My window spans two floors which was a big advantage in this case. The window cannot open far enough for any intruder to go in and i could install the steppermoter on the second floor where it is out of the way.I installed the stepper motor upstairs and attached it with only one screw so it can rotate a bit. This is neccesary because the window frame doesn't move along a line but along a curve. It also enables you to move the threaded rod out off the nutholder so you can manually open and close the window.To attach the threaded rod to the axis of the stepper you'll need couplings. My Stepper motor has a pulley fixed on it so i had to use the couplings that i also uploaded. For a normal stepper motor you can find lots of couplings by searching for Z coupling on Thingiverse.I used a lasercutter to cut the nutholderpieces from 5mm birch plywood. I drilled a little hole in the top so i could use a pin to keep the threaded rod inside the nutholder. If i want to manually open the window i can take the pin out and move the threaded rod out of the nutholder.Now you have a motor that can open and close a window. Next thing is the electronics to control the motor. I used an Arduino uno  with a shield that allowed me to connect a steppermotor, 2 sensors ,a 12v powersupply, some leds and a stepper motor driver. I made the shield from a piece of prototyping boardYou can find the schematics and the code for the uno in the documents.To connect the pir motion sensors to the shield i used an old network cable, it has 8 threads of which i only needed 6.