I want to be able to detect if a garage door is opened or closed with my Arduino. I don't know what kind of sensor would be suitable for this task. The Arduino and the unknown sensor will be placed approximately 4 meters from the garage door.
I live at the second floor of a house and last weekend the garage door left opened the whole night by mistake. I want a method to detect the door is opened. My first thought (I'm newbie in electronics) has been to place the Arduino I own in the garage, and putting some sensor detect the door is opened and then send via wireless to another Arduino place at the second floor that the door is opened and reproduce some sound to warn the garage door is opened.