Patent ID: 7992862

Claim:
A sheet size detecting mechanism for use in a sheet input tray of an automatic document feeder, said sheet size detecting mechanism comprising: an adjustable module disposed on said sheet input tray, and comprising: a first sheet guide arranged at a first side of said adjustable module; a second sheet guide arranged at a second side of said adjustable module; an adjustable part comprising a rack member, wherein said rack member is moved to a position where said first sheet guide and said second sheet guide are respectively in contact with two parallel edge sides of a sheet; and a circular gear engaged with said rack member and rotated as said adjustable part is moved; a sensing module for sensing a rotating angle of said circular gear, said sensing module comprising: a sustaining element connected to an underside of said circular gear, and synchronously rotated with said circular gear wherein said sustaining element comprises a first pin and a second pin; a circuit board comprising a circular sensing part with plural sensing regions, which are discretely arranged around said circular sensing part and where said circular sensing part further includes an annular region, wherein when said circular gear is rotated by an angle, said sustaining element is in contact with a specified one of said plural sensing regions, so that said circuit board generates a corresponding sensing signal and where said sensing signal is a current value; and a controlling unit for receiving said sensing signal and acquiring a distance between said two parallel edge sides of said sheet according to said sensing signal and where said controlling unit provides a voltage to said circuit board, wherein when said first pin is in contact with said annular region and said second pin is in contact with said specified one of said plural sensing regions, said circular sensing part provides a resistance value, so that said circuit board generates a corresponding current value.