Build a Simple Robotic Metal Detector (part two)

Building a BFO Metal Detector

This is part two of this set of instructions for building a BFO Metal detector for your robot. If you missed part one check here:

Making the Coils

You Will Need:
    • 1 role of 0.25 mm enamel coated copper wire
    • 1 plastic sheet that is at least 0.26 mm thick and 16 cm in diameter
    • 2 sheets of plywood wood or sturdy plastic that are at least 20 cm in diameter and 3mm thick
    • wood or plastic dowel  12 mm in diameter and 60 mm long
    • a 1 cm length of brass pipe about 14 mm in diameter
    • a cutting tool and a way to draw a circle 15 cm in diameter
    • standard tools
    • some nuts and bolts, tape and perhaps some glue

Smaller disk mounted on larger disk

side view of the completed disk

enamel coated copper wire
  1. You will need a form to hold the search coil. Begin by cutting your two sheets of plywood into circles 16 cm in diameter. Cut the plastic sheet into a circle that is 15 cm diameter. Essentially any material will work for the circles as long as it does not conduct electricity and stays fairly rigid.
  2. Center the smaller circle one of the two larger ones. Drill a small hole through the larger circle that you are using (a 1/16 drill-bit or smaller will work fine) the hole should be located exactly at the edge of the smaller circle when it is centered on the larger one. Make sure that the smaller disk will stay centered while you bolt the two halves together. If the disks will not stay then you will have to use a small amount of glue. Then drill some holes and tightly bolt the two halves together. When adding the bolts you must make sure that you do not put them any closer than 2 cm from the edge of the circle because your goal is to make a disk with a single thin channel along the outside.
  3. Wind 10 turns of the enamel coated copper wire around the groove in the edge of the former. Connect this coil when finished to the points marked coil 1 on the schematic.
  4.  Begin winding the coil for the reference oscillator by wrapping 125 turns of the enamel coated copper wire around the piece of dowel. Drill a small hole through each end of the wooden dowel so that you can pass the beginning and end of the wire through these holes to keep the windings in place. Wrap some tape around the coil until the piece of brass pipe will fit snugly over it without falling off. You should also be able to slide the brass pipe without damaging the coil as this will be important for tuning the metal detector later.
This concludes part two of how to 'Build a Simple Robotic Metal Detector'.