AVR Analog to digital conversion Atmega32

Question

I'm making some system that measure the environment light and turn off or on the light switch. To do this I have to use Atmega micro controller. The light measuring is done using LDR. LDR always output an Analog value and I have to convert it to digital value using AVR's ADC feature. I only have small knowledge in micro-controller programming. I write some code but I have no idea how to turn on relay switch using AVR.

this is my code

#ifndef F_CPU
#define F_CPU 8000000UL
#endif

#include <avr/io.h>
#include <stdlib.h>
#include <avr/interrupt.h>


int main(void)
{

    ADCSRA |= 1<<ADPS2;
    ADMUX |= 1<<ADLAR;
    ADCSRA |= 1<<ADIE;
    ADCSRA |= 1<<ADEN;
    sei();
    ADCSRA |= 1<<ADSC;
    while(1)
    {


    }
}   

ISR(ADC_vect)
{

    char adcres[4];
    itoa (ADCH, adcres, 10);

    PORTC=0x01; // Turn ON relay switch

    ADCSRA |= 1<<ADSC;
}

I want to measure analog values using attached LDR and convert them in to digital values. Then after some per-define number relay should turn on and

I need somethins like this

lux = ldr_digital_value

if (lux > 5 )
   { PORTC=0x00; }
else
   { PORTC=0x01; }

How can I do that ?

Answer 1

assuming an ATmega8 (there are some differences between avrs)

#ifndef F_CPU
#define F_CPU 8000000UL
#endif

#include <avr/io.h>
#include <stdlib.h>
#include <avr/interrupt.h>

volatile unsigned char lux=0; // the trick is the volatile.

int main(void)
{

    ADCSRA = 1<<ADPS2; //slowest clock, ok
    ADMUX  = 1<<ADLAR; // you want 8 bit only? also channel 0 selected and external VREF.
    // I suggest you to use Internal AVCC or internal 2.56V ref at first
    ADCSRA |= 1<<ADIE; // with interrupts wow!
    ADCSRA |= 1<<ADEN; // enable!
    sei();
    ADCSRA |= 1<<ADSC; // start first convertion
    while(1)
    {
         if (lux > 5 ) //please choose an appropiate value.
            { PORTC=0x00; }
         else
            { PORTC=0x01; }

    }
}   

ISR(ADC_vect)
{    
    lux =ADCH;    
    ADCSRA |= 1<<ADSC;
}