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My sneaky cat can open all doors of the apartment, even the fridge door. No meat is safe any more.
To prevent that I built a cat repeller using an Atmel ATmega328P Arduino Nano 3.0 board:
The firmware is available on git.
The device is placed in front of the fridge and is battery powered using 3xAAA batteries and a 5V boots regulator. This last only around two days before the batteries need to be recharged.
Why does a 24-25 kHz sound annoy the cat? There is no psych-acoustic secret behind it. It's just a loud ultra-sound (> 20 kHz) in a range the cat can hear, but not humans can't. Else it would annoy us too.