Tachometer

// Tachometer with lcd i2c screen
#include <Wire.h>
#include <LiquidCrystal_I2C.h>
LiquidCrystal_I2C lcd(0x27, 16, 2);
#define sensor_pin 2
int READ = 0;
int i = 0;
bool mask = 0;
unsigned long timer = 0;
float Time = 0.0;
int RPM = 0;
const int revnum = 20;
void setup()
{
  pinMode(sensor_pin, INPUT);
  //  Wire.begin();
  Serial.begin(115200);
  lcd.begin();
  lcd.backlight();
  timer = millis();
}

void loop()
{
  READ = digitalRead(sensor_pin);
  if (READ == 0 && mask == 0) {
    i++;
    mask = 1;
  }
  if (READ == 1 && mask == 1) mask = 0;
  if (i == revnum) {
    Time = millis() - timer;
    Time = Time / 1000;
    RPM = ( revnum / Time ) * 60;
    lcd.clear();
    lcd.setCursor(0, 0);
    lcd.print(revnum);
    lcd.setCursor(3, 0);
    lcd.print( "revs / ");
    lcd.setCursor(10, 0);
    lcd.print(Time);
    lcd.setCursor(15, 0);
    lcd.print("s");
    lcd.setCursor(0, 1);
    lcd.print("RPM = ");
    lcd.setCursor(6, 1);
    lcd.print(RPM);
    lcd.setCursor(12, 1);
    lcd.print("rpm");
  }
  if (i >= revnum) {
    i = 0;
    timer = millis();
  }
  Serial.print(i);
  Serial.print("  ");
  Serial.print(mask);
  Serial.print("  ");
  Serial.print(Time);
  Serial.print("S");
  Serial.print("  ");
  Serial.print(RPM);
  Serial.println("RPM");

}