Sheet size detecting mechanism

A sheet size detecting mechanism for a sheet input tray of an automatic document feeder, the sheet size detecting mechanism comprising:

• a sheet guiding module installed on the sheet input tray, and movable relative to the sheet input tray;
• a sensing module installed on the sheet input tray, and comprising a contact element and a circuit board, wherein the contact element is connected with the sheet guiding module, the circuit board comprises a first conducting part, a second conducting part and plural fixed resistors, and the first conducting part comprises plural conductive segments, wherein the plural conductive segments are spaced apart, and the plural conductive segments are connected with the corresponding fixed resistors, respectively, wherein while the sheet guiding module is moved relative to the sheet input tray and moved in a direction to be contacted with two parallel edge sides of a first paper sheet, the contact element is moved with the sheet guiding module, wherein when the contact element is contacted with a first conductive segment of the plural conductive segments of the first conducting part and the contact element is contacted with the second conducting part, a first fixed resistor of the plural fixed resistors connected with the first conductive segment provides a first resistance value, and the circuit board issues a first sensing signal according to the first resistance value;
• and
• a control unit receiving the first sensing signal and acquiring a distance between the two parallel edge sides of the first paper sheet according to the first sensing signal.