main.c

#define F_CPU 16000000UL

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

volatile unsigned int limit = 2;
volatile unsigned int count = 0;

int main(void)
{

  //Setup the clock
  cli();			//Disable global interrupts
  TCCR1B |= 0<<CS11 | 1<<CS10 | 1<<CS12;	//Divide by 1024
  OCR1A = 15625;		//Count 15624 cycles for 1 second interrupt
  TCCR1B |= 1<<WGM12;		//Put Timer/Counter1 in CTC mode
  TIMSK1 |= 1<<OCIE1A;		//enable timer compare interrupt
  sei();			//Enable global interrupts

  //Setup the I/O for the buzzer
  DDRD |= 0xFF;		//Set PortD Pin0 as an output

  while(1) { }			//Loop forever, interrupts do the rest
}

ISR(TIMER1_COMPA_vect)		//Interrupt Service Routine
{
  count++;


  if (count > limit) {
    count = 0;

    // buzz once
    PORTD = 0x01;
    _delay_ms(200);
    PORTD = 0x00;

    _delay_ms(300);

    // buzz again
    PORTD = 0x01;
    _delay_ms(200);
    PORTD = 0x00;

    limit = ((rand()) & 0x1F) + 5;
  }
}