How to make RPM Counter with Arduino

           CODE 

#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x3F, 2, 1, 0, 4, 5, 6, 7, 3, POSITIVE);  // use lcd address 0x27 in the place of 0x3F ,if LCD not workd

float value=0;

float rev=0;

int rpm;

int oldtime=0;

int time;

int sec;

void isr() //interrupt service routine

{

rev++;

}

void setup()

{

lcd.begin(16,2);                //initialize LCD

attachInterrupt(0,isr,RISING);  //attaching the interrupt

}

void loop()

{

delay(1000);

detachInterrupt(0);           //detaches the interrupt

time=millis()-oldtime;        //finds the time 

rpm=(rev/time)*60000;         //calculates rpm

oldtime=millis();             //saves the current time

rev=0;

lcd.clear();

lcd.setCursor(0,0);

lcd.print("   RPM COUNTER  ");

lcd.setCursor(0,1);

lcd.print(     rpm);

lcd.print(" RPM");

lcd.print(" ");

sec=rpm/60;

//lcd.print(   sec);

//lcd.print("/s");

attachInterrupt(0,isr,RISING);

}