the device comprises at least one lead screw driving a sliding yoke whose guide bar is mounted on a rocker arm . the lead screw actuates a switch by means of a cam via the sliding yoke . a deliberate action on the rocker arm moves away the sliding yoke which is brought instantaneously into initial position by a spring . the lead screw is for preference disengageable by means of an arm driven by the axle of the sliding yoke . this arm provides an angular positioning of the lead screw by acting on a heart - shaped cam . the device permits rapid , sure and precise setting of the stopping points of a motor .