Velocimetro

#include <Stepper.h>
#define STEPS 32 // 40 - 60 Steps for Opel Tacho
#define SPEED 7000 // Speed in Rounds per Minute
#define Reed 2                  // Port do reed switch
Stepper stepperSPEED(STEPS, 8, 10, 9, 11);
float rev = 0;
int rpm;
unsigned long Tempo;
int diferenca = 0;
int rpm_old = 0;
//------------------------------
void isr() //interrupt service routine
{
  rev++;
}
//-------------------------------
void setup()
{
  pinMode(Reed, INPUT_PULLUP);                          // Define o  port reeed como entrada e liga o pull up
  Serial.begin(9600);
  stepperSPEED.setSpeed(60);
  attachInterrupt(0, isr, RISING);                   // Interrupt do reed
  Tempo = millis();
}
//------------------------------
void loop()
{
  if (millis() - Tempo > 1000)
  {
    rpm = rev;
    Tempo = millis();
    rev   = 0;
 //   Serial.println(rpm);
  }
  diferenca = rpm - rpm_old;
  stepperSPEED.step(diferenca);
  rpm_old = rpm;
//  Serial.println(rpm);
//  Serial.println(rpm_old);
  if (diferenca != 0)
  Serial.println(diferenca);
}