JB P3.4,WAIT

CLR P3.7

CLR P3.6

MOV A,P1

MOV R5,A

SUBB A,#86

JC NEXT

SETB P3.3

CLR PSW.7

NEXT: MOV A,R5

SUBB A,#115D

JNC LABEL

MOV A,#00000000B

CLR PSW.7

LABEL: MOV B,#5D

MUL AB

MOV B,#8D

DIV AB

MOV B,#10D

DIV AB

MOV R6,A

MOV R7,B

DLOOP:SETB P3.0

MOV P0,#01000000B

ACALL DELAY

CLR P3.0

SETB P3.1

MOV A,R6

ACALL DISPLAY

MOV P0,A

ACALL DELAY

CLR P3.1

SETB P3.2

MOV A,R7

ACALL DISPLAY

MOV P0,A

ACALL DELAY

CLR P3.2

DJNZ R4,DLOOP

SJMP MAIN

DELAY: MOV R3,#255D

LABEL1: DJNZ R3,LABEL1

RET

DISPLAY: MOVC A,@A+DPTR

CPL A

RET

LUT: DB 3FH

DB 06H

DB 5BH

DB 4FH

DB 66H

DB 6DH

DB 7DH

DB 07H

DB 7FH

DB 6FH

END

Notes.

The MQ135 gas sensor requires around 5 minutes of preheat before the first use.
The MQ135 takes few minutes to retrace back to its normal condition after a positive test (alcohol present in the breath).
If there is no alcohol in the breath the sensor output will swing back to its normal condition very fast.
Read these articles Interfacing seven segment display to 8051 microcontroller , Interfacing ADC to 8051 microcontroller before attempting this project.
This breathalyzer circuit is just an entry level one and is not suitable for high end applications such as law enforcement or laboratory application.
The logic for converting the digital output of ADC into BAC percentage was obtained using approximation techniques.