arduino uno R3 input pins with gsm shield

In my project i need to read input data from water flow sensor. It works well when i wire water flow with pin 2 or 3 , but does not work work when i wire it to any other pins. It becomes a problem because i need to use GSM shield and you know pins 2,3 and 7 reserved for arduino and modem

the code:

#include <LiquidCrystal.h>    // initialize the library with the numbers of the interface pins 

LiquidCrystal lcd(5, 6, 9, 10, 11, 12);

volatile int  TopsFan; //measuring the rising edges of the signal

int result;                               

int pin = 2;    //The pin location of the sensor



void rpm ()     //This is the function that the interrupt calls 

{ 

  TopsFan++;  //This function measures the rising and falling edge of the hall effect sensors signal

}

// The setup() method runs once, when the sketch starts

void setup()  

{ 

  pinMode(pin, INPUT); //initializes digital pin 2 as an input

  Serial.begin(9600); //This is the setup function where the 

serial port is initialized(USB port)

  attachInterrupt(0, rpm, RISING); // the interrupt is attached

} 

 lcd.begin(16, 2);     // set up the LCD's number of columns and rows

lcd.print("The water flow: ");    // Print a message to the LCD.


// the loop() method runs over and over again,

// as long as the Arduino has power

void loop ()    

{

  TopsFan = 0;   //Set TopsFan to 0 ready for calculations

  sei();      //Enables interrupts

  delay (1000);   //Wait 1 second

  cli();      //Disable interrupts

  result =  (TopsFan * 60 / 7.5); //(Pulse frequency x 60) / 

7.5Q, = flow rate in L/min 

 lcd.setCursor(0, 1);   //prepare the cursor on the screen

 lcd.print(result, DEC);  //Prints the number calculated above

 lcd.print(" L/min\r\n");  //Prints "L/min" and returns a  new 

line

}

Pls read https://www.arduino.cc/en/Reference/AttachInterrupt :

...

The first parameter to attachInterrupt is an interrupt number. Normally you should use digitalPinToInterrupt(pin) to translate the actual digital pin to the specific interrupt number. For example, if you connect to pin 3, use digitalPinToInterrupt(3) as the first parameter to attachInterrupt.

Board                               Digital Pins Usable For Interrupts
Uno, Nano, Mini, other 328-based    2, 3

...

It would seem you need to try a different approach, perhaps bypassing interrupts altogether. Something like the following:

#include <LiquidCrystal.h>    // initialize the library with the numbers of the interface pins 

int count_rpm(void) ;

LiquidCrystal lcd(5, 6, 9, 10, 11, 12);

int flow_pin = 5 ;    //The pin location of the sensor (5 or whatever suits you)

// The setup() method runs once, when the sketch starts

void setup()  

{ 
  pinMode(flow_pin, INPUT); //initializes digital pin 2 as an input
  Serial.begin(9600); //This is the setup function where the serial port is initialized(USB port)

  lcd.begin(16, 2);     // set up the LCD's number of columns and rows
  lcd.print("The water flow: ");    // Print a message to the LCD.
} 



// the loop() method runs over and over again,

// as long as the Arduino has power

void loop ()    
{
int result;                               

 result =  (count_rpm() * 60 / 7.5); //(Pulse frequency x 60) / 7.5Q, = flow rate in L/min 
 lcd.setCursor(0, 1);   //prepare the cursor on the screen
 lcd.print(result, DEC);  //Prints the number calculated above
 lcd.print(" L/min\r\n");  //Prints "L/min" and returns a  new line
}

int count_rpm(void)
{
  long start = millis() ;
  int count = 0 ;
  boolean current_state ;
  boolean previous_state = digitalRead(flow_pin) ;
  for( ; millis()-start <= 1000 ; )
  {
     current_state = digitalRead(flow_pin) ;
     if (current_state && !previous_state)
        count++ ;
     previous_state = current_state ;
  }
  return count ;
}