This cheap, small, 3d-printable, battery powered node allows you to check if your windows or doors are open or close!
As many other commercial device it uses a Reed switch and a magnet to detect the opening of a door or window.
The cost of the entire build is low (about 5€) and it can be used in many different cases like for example to detect the opening of a drawer or fridge door.
RFM69 (868 Mhz) and NRF24L01 (2.4 Ghz) radio modules are both supported and the behavior of the 4 LEDs can be configured in the firmware.
The expected battery life with a single CR2032 coin cell is more than 1 year.
Visit the dedicated wiki page to build yourself one of this sensor.