I was looking for a solution to open and close the window when king Cat or queen Cat decided to go outside without me having to get up and open the window by myself.

I first thought to motorize the window with something like a linear actuator or some kind of belt.

The window would open when a cat is detected, I found some animal RFID Arduino module directly from China. Oddly, animal chip RFID is not on the same frequency as mostly found RFID.

The two parts, detector and motor would be linked via wifi, MQTT and the rule engine of the Openhab instance currently running at home.

I had every mechanical parts gathered but I was worried about two points : 

- the whole system has to be inconspicuous as the windows is in the living room and I don't want to live in an engineering lab.

- the system has to be very robust, I don't want a cat locked outside, so availability : 100%

Then I realized, what is invisible and never break down  ? Gravity !  I gave up my first idea and imagine a (much) simpler solution : 

I installed a cable with a weight to close the door when it is open.

 As cats are not so heavy, the closing force has to be carefully chosen. Enough force to close the relatively heavy window but not too much to enable the cats to open by themselves. Putting pulleys drastically reduced friction and some spare washers allow to tweak the weight and force.

By installing the cable very close to the edge of the window and close to the wall, behind the pipes, everything is invisible when the window is closed.

And voila ! Low tech automated window closing system.

Now, it has to accepted by the users :

1st user test

Success

2nd user test :

Success

It's been 2 years and I only had to oil once the pulley ball bearing so the robustness is here. Sometimes, you don't need high tech to simplify your life at home !