Contactless Metal Detector With Reed Contact

Description:

This is a metal detector, that is capable of detecting metals that can be magnetized like mild steel. The detection distance to a metal sheet is about 5mm. A pinball is detected till about 4.5mm.
The magnets uses are 5x2mm in size. I also tried larger magnets, but that did not increase the detection distance. Smaller ones did reduce the detection range. The 5mm seems to optimal.
The metal is detected, because its presence disturbs the magnetic field. The field is no longer ba