Abstract: A hopper for receiving documents, such as checks, fed on edge from a conveyor and adapted to be stacked one atop the other inside the hopper includes a slot in one wall of the hopper through which a portion of a check projects when the stack reaches a predetermined height. A detector in the form of a light emitting member and light receiver is disposed on respective sides of the slot outside the hopper to provide a control signal when the light beam is interrupted due to projection of a portion of a check to signal when the stack has reached a predetermined height.