Digital thermometer using 8051

Thermometer using 8051.

This article is about a simple 0-100°C digital thermometer with 1°C resolution using 8051. The circuit is based on LM35 analog temperature sensor, ADC0804 and AT89S51 microcontroller. LM35 is an analogue temperature sensor IC which can measure a temperature range of -55 to 150°C. Its output voltage varies 10mV per °C change in temperature.

For example, if the temperature is 32°C, the output voltage will be 32 x 10mV = 320mV. ADC 0804 is used to convert the analogue output voltage of the LM35  to a proportional 8 bit digital value suitable for the microcontroller. The microcontroller accepts the output of ADC, performs necessary manipulations on it and displays it numerically on a 2 digit seven segment LED display.

Out put of the LM35 is connected to the +Vin (pin 6) of the ADC0804. Resistor R13 and preset R14 is used to provide an external reference voltage of 1.28V to the Vref/2 pin  ( pin 9) of the ADC0804 and with this reference voltage, the step size of the ADC will be 10mV and span will be 0-1 V. This means that for a 10mV input the digital out of ADC will be 1 (1 in decimal also), for 20mV it will be 10 (2 in decimal), for 30mV it will be 11 (3 in decimal) and so on. The microcontroller accepts this data and puts it on the seven segment display.

Circuit diagram.

Digital thermometer using 8051

Digital out of the ADC (D0 to D7) are connected to P1 (P1.0 to P1.7) of the microcontroller. This is the line through which the microcontroller accepts data from the ADC. The control pins CS, RD, WR and  INTR are connected to P3.7, P3.6, P3.5 and P3.4 of the microcontroller. This is the data path through which the microcontroller sends chip select (CS), read (RD) write (WR) signals to the ADC and receives INTR signal from the ADC. Data lines (a to h) of the multiplexed seven segment display are interfaced to P0 (P0.0 to P0.7) of the microcontroller. Activation signals for the segment driver transistors Q1 and Q2 are available from P3.2 and P3.1 pins of the microcontroller. 

Program

ORG 00H

MOV P1,#11111111B   // initializes P1 as input port

MOV P0,#00000000B   // initializes P0 as output port

MOV P3,#00000000B   // initializes P3 as output port

MOV DPTR,#LABEL     // loads the address of "LABEL" to DPTR

MAIN: MOV R4,#250D  // loads register R4 with 250D

      CLR P3.7      // makes Cs=0

      SETB P3.6     // makes RD high

      CLR P3.5      // makes WR low

      SETB P3.5     // low to high pulse to WR for starting conversion

WAIT: JB P3.4,WAIT  // polls until INTR=0

      CLR P3.7      // ensures CS=0

      CLR P3.6      // high to low pulse to RD for reading the data from ADC

      MOV A,P1      // moves the digital output of ADC to accumulator A

      MOV B,#10D    // load B with 10D

      DIV AB        // divides the content of A with that in B

      MOV R6,A      // moves the quotient to R6

      MOV R7,B      // moves the remainder to R7

DLOOP:SETB P3.2     // sets P3.2 which activates LED segment 1

      MOV A,R6      // moves the quotient to A

      ACALL DISPLAY // calls DISPLAY subroutine

      MOV P0,A      // moves the content of A to P0

      ACALL DELAY   // calls the DELAY subroutine

      CLR A         // clears A

      MOV A,R7      // moves the remainder to A

      CLR P3.2      // deactivates LED segment 1

      SETB P3.1     // activates LED segment 2

      ACALL DISPLAY

      MOV P0,A

      ACALL DELAY

      CLR A

      CLR P3.1      // deactivates LED segment 2

      DJNZ R4,DLOOP // repeats the loop "DLOOP" until R4=0

      SJMP MAIN     // jumps back to the main loop

DELAY: MOV R3,#255D // produces around 0.8mS delay

LABEL1: DJNZ R3,LABEL1

        RET

DISPLAY: MOVC A,@A+DPTR // converts A's content to corresponding digit drive pattern

         RET

LABEL: DB 3FH       // LUT (look up table) starts here

       DB 06H

       DB 5BH

       DB 4FH

       DB 66H

       DB 6DH

       DB 7DH

       DB 07H

       DB 7FH

       DB 6FH

END 

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