Make an alcohol tester based on 8051 microcontroller

We often see traffic police using breathalyzers to check whether drivers are under the influence of alcohol when checking for drunk driving. In fact, the alcohol tester is actually very easy to make. Here is how to make a homemade alcohol tester with a 3-digit display output. Of course, after all, it is used as a simple case, so the accuracy is slightly worse.

In this article, a breath analyzer circuit using 8051 microcontroller is used which outputs the blood alcohol content (BAC) from the breath. The "percentage of alcohol content" is displayed as a percentage on the 3-digit seven-segment display. The microcontroller used is AT89S51 of 8051 series and alcohol sensor MQ135 gas sensor.

1. MQ135 gas sensor circuit

MQ135 is a stable and sensitive gas sensor that can detect ammonia, carbon dioxide, alcohol, smoke, nitrogen dioxide, etc. The sensor consists of a tin dioxide sensitive layer inside an aluminum oxide microtube, a measuring electrode and a heating element inside a tubular aluminum shell. The front end of the sensor is covered with a stainless steel mesh, and the connection terminals are fixed on the back.

The ethanol present in the breath is oxidized to acetic acid as it passes through the heating element. This ethanol lands on the tin dioxide sensing layer, so its resistance decreases. Use an external load resistor to convert this resistance change into a suitable voltage change. MQ135

A typical connection arrangement for an alcohol sensor is as follows:

MQ135 has different resistance values ​​at different temperatures and different gas concentrations. The manufacturer recommends 100 ppm ammonia or 50

Calibrate the sensor in ppm alcohol. The recommended value of the load resistor is between 10K and 47K.

2. Schematic diagram of alcohol tester

List of required components:

Alcohol sensor (MQ135) – 1 pc.

IC (ADC 0804) – 1 pc.

Microcontroller (AT89S51) – 1 pc.

7-segment display – 3 pcs.

Transistor (2N2222) – 3 pcs.

Switch – 1 pc.

Capacitor (150pF) – 1 pc.

Capacitor (22uF/10V) – 1 pc.

Capacitors (10uF/10V) – 3 pcs.

Resistors: 100 – 11 nos, 10k – 3nos, 22k – 1 nos, 8.2k – 1 nos, 330 ohm – 1

nos.

3. Working principle of alcohol detector circuit

The voltage output of the alcohol detector unit is converted to digital format using ADC0804 (IC1). The V ref/2 pin of the ADC is used by R14 and R15

The resulting voltage divider network remains at 1.28V. V ref/2 =1.28V means that the step size of the ADC is 10mV. Every time the analog input increases by 10mV, the output of the ADC will increase by one bit. See

Datasheet for the ADC0804 to get a better grasp. The digital outputs of the ADC (D0 to D7) are connected to port 1 of the microcontroller. The control signals CS, RD, WR, and INTR are respectively sent from the microcontroller

P3.7, P3.6, P3.5, P3.4 pins are obtained. R9 and C1 are related to the clock circuit of ADC0804.

Capacitor C3 is connected between V in+ and V in- of the noise filter (if present) in the ADC0804 sensor output. If C3 is not used, the ADC

The digital output will be unstable. The filter capacitor certainly causes some hysteresis in the ADC response, but it is not very important in this entry-level application. Microcontroller to ADC

The digital output performs the required operations in order to convert it to "percent alcohol content" and display it on a three-digit, seven-segment display. Port 0 of the microcontroller

Connected to multiplexed three-digit seven-segment display. The three-digit driving signals are obtained from the P3.0, P3.1, and P3.2 pins of the microcontroller.

4. 8051 microcontroller alcohol tester 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

     DB4FH

     DB66H

     DB 6DH

     DB 7DH

     DB07H

     DB7FH

     DB 6FH