Header file:

 

/*
ArduinoTestIO
 Auteur     : Henk Siewert
 Datum      : 31 oktober 2022
 Versie	      : 001
 MCU	      : ATMega328
 Language : Arduino C++
 Compiler  : Arduino 1.8.19
*/
 
#include

void setup()
{
  // Initialize digital pin LED_BUILTIN as output.
  pinMode( LED_BUILTIN, OUTPUT );
  Serial.begin( 115200 );                         // Data to Serial port
}
void loop()
{
  digitalWrite( LED_BUILTIN, HIGH );   // Turn the LED on
  Serial.println( "Let is aan" );
  delay(1000);                                           // Wait for a second
  digitalWrite( LED_BUILTIN, LOW );    // Turn the LED off
  Serial.println( "Let is uit" );
  delay(1000);                                          // Wait for a second
}