Digital Thermometer Using PIC Microcontroller

Introduction

A digital thermometer is a device that measures temperature and displays it in a readable format, typically on an LCD screen. Digital thermometers are widely used in various applications, from medical devices to environmental monitoring systems. In this article, we will discuss the development of a simple yet effective digital thermometer using a PIC microcontroller. This project involves interfacing a temperature sensor with the microcontroller and displaying the temperature readings on an LCD.

Overview of the Project

The project involves using a temperature sensor, such as the LM35, which provides an analog voltage output proportional to the temperature. The PIC microcontroller reads this analog signal, converts it to a digital value using its ADC (Analog-to-Digital Converter), and processes the data to display the temperature in degrees Celsius on an LCD.

Components Required

• PIC Microcontroller (PIC16F877A): The central processing unit that controls the entire system.

• Temperature Sensor (LM35): Provides an analog voltage proportional to the temperature.

• LCD Display (16x2): To display the temperature readings.

• Resistors and Capacitors: For circuit stability and protection.

• Oscillator (Crystal and Capacitors): Provides the necessary clock signal to the microcontroller.

• Power Supply: To power the microcontroller and other components.

• Connecting Wires: For making connections between different components.

• Breadboard/PCB: For assembling the circuit.

Circuit Diagram

Below is a basic circuit diagram for the digital thermometer:

Temperature Sensor (LM35)  -->  PIC16F877A  -->  LCD Display

                |                 |

        Power Supply        Oscillator (XTAL)

                |

        Resistors/Capacitors

1. Temperature Sensor (LM35) to PIC Microcontroller:

2. The output pin of the LM35 is connected to an analog input pin of the PIC microcontroller (e.g., AN0).

3. The sensor’s Vcc and GND are connected to the power supply.

4. PIC Microcontroller to LCD Display:

5. The microcontroller’s output pins (data and control pins) are connected to the corresponding pins of the LCD display.

6. Power Supply:

7. The PIC microcontroller and other components are powered by a suitable DC power supply, typically 5V.

Working Principle

1. Temperature Measurement:

2. The LM35 temperature sensor provides an output voltage that is linearly proportional to the temperature. For instance, the LM35 outputs 10 mV per degree Celsius.

3. ADC Conversion:

4. The PIC microcontroller reads the analog output from the LM35 sensor using its ADC. The ADC converts the analog signal into a digital value that the microcontroller can process.

5. Temperature Calculation:

6. The microcontroller processes the ADC value to calculate the corresponding temperature. Since the LM35 provides 10 mV/°C, the temperature in degrees Celsius can be calculated as: Temperature (°C)=ADC Value×(Vref1024)×100\text{Temperature (°C)} = \text{ADC Value} \times \left(\frac{\text{V}_{\text{ref}}}{1024}\right) \times 100Temperature (°C)=ADC Value×(1024Vref​​)×100

7. Display:

8. The calculated temperature is displayed on the 16x2 LCD display.

Programming the PIC Microcontroller

The microcontroller is programmed using Embedded C or Assembly language. Below is an example of a basic code snippet in Embedded C for this project:

#include <pic16f877a.h>

#include <lcd.h> // Assumed LCD library for easy interfacing

#define TEMP_SENSOR_CHANNEL 0 // AN0

// Function prototypes

void init_system();

unsigned int read_adc(unsigned char channel);

float calculate_temperature(unsigned int adc_value);

void delay(unsigned int ms);

void main() {

    unsigned int adc_value = 0;

    float temperature = 0;

    init_system();

    while(1) {

        adc_value = read_adc(TEMP_SENSOR_CHANNEL);

        temperature = calculate_temperature(adc_value);

        lcd_clear();

        lcd_set_cursor(1,1);

        lcd_print("Temp:");

        lcd_print_float(temperature);

        lcd_print("C");

        delay(1000);

    }

}

void init_system() {

    TRISA = 0xFF; // Set PORTA as input for ADC

    ADCON1 = 0x80; // Configure ADC

    lcd_init(); // Initialize LCD

}

unsigned int read_adc(unsigned char channel) {

    ADCON0 = (channel << 3) | 0x01; // Select ADC channel and enable ADC

    ADCON0 |= 0x02; // Start conversion

    while(ADCON0 & 0x02); // Wait for conversion to complete

    return (ADRESH << 8) | ADRESL; // Return 10-bit ADC value

}

float calculate_temperature(unsigned int adc_value) {

    // Assuming Vref = 5V, LM35 provides 10mV per degree Celsius

    return ((adc_value * 5.0) / 1024.0) * 100.0;

}

void delay(unsigned int ms) {

    unsigned int i, j;

    for(i = 0; i < ms; i++)

        for(j = 0; j < 1275; j++);

}

Explanation of the Code

• TRISA: Configures PORTA pins as input for reading the analog signal from the temperature sensor.

• ADCON1: Configures the ADC module for proper operation.

• ADCON0: Selects the ADC channel and starts the conversion process.

• read_adc(): Reads the analog value from the specified ADC channel and returns the 10-bit digital value.

• calculate_temperature(): Converts the ADC value to temperature in degrees Celsius.

• The main loop continuously reads the temperature, calculates the value, and displays it on the LCD.

Advantages of a Digital Thermometer

• Precision: Offers accurate and consistent temperature readings.

• Ease of Use: The LCD display provides an easy-to-read temperature output.

• Scalability: The system can be expanded to include additional features such as data logging or remote monitoring.

Conclusion

A digital thermometer using a PIC microcontroller is a practical and useful project for beginners and experienced developers alike. By following the steps outlined in this article, you can build your own digital thermometer, which can be used in various applications, including environmental monitoring, HVAC systems, and laboratory equipment. This project demonstrates the practical application of microcontrollers in sensor interfacing and data processing, providing a solid foundation for more complex embedded systems.

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