#include <PinChangeInterrupt.h>

#define LED_PIN 12
#define FASTER_BUTTON 3
#define SLOWER_BUTTON 2

int ledDelay = 200;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  pinMode(LED_PIN, OUTPUT);
  pinMode(FASTER_BUTTON, INPUT_PULLUP);
  pinMode(SLOWER_BUTTON, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(FASTER_BUTTON), faster, CHANGE);
  attachInterrupt(digitalPinToInterrupt(SLOWER_BUTTON), slower, CHANGE);
}

void loop() {
  // put your main code here, to run repeatedly:
    digitalWrite(LED_PIN, HIGH);
    delay(200);
    digitalWrite(LED_PIN, LOW);
    delay(ledDelay);
    /*
      I have made the LED to be ON for 200ms (constant)
      Only the OFF state will vary.
      The LED will blink faster or slower.
    */
}

void faster() {
  if(digitalRead(FASTER_BUTTON) == HIGH) {
    Serial.println("Frequency Increased");
    if(ledDelay > 200) {
      ledDelay -= 200;
    }
    else {
      Serial.println("Caannot lower the delay than this");
    }
  }
}

void slower() {
  if(digitalRead(SLOWER_BUTTON) == HIGH) {
    Serial.println("Frequency Decreased");
    ledDelay += 200;
  }
}