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.
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.
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