int bLed = 3;
int gLed = 4;
int rLed = 6;
int pin = 9;
unsigned long duration;
float T = 0;
int F = 1/T;

void setup() {
  // put your setup code here, to run once:
  pinMode(pin, INPUT);
  Serial.begin(9600);
  pinMode(bLed, OUTPUT);
  pinMode(gLed, OUTPUT);
  pinMode(rLed, OUTPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
  delay(1000);
  duration = pulseIn(pin, HIGH);
  T = 2*duration;
  Serial.print("F: ");
  F = 1000000*1/T;
  Serial.println(F, DEC);

if (F<1000)
{  
  digitalWrite(bLed, HIGH);
  digitalWrite(gLed, HIGH);
  digitalWrite(rLed, HIGH);
  delay(1000);
}

if (F>1000)
{
  digitalWrite(bLed, LOW);
  digitalWrite(gLed, LOW);
  digitalWrite(rLed, LOW);
  delay(1000);
}
}