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.

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.

A differential transformer magnetic permeability sensor includes a substrate, a drive coil, a first differential coil, a second differential coil, a first interconnection pattern, and a second interconnection pattern. The first differential coil is disposed at a side of a first surface of the substrate, and an induced voltage is generated therein when the drive coil is driven. The second differential coil is disposed at a side of a second surface of the substrate, and an induced voltage is generated therein when the drive coil is driven. The first interconnection pattern is located on the first surface and allows the first differential coil to serve as a reference coil, and the second differential coil as a sensing coil. The second interconnection pattern is located on the second surface and allows the second differential coil to serve as a reference coil, and the first differential coil as a sensing coil.