A document feeder may include a media transporter to feed a number of sheets of media through a feed path and a detector to detect the presence of a metal object including a number of triangular-shaped inductor coils to detect the metal objects. A method of detecting a metal object coupled to a sheet of media entering a document processing device may include applying an alternating current to a number of triangular-shaped inductor coils to create a magnetic field upstream of a media feed path, detecting a change in a magnetic flux of the magnetic field due to a metal object in the path of said magnetic flux, and producing a warning when the changes in the magnetic flux are detected.