a projection device for selectively projecting one of a large number of images . a stepper motor operatively positions a selected transparency on a cartridge , containing a plurality of transparencies , within a projection system for illumination and projection of the image from the transparency onto a viewing screen . apparatus is provided for automatically identifying the transparency cartridge which is being controlled by the stepper motor and for determining a homing position . the stepper motor always returns the cartridge to a starting or home position before moving the number of steps required to locate the selected transparency within the projection system .