1) CAPTCHA

#include <LiquidCrystal.h>
LiquidCrystal lcd (8 , 9 , 4 , 5 , 6 , 7) ;

void setup (){
  Serial.begin(9600) ;
  lcd.begin (16 ,2) ;
  randomSeed( 3 ) ;
}

char rndChar(){
  switch(random (1,4)){
  case 1: return random (48, 57);
  break;
  case 2: return random (65, 90);
  break;
  case 3: return random (97, 122);
  break;
  default:
  Serial . println ( "greskaUrandChar" );
}}

char* rndString(char* str, int sSize){
  int i = 0;
  for(i = 0; i < sSize; i++) str[i] = rndChar();
  str[sSize - 1] = '\0';
  return str;
}

#define DUZINA_RAND 5
char randString[DUZINA_RAND];

char string [64]; //bafer u koji se smesta primljeni string
int duzina ;

void loop () {
    lcd.clear();
    lcd.setCursor(0, 0);
    lcd . print (rndString(randString, DUZINA_RAND)) ;

    while (!Serial.available()) ;
    delay (100);
    duzina = Serial.available() ;
    Serial.readBytes(string,duzina) ;
    string[duzina] = 0;

    lcd.setCursor(0, 2);

    if(NULL != strstr(string, randString))lcd.print ("HUMAN");
    else  lcd.print ("BOT") ;

    delay (1000);
  }