A simple, sensitive metal detector

This is a implementation of a project 561 from a Electronics Today International.

The Protel MS-DOS freeware software can be downloaded as ZIP-File from Protel Site.

ETI Project 561
Simple, Sensitive Metal Detector
Phil Wait
Electronics Today International

The Search Coil (abbreviated from the original article)

The greater the coil diameter the greater the penetration depth, but the less sensitive it is to small objects. As a general rule the penetration is about equal to the search coil diameter, while the sensitivity is roughly proportional to the cube of the object diameter. Sensitivity is also inversely proportional to the sixth power of the distance.

The chosen coil has 150 mm diameter to give good sensitivity to small objects giving about 100 - 150 mm penetration. Remember that if the coil diameter is increased the number of turns have to be reduced to maintain constant frequency.

Faraday Shield

To remove the effect of capacitance to the seearch coil it is enclosed in a Faraday shield. The sield consists of a wrapping of aluminium foil around the coil but broken at one point so it does not make a shorted turn. This shield is then connected to the ground potential of the detector.

Construction

The search coil is mounted on a 165 mm diameter plastic pot stand. The connection to the search coil is via a length of shielded cable.

The search coil is wound so that it can be tucked inside the rim of the up-turned plastic pot stand. First make a cardboard former of the appropriate diameter. Wind then the coil into this former. The coil contains 70 turns of 0.4 mm enamelled wire. Wind two layers of insulation tape around the coil leading the ends out at the same place.

Next wind the Faraday shield. Cut some aluminium kitchen foil into 15 mm wide strips and wind ovet the coil to make two layers but leaving a small gap about 5 mm to 10 mm wide where the coil ends come out.

Wind a lendth of tinned copper wire around the shield. Wind it tight with around 10 mm between successive turns. The end of this wire is taken out at the same place as the coil connections.

Now wind another two layers of insulation tape around the whole assembly. Drill a hole in the side of the pot stand and the press the coil down into the rim. Pass the wires thru the hole. Pour epoxy over the coil to hold it in place.

Solder the coil connections to the twin shielded cable, tha Faraday shield connecting to the cable's shield, and glue the cable and wires underneath the pot stand to hold them rigid.

pam(at)iki.fi Updated: 2004-12-14