With this genious little triangle you are able to use one optical sensor for a double window. No matter in which constelation you open or close the windows, it will always be detected.

How it works:

The triangle will be applied to one of the windows underneath the sensor. One reflective sticker is on the triangle, the other one on the other window.

I use a homematic HMIP-SWDO with the original reflection stickers, but any other reflective sticker will work. Just be sure to position everything correct.

I upgraded the design but will not update mine for the moment as i would waste all the reflective stickers.

The sensors turn off the radiator when the window is opened and they could also be used for security alarms.