After many mornings being woken up by a cat attacking the living room door to be let out, I decided to find a way to train her not to do it. The market has air spray based trainers but they are very expensive so I decided to spin my own.
It's very simple at its core: a motion activated, battery powered fan pointed at the area you don't want the cat to go. Cat walks in front of the sensor and it makes an annoying noise which, hopefully, makes the cat re-think their decision. It is completely harmless to the cat as it's just a very low powered motor with rubber blades.
Schematic to follow shortly.
PIR detector module
L298N Motor Driver
Interface and IO ICs / Peripheral Drivers and Actuators