My neighbor had a little problem with his cat. It want's to go out and back inside too frequently. Usually he opens just his kitchen window and let's the cat in/out. Usually the cat stays only short periods outside and want's soon to come back inside. And then again out and back in. Okay, he could just let the window open, but here in Finland it can be easily -30 C (-22 F) in winter, so this is not a good solution.
We decided to build a microcontroller to the window to open and close the window automatically. Arduino controls a strong servo motor, which moves the window. It has three proximity sensors to sense where the is cat is.
Step 1: Parts list
1 pc suitable power adaptor to juice the Arduino (input voltage 7-12V dc)
1 pc Arduino microcontroller (or equivalent *duino)
1 pc strong servo motor to move the window. I used Towerpro MG995 Metal Servo with Gears (10kg Torque).
3 pc Pololu 1134 carrier board with Sharp GP2Y0D810Z0F Digital distance sensor 10 cm
1 pc "Third hand" to hold the sensors
1 pc cat (operator)
small window hinge
some metal parts to fix the servo to the window frame, screws, etc
Solder & soldering iron
Computer & usb cable to program the Arduino