Frequency Counter using external interrupt in Arduino Sketch http://www.fypsolutions.com/tutorials/frequency-meter-using-arduino/

frequencyCoutnerArduino.ino

// include the library code:
#include <LiquidCrystal.h>

// initialize the library with the numbers of the interface pins
LiquidCrystal lcd(6, 7, 10, 11, 12, 13);
int freqCounter = 0;
long preMillis = 0;

void isr() //interrupt service routine
{
  freqCounter++;
}

void printCounting(){
  lcd.setCursor(6, 1); //6th column and second row
  lcd.print((freqCounter/1000)%10);
  lcd.print((freqCounter/100)%10);
  lcd.print((freqCounter/10)%10);
  lcd.print(freqCounter%10);
  lcd.print("Hz");
 
}

void setup() {
   // set up the LCD's number of columns and rows:
  lcd.begin(16, 2);
  // Print a message to the LCD.
  lcd.print("Frequency Meter");
  printCounting();
  delay(10);
  attachInterrupt(0,isr,RISING);  //attaching the interrupt

}


void loop() {
 
 while((millis()-preMillis)<1000);
 detachInterrupt(0);           //detaches the interrupt
 printCounting();
 freqCounter = 0;
 preMillis = millis();
 attachInterrupt(0,isr,RISING);  //attaching the interrupt again

}