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 balanced and that activates the reed switch.

I designed these to detect a pinballs in a pinball machine.
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Building:

- Heat the copper wires and push them in the sides of the plastic. The wires need to stick out a couple of mm, so you can solder the reed switch wires to it.
- Push the reed switch through the center hole. If closed, use a drill to open the hole.
- Put the 2 magnets in place in such a way that they attract each other.
- Test sensor with a piece of metal. You can hear the reed switch closing and opening. Sometimes you need to rotate the reed contact to find the position where it reacting correctly.  The magnets need to be of equal strength, otherwise the reed contact will not open . 
- Then bend the wires up against the copper wires and solder them.
- Finally push some isolated wires through the side hole, and solder them to the copper wires.

See the photo's how everything is mounted.
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Materials:

You will need the following parts

- 7mm or 10mm glass reed switch : for example type MKA-10110 from http://www.aliexpress.com/item/2016-Hot-10-pcs-MKA-10110-10-15AT-Magnetic-Sensor-N-O-SPST-Contact-Reed-Switches/32682392705.html 
- 2 pieces of  5cm long  1mm  diameter copper wire
- 2 magnets size 5x2mm or 5x1.8mm from  http://www.banggood.com/100pcs-5mmx2mm-N52-Strong-Round-Magnets-Rare-Earth-NdFeB-Neodymium-Magnets-p-988439.html  . Enough to make 50 sensors!

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Print:

I used ABS, but PLA , PET or other materials also work. 90% to 100% fill. No support needed.