#include <Wire.h> 
#include <LiquidCrystal_I2C.h>
LiquidCrystal_I2C lcd(0x20,16,2);  // LCD address to 0x20 (16 chars on 2 line)
volatile float time = 0;
volatile float time_last = 0;
void setup() {
  lcd.init(); 
  lcd.backlight(); 
  lcd.setCursor(0,0);        // (col,row)
  lcd.print("rpm: "); 
  attachInterrupt(0, rpm_interrupt, RISING);     //Pin 2 As Interrupt ( if signal increase)
}

void loop() {                    // Loop To Calculate RPM and Update the LCD Display
  int rpm = 0;
    while(1){ 
      delay(500);                //Slow Down Display Updates ( half second)
      if(time > 0) { 
        rpm = 60*(1000000/(time));  
        lcd.setCursor(0,1);
        lcd.print(rpm);  
        }   
    }
}

void rpm_interrupt() {              //Capture The OPB704 Beam =>Interrupt
   time = (micros() - time_last); 
   time_last = micros();
}