Project: Automatic Light Control with LDR and PIC16F877A

1. Introduction

In this project, we will build an automatic light control system using a Light Dependent Resistor (LDR) and the PIC16F877A microcontroller. The microcontroller reads ambient light intensity via the LDR and automatically turns an LED or relay-controlled lamp ON in darkness and OFF in brightness.

Real-World Applications:

• Streetlights that turn on at night
• Energy-saving home lighting
• Automatic garden or security lights

2. Learning Objectives

• Learn how to read analog signals using PIC ADC (Analog-to-Digital Converter)
• Understand light sensing with an LDR voltage divider
• Use conditional logic to drive outputs based on sensor input
• Control devices like LEDs or relays with transistors and PIC I/O

3. Tools and Components

Hardware Required:

• PIC16F877A microcontroller
• Light Dependent Resistor (LDR)
• 10kΩ resistor (for LDR voltage divider)
• LED or 5V Relay with NPN transistor (e.g., BC547)
• Flyback diode (e.g., 1N4007) for relay
• Resistors: 330Ω for LED, 1kΩ for transistor base
• Power supply: 5V regulated
• Breadboard, jumper wires
• Crystal oscillator (20MHz) + 22pF capacitors (if external)
• MPLAB X IDE + XC8 Compiler
• PIC Programmer (e.g., PICKIT 3)

4. Background Concepts

• LDR (Light Dependent Resistor): Resistance decreases as light intensity increases.
• Voltage Divider: Converts LDR resistance changes into voltage readable by the PIC ADC.
• ADC (Analog-to-Digital Converter): Converts voltage (0-5V) to a 10-bit digital value (0-1023).
• Thresholding: Define a light level threshold; below it, we turn the light ON.

5. Step-by-Step Assembly and Code

Circuit Wiring Overview

1. LDR Voltage Divider:
   • Connect one leg of the LDR to 5V.
   • Connect the other leg to both:
     • RA0 (AN0) of PIC
     • One leg of the 10kΩ resistor; connect the other end of resistor to GND.
   • Result: Analog voltage at RA0 changes with light intensity.
2. Output Control (LED or Relay):
   • LED Option:
     • Connect LED anode to RB0 via 330Ω resistor.
     • LED cathode to GND.
   • Relay Option:
     • RB0 → 1kΩ resistor → Base of BC547 transistor.
     • Emitter to GND.
     • Relay coil between 5V and collector.
     • Diode across relay (cathode to 5V) to protect from voltage spikes.

Source Code with Comments (XC8)

#include <xc.h>
#define _XTAL_FREQ 20000000  // 20MHz Oscillator

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

void ADC_Init();
unsigned int ADC_Read(unsigned char channel);

void main() {
    unsigned int light_value;
    const unsigned int threshold = 500; // Set threshold (adjust as needed)

    TRISA = 0x01;     // RA0 as input (AN0)
    TRISB0 = 0;       // RB0 as output
    PORTB = 0x00;     // Initialize outputs to 0

    ADC_Init();       // Initialize ADC

    while(1) {
        light_value = ADC_Read(0);  // Read LDR analog value

        if (light_value < threshold) {
            RB0 = 1;  // Turn ON light (LED or relay)
        } else {
            RB0 = 0;  // Turn OFF light
        }

        __delay_ms(500);  // Sampling delay
    }
}

// ADC Initialization Function
void ADC_Init() {
    ADCON1 = 0x80;  // Right justified, Fosc/32
    ADCON0 = 0x01;  // Enable ADC, select AN0
}

// ADC Read Function
unsigned int ADC_Read(unsigned char channel) {
    ADCON0 = (channel << 3) | 0x01;  // Select channel and turn on ADC
    __delay_us(20);                  // Acquisition delay
    GO_nDONE = 1;                    // Start conversion
    while (GO_nDONE);                // Wait until conversion completes
    return ((ADRESH << 8) + ADRESL); // Return result
}

6. Testing and Debugging Tips

• LDR not responding: Verify LDR connections and check voltage at RA0 using a multimeter.
• ADC reads 0 or erratic values: Confirm ADC initialization and sampling time. Add a capacitor (~0.1µF) across LDR for stability.
• Light doesn’t switch ON/OFF properly: Adjust the threshold value based on your room lighting conditions. Test different values between 300–700.
• Relay chattering or no activation: Ensure sufficient base current to the transistor. Check diode placement across relay coil.

7. Project Extensions

• Add an LCD display to show real-time light levels.
• Incorporate manual override switch to force the light ON/OFF.
• Use EEPROM to store the threshold and allow dynamic adjustment.
• Implement PWM dimming instead of ON/OFF switching for adjustable brightness.