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:
  * a HC-SR501 PIR motion detector first checks if the cat is nearby (or any other movement happened)
  * then an E18-D80NK IR range detector verifies if the cat is in front of the fridge (it's pawn is present at the corner of the fridge's door)
  * if this is the case 24-25 kHz PWM signals will generate an ultrasound using a piezoelectric diaphragm, encouraging the cat to leave the premisses
  * if it still opens the fridge, a switch is released, and a human audible alarm will sound until it is closed

The firmware is available on [[https://git.cuvoodoo.info/fridge-cat-repeller/|git]], with connection information.

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.

|  {{cat_repeller:motion.jpg?300}}  |  {{cat_repeller:range.jpg?200}}  |  {{cat_repeller:piezo.jpg?300}}  |
| PIR motion detector | IR range sensor | piezoelectric element |

|  {{cat_repeller:setup.jpg?500}}  |  {{cat_repeller:installation.jpg?300}}  |
| all elements connected | final installation |