#define CLK 3
#define DT  2

int lastClk = HIGH;
int Speed = 0;
unsigned long Old = 0;

void setup() {
  Serial.begin(115200);
  pinMode(CLK, INPUT);
  pinMode(DT, INPUT);
}
void loop() {
  unsigned long New = millis();

  int newClk = digitalRead(CLK);
  if (newClk != lastClk) {

    lastClk = newClk;
    Speed++;
  }

  if (New-Old >= 1000) {
   
    int RPM = Speed * 3;
    Speed = 0;
    Serial.print("RPM = ");
    Serial.println(RPM);
    Old = New;
  }
}