Construction of Thermometer using analog sensor and microcontroller

 A Digital Thermometer development using precise analog Temperature sensor LM 35 and Microcontroller PIC 16F877

This is basic example project for learner and hobbyist to prepare a digital thermometer using a few electronics components along PIC microcontroller. This project will involve use of Temperature sensor, analog to digital converter and a display. To run whole project a regulated +5VDC power supply will be required.

The LM35DZ is a three pins integrated circuit (IC) for the measurement of Temperature and reflects output signal in the form voltages corresponding to the ambient Temperature measured. The output voltage of sensor is directly proportional to the measured temperature. This Temperature sensor is useful for the measurement of Temperature from zero to 125◦ C.  The Two pins of this integrated circuit Temperature sensor are used to give power supply to it, therefore in most of the cases in the microcontroller based application these pins are connected to the power supply of microcontroller i.e. +5V and the ground. The third and last pin is the output pint, the voltage on this pin with reference to ground reflects the measured ambient Temperature linearly Vo = 10 milliVolt/◦ C. Therefore we can say that if measured ambient Temperature is 35 ◦C then output voltage would be around 350milliVolt. The output Pin of this sensor can be connected directly to ADC input pin of microcontroller like PIC, AVR. In the case we are using 8051 Microcontroller the addition ADC would be required to use between LM35 and 8051. In that case ADC0804 single channel ADC having 8 bits output will be enough to start with, later on the better ADC with efficient resolution could be used. But here in this experiment we are going to use PIC16f877 or PIC16f877A MicroChip Microcontroller which have 08 Nos built in ADCs in the microcontroller and each of the ADC channel is have 10 bit resolution, so it would be more than enough at this stage. The PIC microcontroller family is very well known family in the world of microcontroller and microprocessors and PIC microcontrollers are being used in variety of applications around us everywhere.

Designing and Construction of Electronics Circuit diagram:

The designing of this circuit is not much difficult as describe above only a few electronics components are required to build this digital thermometer like sensor, microcontroller and display. Therefore we will directly move to the construction phase of the electronics circuit diagram, first of all we will list down the components of this project in detail:

1. LM35 , 3 PIN IC, used as Temperature Sensor ( Analog), works on +V to +30V DC without any problem.
2. PIC 16F877,40 PIN, MicroChip Microcontroller having very useful features. It may be noted that the hardware is designed so that this project will also work on PIC16f84 equally good. The software also supports it. Although these two microcontroller have difference in construction and number of Pins, even then in some of the application these could replace each other with certain changes in hardware side like IC pin socket and re-routing the electrical connection where required. The PIN names are same where the function of that Pin is identical but the number if correspond to dive and could be different.
3. LCD, 16Ch & 02 Lines, will be used to display the measured value
4. A 04 MHz crystal with 22pF capacitor (02NOs)
5. 10K ohm resistor.
6. +5V DC regulated power supply to power up this circuit.

The circuit diagram of the thermometer:

Circuit Diagram of Digital Thermometer using PIC Microcontroller

The output of LM35 Temperature sensor is directly connected to the Analog channel AN0 of the microcontroller. The LCDs are widely used in microcontroller based projects to display the value of parameters or variable and to show input output user data. There are two main types of LCDs mostly used with microcontrollers for this purposes: text based or graphical. The text LCDs are very easy to handle and writing the software coding is also easy, however the graphic LCD involves more code listing and are lit but complex but not too much. Here in this project we are going to use a small text LCD. In this regards the most popular LCD is HD44780 based controller.

LCD PIN CONNECTIONS and FUNCTIONS

The more pronounced models of LCDs using HD44780 controller are following as example:

• LM016L:  It is consist of 2 rows × 16 characters in each row, total 32 characters can be display at a time.
• LM017L:  It is consist of 2 rows × 20 characters in each row, total 40 characters can be display at a time.
• LM018L:  It is consist of 2 rows × 40 characters in each row, total 80 characters can be display at a time.
• LM044L:  It is consist of 4 rows × 20 characters in each row, total 80 characters can be display at a time.

The PICC language have built in libraries to handle LCDs based on controller HD44780 and the data/ text can be very easily displayed it using built in functions. The following built in library functions are used to control the LCD with PIC microcontroller using PICC Compiler:

• lcd init: To initialize the LCD at start of program, when controller is power ON, the code inside this function is executed to initiate the LCD to make it ready for accepting for commands to display user text on user define location of LCD etc.
• lcd clear: To clear the screen of whole LCD , all previously display text will b washed and the cursor will point to the home location means at the start of LCD.
• lcd goto: This function is used to bring the cursor at desired location/position
• lcd write: It is used to send a character to the LCD for display of a single character.
• lcd puts: It is used to send a text string to the LCD for display of test.

The LCDs based on HD44780 type controller have 14 pins. The following figure shows the pin numbers and their function. The Pin 3 of LCD is used to control the contrast of the display, normally a variable resistor is used with this, but for simplicity it can be directly tied to ground for full contrast. The RS pin of LCD is used to send a control or a text depends on the logic on this pin. When the read or write (R/W) pin of LCD is at logic level 0, a command or a text message can be sent to the LCD. When R/W is at logic level 1, the LCD status can be read. The LCD is enabled when the E pin of LCD is at logic level 0, therefore it is also normally connected to ground for permanent enable condition. Pins D0 to D7 of LCD are the data inputs pins. There are two modes of interface 8 bit or 4 bit mode. The LCD in full 8-bit mode is connected to 8 lines of microcontroller, whereas, or in 4-bit half mode only the upper four data pins are connected to four i/o lines of microcontroller. Usually the 4-bit mode is preferred as it uses fewer pins of microcontroller. In order to use the PICC, pre written library LCD functions, the LCD shall be connected in a predefine hardware connection with microcontroller which are as follows,

Microcontroller Port pin                     LCD pin

RB0                                                        D4

RB1                                                        D5

RB2                                                       D6

RB3                                                       D7

RA2                                                       RS

RA3                                                        E

Software of The Thermometer

//*******************************************************************

//

// DIGITAL THERMOMETER USING LM35 AND PIC 16F877

// ============================

// Author: Dr. Rana

// Date: 17 August 2022

// File: DIGITAL-THERMOMETER.C

// The coding listing in this program will read the raw-data Temperature form a LM35DZ analog

// sensor after one second and then displays the measured raw-data Temperature on an Alphanumeric LCD.

// The microcontroller PIC 16f877 is configured to operate on a 4MHz crystal.

//

//*******************************************************************

#include <pic.h> // PIC Library having all basic definition of PIC Microcontroller

#include <delay.c> // The Delay Libray used to introduce some delay functions in the code listing

#include <lcd.c> // To control LCD, this built in Library or header file is included

#include <stdio.h> // the compiler’s built in library to handle text strings efficiently

// Delay Function of about one second

void wait a second()

{

unsigned int j;

for(j = 0; j < 4; j++)DelayMs(250);

}

// Main Program start here

main(void)

{

const float Volt2number = 5000.0/1024.0; // declaration and unitization of variable

float milliVolt, raw-data, raw-data-one, raw-data-high;

unsigned int raw-data-temp; // declaration and unitization of variable

unsigned char disp[] = "RAW-DATA = ";// declaration and unitization of variable

TRISA = 1; /* RA0 is input, others output */

TRISB = 0; /* PORT B is output */

ADCON1 = 0x8E; /* RA0=analog, RA2,RA3=digital */

ADCON0 = 0x41; /* Configure A/D clock and select channel 0 */

for(;;)// While One function, it be executed as long as the MCU is power ON

{

ADCON0 = 0x45; /* Start A/D conversion */

while(ADCON0 & 4) != 0); /* Wait for conversion */

raw-data-high = ADRESH; /* Read upper 2 bits */

raw-data-one = ADRESL; /* Read lower 8 bits */

raw-data = 256.0*raw-data-high + raw-data-one; /* Raw-data corresponds to Temperature in digital */

milliVolt = raw-data *Volt2number; /* Raw-data corresponds to Temperature in milli-Volt */

raw-data-temp = milli-Volt / 10.0; /* Raw-data corresponds to Temperature in Centigrade. (10milliVolt/C) */

sprintf(disp+7, "%d",raw-data-temp); /* Convert raw-data to Temperature to a string */

lcd puts(raw-data-temp); /* Display raw-data Temperature on LCD */

wait a second(); /* delay of one second */

lcd clear(); /* Clear display */

}// end of while one function

}// end of main function and also end of code listing

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