If you want to hang a picture, install a shelf, or accomplish many other household tasks, you need a level to make it look professional and straight. While spirit levels with bubbles in the middle work very well, if trusting your eye to judge whether or not the air pocket is actually in the middle of the window bothers you, then GreatScott! has a solution with his digital version.
His device uses an MPU-9250 accelerometer to measure the angle with respect to the Earth, selected after the popular MPU-6050 turned out to generate too much noise for this application. A very simple setup is first produced using a breadboard and an Arduino Nano with a serial output. From there, a more refined version uses an OLED display attached to the breadboard itself.
While useful in a pinch, a breadboard configuration isn’t what you would want for everyday use, so GreatScott! set to work measuring the components, then designing a housing for his device in Fusion360. The resulting build should have a run time of 28 hours on the LiPo used, and he reports that while there are some potential accuracy problems, it does work decently. It looks like a great starting point for a really refined instrument, and you can see his digital level in the video below!