LCD1602A LCD 5V display module (IIC/I2C interface)
Experimental wiring: Uno D2 connected to flow sensor OUT, relay connected to D4
Arduino------LCD1602
5V-------------VCC
GND-----------GND
A4-----------SDA IIC data cable
A5-----------SCL IIC clock line

Experimental open source code

/*
   [Arduino] 168 sensor module series experiments (data code + simulation programming + graphics programming)
   Experiment 88: LCD1602A LCD 5V display module (IIC/I2C interface)
   Project 21: Water flow sensor, 5V relay module and LCD1602 I2C module
   Experimental wiring: Uno D2 connected to flow sensor OUT, relay connected to D4
   Arduino------LCD1602
   5V-------------VCC
   GND-----------GND
   A4-----------SDA IIC data cable
   A5-----------SCL IIC clock line
*/

#include <LiquidCrystal_I2C.h> //include LiquidCrystal Library

LiquidCrystal_I2C lcd(0x27, 16, 2);
#define FLOWSENSORPIN 2 //Water flow sensor connected to Arduino digital pin 2
#define relayPin 4 // 5v relay module connected to Arduino digital pin 4
volatile uint16_t pulses = 0; // count how many pulses
volatile uint8_t lastflowpinstate; // track the state of the pulse pin
volatile uint32_t lastflowratetimer = 0; // you can try to keep time of how long it is between pulses
volatile float flowrate; // and use that to calculate a flowrate
// Interrupt is called once a millisecond, looks for any pulses from the sensor!

SIGNAL(TIMER0_COMPA_vect) {
   uint8_t x = digitalRead(FLOWSENSORPIN);

   if (x == lastflowpinstate) {
     lastflowratetimer++;
     return; // nothing changed!
   }

   if (x == HIGH) {
     //low to high transition!
     pulses++;
   }
   lastflowpinstate = x;
   flowrate = 1000.0;
   flowrate /= lastflowratetimer; // in hertz
   lastflowratetimer = 0;
}

void useInterrupt(boolean v) {
   if (v) {
     // Timer0 is already used for millis() - we'll just interrupt somewhere
     // in the middle and call the "Compare A" function above
     OCR0A = 0xAF;
     TIMSK0 |= _BV(OCIE0A);
   } else {
     // do not call the interrupt function COMPA anymore
     TIMSK0 &= ~_BV(OCIE0A);
   }
}

void setup() {
   Serial.begin(9600);
   Serial.print("---Water flow sensor---");
   lcd.init(); // Initialize the LCD display
   lcd. backlight();
   lcd.begin(16, 2); //16X2 lcd display
   lcd.setBacklight(HIGH);
   lcd.setCursor(0, 0); //setting display position
   lcd. print("Aqua counter");
   pinMode(FLOWSENSORPIN, INPUT); //sets the FLOWSENSORPIN as an INPUT
   pinMode(relayPin, OUTPUT);//sets the relayPin as OUTPUT
   digitalWrite(relayPin, LOW);
   digitalWrite(FLOWSENSORPIN, HIGH);//optional Internal Pull-Up
   lastflowpinstate = digitalRead(FLOWSENSORPIN);
   useInterrupt(true);
   delay(2000);
   lcd. clear();
}

void loop()
{
   lcd.setCursor(0, 0);
   lcd. print("Pulses:"); lcd. print(pulses, DEC);
   lcd. print("Hz:");
   lcd. print(flowrate);
   //lcd. print(flowrate);
   Serial.print("Freq: "); Serial.println(flowrate);
   Serial.print("Pulses: "); Serial.println(pulses, DEC);

   // if a plastic sensor use the following calculation
   // Sensor Frequency (Hz) = 7.5 * Q (Liters/min)
   // Liters = Q * time elapsed (seconds) / 60 (seconds/minute)
   // Liters = (Frequency (Pulses/second) / 7.5) * time elapsed (seconds) / 60
   // Liters = Pulses / (7.5 * 60)
   float liters = pulses;
   liters /= 7.5;
   liters /= 60.0;

   /*
     // if a brass sensor...