#include <Keypad.h>

const byte ROWS = 4;

const byte COLS = 4;

const byte LED_PIN = 31;

byte ledState = LOW;

char keys[ROWS][COLS] = {
  
  {'1', '2', '3', 'A'},
  
  {'4', '5', '6', 'B'},
  
  {'7', '8', '9', 'C'},
  
  {'*', '0', '#', 'D'}

};

byte rowPins[ROWS] = {9, 8, 7, 6};

byte colPins[COLS] = {5, 4, 3, 2};


Keypad keypad = Keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS );

void setup() {
  
  pinMode(LED_PIN, OUTPUT);

}
  
void loop(){
  
  char key = keypad.getKey();
  
  if(key == '0') {
    
    toggleLED();

  }

}

void toggleLED() {

  if(ledState == LOW) {

    ledState = HIGH;

  }

  else {

    ledState = LOW;

  }

  digitalWrite(LED_PIN, ledState);

}