#define LED_pin 9

// Track LED state
bool LED_state = false;

// Set frequency of LED
float blinkrate = 5; // In Hz
float delay_between_switch = 500.0/blinkrate; // Blink at N Hz means you switch every 1s/(N/2)

// Track blinking cycle times of LED
long lastBlinkedTime;

void setup() {
  pinMode(LED_pin, OUTPUT);
  lastBlinkedTime = millis();
}

void loop() {
  long now = millis();
  if (now - lastBlinkedTime > delay_between_switch){
    digitalWrite(LED_pin, LED_state);
    LED_state = !LED_state;
    lastBlinkedTime = now;
  }
}