Abstract: A motion sensor includes an array of interdigitated zig zag shaped conductors, a metal sphere for bridging adjacent conductors and a housing formed of an electrically insulative material in a one-piece molded assembly defining a chamber in which the sphere rolls upon the indigitated conductors. The chamber contains a cylindrical wall and an annular wall, the latter of which is of the geometery of a right conic section coaxial with the cylindrical wall. Movement of the ball bridges and unbridges adjacent conductors to cause electrical current surges. Electronic detector provides audible signal upon detection of such current surges. The cylindrical wall serves to rigidize the housing and the tapered inner wall renders the sensor more tolerant of positioning on an installed object.