Stack height sensing system

An electrophotographic printing machine of the type in which a latent image is developed on a photoconductive member and the developed image is transferred to a copy sheet with successive copy sheets being supplied from a stack thereof, wherein the improvement includes:

• a sheet feeder;
• a tray arranged to have a stack of copy sheets disposed thereon, said tray being adapted to move between a first position with an outermost copy sheet of a stack being adjacent the sheet feeder and a second position remote therefrom, said sheet feeder being adapted to advance successive outermost copy sheets from the stack thereof with said tray being in the first position;
• means for moving the tray to control the position thereof;
• means for generating a flow of air;
• and
• means for receiving the flow of air from said generating means, said receiving means being enabled to transmit a signal indicative of the absence of the stack of copy sheet at the first position in response to receiving the flow of air and being inhibited from transmitting the signal in response to the stack of copy sheets blocking the flow of air to indicate the presence of the outermost copy sheet of the stack of sheets being at the first position adjacent said sheet feeder, said moving means, in response to the signal from said receiving means, moving said tray to position the outermost copy sheet of the stack at the first position adjacent said sheet feeder.