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.

8051 digital thermometer

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. 

Before attempting this circuit go through these articles Voltmeter using 8051  , Interfacing ADC to 8051 , Interfacing seven segment display to 8051


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
      MOV P0,A      
      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          
DISPLAY: MOVC A,@A+DPTR // converts A's content to corresponding digit drive pattern 
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
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18 Responses to “Digital thermometer using 8051”

  • rocky says:

    can anybody please tell me that how to measure water temperature using thermometer? what will be the changes?

  • chatchai.ribruamsup says:

    I need c code

  • naresh says:

    in theory the ADC terminals rd,wr,cs,intr are different compare to above diagram all are complementry please tell the anthour program why becuse the ADC is not working

  • naresh says:

    please tell the project program in c language

  • gautam says:

    can anyone please send me any simulated version of working one. urgently needed..thanks in advance

  • manish says:

    can anyone give me pcb layout?
    since adc0804 is not present in eagle

  • Rinal says:

    Hello, my name rinal and im a student.

    of course, I am very grateful for the knowledge that you give. This adds to my knowledge.

    I have a bit question, I hope you do not mind to answer it..
    bif you ever make this circuit? whether these circuits have been tested and work properly?
    I would be happy if you guide with answering my questions,
    thank you. :)

  • dare says:

    i am gng to start working on this project .. evry thng is fine with it?coz i hav read only quest in replies pls pls help

  • kaothang says:

    it would be more helpful if you provide me the proteus file.thanks…:v

  • abdul rauf says:

    CAN I USE 89C51 INSTEAD OF 89S51.

  • 8051zz says:

    Can anyone provide a c program version of the above code??
    It would be very helpful for KEIL users. Thanks!

  • prutha says:

    can anyone please provide me the complete layout of this circuit and also the approximate cost? It would be of great help.

  • f10 says:

    can i get the project report on digital thermometer using at89s51 with lcd interface?? plz

  • Jaya says:

    Can anyone please provide me the layout of this circuit. Please do help someone..its very urgent..!!

  • EZE ROBERT says:

    what is the functimn of mov R4,#254 is a delay explaine more on it

    • admin says:

      MOV R4,#250D initially loads R4 with 250D ,this instruction in conjunction with DJNZ R4,DLOOP repeats the display cycle 250 times (or for roughly 0.8mS). This means that the “MAIN” loop which fetches the data from the ADC is repeated only once in every 0.75mS.

  • EZE ROBERT says:

    thank u for helping students. Yes i would suggest that post ur work with proteus version of ur work, what i mean is design it with proteus and up-load it will actracts many users to vist tis site. This is want they are doing thus in this site,

  • EZE ROBERT says:

    yes that is a good work. Thanks but it will be more professional if u configure the cct using 3 7-segment display in showing the meter type and in the temperature form type as u up-loaded here inshort i personally will be glad likewise others…