Cool Idea.....
Here is a thought, maybe someone could come up with some sort of knocking device that was attached to the door. A small hammer, possibly on a pivot and a plate of metal so that it made a sharper noise.
Alternatively a high-tech approach could be to have a battery, button, a bit of dual-core wire and then some sort of noise making device. I don't know, maybe a dome shaped bell with a small striker that rotated with the button was pressed, or an electromagnet then when activated hit a chime bar.
Maybe I should look at patenting these ideas for a reliable door alert system that does not require an Internet connection to work.
Biting the hand that feeds IT
