🔧 PIC Microcontroller Project: Digital Temperature Monitor with LM35 and LCD

1. Introduction

In this project, we’ll build a simple digital thermometer using a PIC16F877A microcontroller. It will read temperature from an LM35 sensor and display the value in Celsius on a 16×2 LCD. This kind of setup is commonly used in temperature monitoring systems like weather stations, greenhouses, or home automation systems.

2. Learning Objectives

By completing this project, you will:

Learn how to connect and use an analog temperature sensor with a PIC microcontroller.
Understand how the Analog-to-Digital Converter (ADC) works on the PIC16F877A.
Interface a 16×2 character LCD using 4-bit mode.
Write and structure embedded C code using MPLAB X and the XC8 compiler.
Test, troubleshoot, and modify your embedded system for real-world use.

3. Components and Tools Needed

You’ll need the following components and tools:

One PIC16F877A microcontroller
One LM35 analog temperature sensor
One 16×2 character LCD (HD44780 compatible)
One 10k potentiometer (for adjusting LCD contrast)
One 20MHz crystal oscillator
Two 22pF ceramic capacitors (for the crystal)
Breadboard and jumper wires
Regulated 5V power supply
MPLAB X IDE and XC8 compiler
PIC programmer (such as PICkit3)

4. Concept Overview

The LM35 outputs a voltage that increases linearly with temperature. For example, 250 millivolts means 25 degrees Celsius. The microcontroller reads this analog signal through its ADC module and converts it into a digital number. We then convert that number into a readable temperature and show it on the LCD screen.

The LCD is operated in 4-bit mode to save microcontroller pins. This means we send data in two parts (high nibble and low nibble) using only four data lines instead of eight.

5. Step-by-Step Instructions

Hardware Setup

Connecting the LCD:

Connect the LCD’s VSS pin to ground and VDD pin to 5V.
The VO pin (for contrast) should connect to the middle pin of a 10k potentiometer, with the outer pins connected to 5V and ground.
RS (Register Select) connects to RC0 on the PIC.
RW (Read/Write) connects to ground (since we only write to the display).
E (Enable) connects to RC1.
Data pins D4 to D7 on the LCD connect to RC2 to RC5 on the PIC.
Connect the LCD’s backlight (if available) through a current-limiting resistor to 5V.

Connecting the LM35:

Pin 1 of the LM35 connects to 5V.
Pin 2 (the output) connects to RA0 on the PIC (analog channel 0).
Pin 3 connects to ground.

Connecting the crystal:

Place the 20MHz crystal between the OSC1 and OSC2 pins (pins 13 and 14).
Add a 22pF capacitor from each of these pins to ground.

Powering the system:

Make sure your PIC microcontroller receives a stable 5V regulated power supply.
Ensure all grounds are connected together.

Code (XC8 for MPLAB X)

Below is the full C code with comments:

#define _XTAL_FREQ 20000000

#include <xc.h>
#include <stdio.h>

// Configuration bits
#pragma config FOSC = HS
#pragma config WDTE = OFF
#pragma config PWRTE = OFF
#pragma config BOREN = ON
#pragma config LVP = OFF
#pragma config CPD = OFF
#pragma config WRT = OFF
#pragma config CP = OFF

// Function declarations
void ADC_Init(void);
unsigned int ADC_Read(unsigned char);
void LCD_Init(void);
void LCD_Command(unsigned char);
void LCD_Char(unsigned char);
void LCD_String(const char*);

void main(void) {
    char buffer[16];
    unsigned int adc_value;
    float temperature;

    TRISA = 0x01; // RA0 as input
    TRISC = 0x00; // PORTC as output for LCD

    ADC_Init();
    LCD_Init();

    while (1) {
        adc_value = ADC_Read(0); // Read from analog channel 0
        temperature = (adc_value * 5.0 / 1023.0) * 100.0;

        LCD_Command(0x80); // Move to first line
        sprintf(buffer, "Temp: %.2f C", temperature);
        LCD_String(buffer);
        __delay_ms(1000);
    }
}

void ADC_Init() {
    ADCON0 = 0x41; // Turn on ADC, select channel 0
    ADCON1 = 0x80; // Right-justified result, Vref = VDD
}

unsigned int ADC_Read(unsigned char channel) {
    if (channel > 7) return 0;

    ADCON0 &= 0xC7; // Clear current channel
    ADCON0 |= (channel << 3); // Select desired channel
    __delay_ms(2); // Wait for acquisition
    GO_nDONE = 1; // Start conversion
    while (GO_nDONE); // Wait until done
    return ((ADRESH << 8) + ADRESL); // Combine 10-bit result
}

void LCD_Command(unsigned char cmd) {
    PORTC &= 0xF8; // Clear control bits
    PORTC |= (cmd >> 4); // Send high nibble
    PORTC |= 0x04; __delay_ms(2); PORTC &= ~0x04;

    PORTC &= 0xF8;
    PORTC |= (cmd & 0x0F); // Send low nibble
    PORTC |= 0x04; __delay_ms(2); PORTC &= ~0x04;
}

void LCD_Char(unsigned char data) {
    PORTC |= 0x01; // RS = 1 for data
    PORTC &= ~0x02; // RW = 0 to write
    PORTC |= (data >> 4); PORTC |= 0x04; __delay_ms(2); PORTC &= ~0x04;
    PORTC &= 0xF8;
    PORTC |= (data & 0x0F); PORTC |= 0x04; __delay_ms(2); PORTC &= ~0x04;
}

void LCD_String(const char *str) {
    while (*str) {
        LCD_Char(*str++);
    }
}

void LCD_Init() {
    __delay_ms(20);
    LCD_Command(0x02); // 4-bit mode
    LCD_Command(0x28); // 2-line display, 5x7 font
    LCD_Command(0x0C); // Display ON
    LCD_Command(0x06); // Entry mode set
    LCD_Command(0x01); // Clear display
}

6. Testing and Debugging Tips

If the LCD only shows blocks, check the contrast by adjusting the potentiometer and confirm the Enable and RS pins are correctly connected.
If no temperature reading appears or the value is always zero, check the LM35 sensor’s output with a multimeter and ensure the RA0 pin is set as an input.
If temperature readings flicker, try placing a small capacitor (like 100nF) between the LM35 output and ground to reduce noise.
Confirm the microcontroller’s oscillator is working by measuring the OSC pins with an oscilloscope if available.
Double-check ADC configuration and that the ADCON0 and ADCON1 settings match the hardware.

7. Project Extensions

Once the basic version is working, you can add more features:

Add a push button to switch between Celsius and Fahrenheit.
Trigger a buzzer or LED when the temperature exceeds a limit.
Log temperature values to EEPROM or an SD card.
Send temperature data wirelessly using an ESP8266 Wi-Fi module.
Optimize for low power by putting the microcontroller to sleep between readings.

🔧 PIC Microcontroller Project: Digital Temperature Monitor with LM35 and LCD

ByEducational Engineering Team