Accelerometer on a cell phone.  There are apps to measure cell phone acceleration. The accelerometer is used to determine “up” for the orientation of the screen.  The accelerometer is probably a microscopic piezoelectric beam or beams that bend under the force of gravity or equivalently under acceleration.  The bending of a piezoelectric produces a charge that can be detected electronically.  Talk about the equivalence principle up to and including “black holes.”

Cell Phone Accelerometer