Centigrade and Fahrenheit Scale Digital Thermometer with LCD Display

Digital thermometers are cool devices as they show temperatures in human readable formats. This digital thermometer project is based on a PIC16F688 microcontroller and a DS1820 temperature sensor, and it displays temperature on a character LCD screen in both Celsius and Fahrenheit scales. I selected PIC16F688 for this project because it is cheap (I bought one for $1.50). DS1820 is a 3-pin digital temperature sensor from Dallas semiconductors (now Maxim) which is designed to measure temperatures ranging from -55 to +125 °C in 0.5 °C increments. The firmware I have written is able to read and display the entire temperature range of DS1820. In order to test for temperature measurements below 0°C, I put the sensor inside my freezer. While trying this, don’t put the whole unit inside the freezer as LCD display unit may stop working at the freezer temperature. Similarly, bringing a soldering iron tip close to the sensor can do testing for the higher range temperature values.



About DS1820
I suggest reading my previous articles on DS1820 for getting details about this 1-wire temperature sensor. The firmware that I wrote works only for DS1820 (it may work for DS18S20 making some change in the temperature conversion time, read http://www.maxim-ic.com/datasheet/index.mvp/id/3021). It will definitely not work for DS18B20.

DS1820 converts temperature in to a 9-bit digital word, which is read by PIC16F688 as two bytes (TempH and TempL). The value of the least significant bit is 0.5°C. For positive temperature, the next eight bits after the LSB gives the integer part of the temperature. For example, 000110011 represents 25.5°C. Further, the floating-point math can be avoided during C to F conversion by using a scale factor of 10. Therefore, 25.5°C (scaled 255) is converted to F as,

TempinF = 9*TempinC/5 + 320 = 9*255/5 +320 = 779 (which is 76.9 F)

However, the negative temperatures are stored in 2's complement form. So the most significant bit of the 2-byte temperature reading from DS1820 is 1 if the temperature is below 0°C. The firmware takes care of all negative temperature readings (in both C and F scales). The computed temperature is displayed on LCD as a 5-digit string array, xxx.x (e.g., 24.5, 101.0, -12.5, etc).

Circuit Diagram
The circuit diagram of this project is shown below. DS1820 sensor output is read by PIC16F688 through RA5 port. The computed temperature is converted to a string and sent to the LCD to display. The LCD is operating in 4-bit mode, and ports RC0-RC3 serves D4-D7 data pins of the LCD. The Register Select (RS) and Enable (E) signals for LCD are provided through ports RC4 and RC5. The Read/Write pin of the LCD is permanently grounded, as there is no data read from the LCD in this project. The contrast adjustment of LCD is done with the 10K potentiometer shown in the circuit diagram.

Besides, there are two tact switches in the design. The first one serves as the system reset that is to reset the whole system and reinitialize the LCD. The another tact switch is connected to the external interrupt pin of PIC16F688. This is the toggle switch for the LCD back light. This is helpful in reading the temperature display in low illumination conditions. An interrupt service routine is written for back light toggling. When the system is first turned ON, the LCD back light turns ON by default.

The following circuit can be used to get +5V regulated power supply required for the circuit.

Firmware
The firmware was developed with mikroC compiler from mikroelektronika. The in-built library routines for DS1820 make the firmware development easier. The code is provided with adequate comments so that the reader won't have much difficulty in understanding programming logic.

/*

Digital Room Thermometer using PIC16F688

Copyright@Rajendra Bhatt

July 13, 2010

*/

// LCD module connections

sbit LCD_RS at RC4_bit;

sbit LCD_EN at RC5_bit;

sbit LCD_D4 at RC0_bit;

sbit LCD_D5 at RC1_bit;

sbit LCD_D6 at RC2_bit;

sbit LCD_D7 at RC3_bit;

sbit LCD_RS_Direction at TRISC4_bit;

sbit LCD_EN_Direction at TRISC5_bit;

sbit LCD_D4_Direction at TRISC0_bit;

sbit LCD_D5_Direction at TRISC1_bit;

sbit LCD_D6_Direction at TRISC2_bit;

sbit LCD_D7_Direction at TRISC3_bit;

// End LCD module connections

// Back Light Switch connected to RA1

sbit BackLight at RA1_bit;

// Define Messages

char message0[] = "LCD Initialized";

char message1[] = "Room Temperature";

// String array to store temperature value to display

char *tempC = "000.0";

char *tempF = "000.0";

// Variables to store temperature register values

unsigned int temp_whole, temp_fraction, temp_value;

signed int tempinF, tempinC;

unsigned short C_Neg=0, F_Neg=0, TempH, TempL;

void Display_Temperature() {

// convert Temp to characters

if (!C_Neg) {

if (tempinC/1000)

// 48 is the decimal character code value for displaying 0 on LCD

tempC[0] = tempinC/1000 + 48;

else tempC[0] = ' ';

}

tempC[1] = (tempinC/100)%10 + 48; // Extract tens digit

tempC[2] = (tempinC/10)%10 + 48; // Extract ones digit

// convert temp_fraction to characters

tempC[4] = tempinC%10 + 48; // Extract tens digit

// print temperature on LCD

Lcd_Out(2, 1, tempC);

if (!F_Neg) {

if (tempinF/1000)

tempF[0] = tempinF/1000 + 48;

else tempF[0] = ' ';

}

tempF[1] = (tempinF/100)%10 + 48; // Extract tens digit

tempF[2] = (tempinF/10)%10 + 48;

tempF[4] = tempinF%10 + 48;

// print temperature on LCD

Lcd_Out(2, 10, tempF);

}

// ISR for LCD Backlight

void interrupt(void){

if (INTCON.INTF == 1) // Check if INTF flag is set

{

BackLight =~BackLight; // Toggle Backlight

Delay_ms(300) ;

INTCON.INTF = 0; // Clear interrupt flag before exiting ISR

}

}

void main() {

TRISC = 0x00 ;

TRISA = 0b00001100; // RA2, RA3 Inputs, Rest O/P's

ANSEL = 0b00000000;

PORTA = 0b00000000; // Start with Everything Low

PORTC = 0b00000000; // Start with Everything Low

CMCON0 = 0b00000111;

Lcd_Init(); // Initialize LCD

Lcd_Cmd(_LCD_CLEAR); // CLEAR display

Lcd_Cmd(_LCD_CURSOR_OFF); // Cursor off

BackLight = 1;

Lcd_Out(1,1,message0);

Delay_ms(1000);

Lcd_Out(1,1,message1); // Write message1 in 1st row

// Print degree character

Lcd_Chr(2,6,223);

Lcd_Chr(2,15,223);

// different LCD displays have different char code for degree

// if you see greek alpha letter try typing 178 instead of 223

Lcd_Chr(2,7,'C');

Lcd_Chr(2,16,'F');

// Interrupt Setup

OPTION_REG = 0x00; // Clear INTEDG, External Interrupt on falling edge

INTCON.INTF = 0; // Clear interrupt flag prior to enable

INTCON.INTE = 1; // enable INT interrupt

INTCON.GIE = 1; // enable Global interrupts

do {

//--- perform temperature reading

Ow_Reset(&PORTA, 5); // Onewire reset signal

Ow_Write(&PORTA, 5, 0xCC); // Issue command SKIP_ROM

Ow_Write(&PORTA, 5, 0x44); // Issue command CONVERT_T

INTCON.GIE = 1; // 1-wire library disables interrpts

Delay_ms(600);

Ow_Reset(&PORTA, 5);

Ow_Write(&PORTA, 5, 0xCC); // Issue command SKIP_ROM

Ow_Write(&PORTA, 5, 0xBE); // Issue command READ_SCRATCHPAD

// Read Byte 0 from Scratchpad

TempL = Ow_Read(&PORTA, 5);

// Then read Byte 1 from Scratchpad

TempH = Ow_Read(&PORTA, 5);

temp_value = (TempH << 8)+ TempL ;

// check if temperature is negative 

if (temp_value & 0x8000) { 

C_Neg = 1; 

tempC[0] = '-'; 

// Negative temp values are stored in 2's complement form 

temp_value = ~temp_value + 1; 

}

else C_Neg = 0;

// Get temp_whole by dividing by 2 

temp_whole = temp_value >> 1 ;

if (temp_value & 0x0001){ // LSB is 0.5C

temp_fraction = 5;

}

else temp_fraction = 0;

tempinC = temp_whole*10+temp_fraction;

if(C_Neg) {

tempinF = 320-9*tempinC/5;

if (tempinF < 0) {

F_Neg = 1;

tempF[0] = '-';

tempinF = abs(tempinF);

}

else F_Neg = 0;

}

else tempinF = 9*tempinC/5 + 320;

//--- Format and display result on Lcd

Display_Temperature();

} while(1);

}

Snapshots of temperature measurements

Back Light ON

Back Light OFF

Freezer Temperature

Soldering Iron Temperature