pressing a 34 title command 34 push button 34 drum present 34 sensor 46 is positioned to go open circuit when a drum 1 is in place upon hub 10 it would be futile to operate the auxiliary unit if the operator had neglected to place a title drum into it this sensor may also be the known light pressure operating switch as the microswitch it is positioned so that rim 5 bears upon the operating plunger of the switch and causes it to remain open as long as the drum is in place either stationary or rotating this switch is also electrically connected as an input to block 45 in the main camera film 3 acts as a damping mechanical load upon stepper motor 22 it is important that an equivalent damping load be provided for stepper motor 20 so that strict motional synchronism between the two is maintained accordingly a spring loaded pressure pad 47 is provided to bear against the rear surface of hub 10 this pad has an adjustable tension device comprised of compression spring 48 and hollow bolt 49 by adjusting the bolt the pressure can be altered to give the required mechanical damping to the auxiliary projector system in a typical embodiment the increment of motion of film 3 and drum 1 is 1 4 mm per step of.