#include <LiquidCrystal_I2C.h>
LiquidCrystal_I2C lcd(0x27, 16, 2);

float value = 0;
float rev = 0;
int rpm;
int oldtime = 0;
int time;

void isr()
{
  rev++;
}

void setup()
{
  lcd.init();
  lcd.init();
  lcd.backlight();
  attachInterrupt(0, isr, RISING);
}

void loop()
{
  delay(1000);
  detachInterrupt(0);
  time = millis() - oldtime;
  rpm = (rev / time) * 60000;
  oldtime = millis();
  rev = 0;
  lcd.clear();
  lcd.setCursor(0, 0);
  lcd.print("   Tachometer   ");
  lcd.setCursor(0, 1);
  lcd.print("RPM: ");
  lcd.setCursor(5, 1);
  lcd.print(rpm);
  lcd.print("   ");
  attachInterrupt(0, isr, RISING);

}