Hi,

brilliant and very well documented instructable, I wish you well in the constest!

As a builder of various types of Metal Detector since I was a teenager (around 60 years ago!), with varying degrees of success in the early years, I hope that I can help you a little with your design mechanics.

I don't use MDF as its too heavy I feel, what I used is thinner, but far stronger plywood, which can be bought in water resistant versions as well.

A further change that I have done for my versions, is that I cut out "troughs" in the ply for the coils to sit in, with a woodwork electric router.

These "troughs" can be almost through the ply, as I use epoxy to hold the coils in place, and once everything is working, I fill the "troughs" with epoxy resin and sand down flat when it is fully hard, this returns the full strength and more to the ply!

This also protects the coils better, as they sit 1mm below the surface of the ply, from stones and other debris, that may damage a winding that is more exposed.

I still have an MD that I built more than 20 years ago using this method.

I also cut out any parts of the plywood "not needed" to further reduce weigh! Easier on a single coil design of course!

But do keep the "troughs" as narrow as possible as this reduces the amount of Epoxy/Polyester resin needed, as it is also heavier than plywood!

Once painted, you cannot see where the coils are on the lower surface of the search head.

Also, although your design is definitely great, but due to the narrow diameter/width of each sending coil, you will reduce the maximum depth for small object detection quite dramatically, as the field strength of the send/receive signal, is apparently "Cone" shaped downwards (and of course upwards as well!).

So deeper objects will only be seen more and more only on a vertical center line, through the center of each coil, till the signal strength is so weak, they would not be seen anymore.

I myself, always assume a coil with the field tapering off at about 45° angle towards the center, which means that a 10 cm coil, will detect well down to about 10 cm, all else being well designed, assuming good circuitry on both send and receive sides. I have not seen any data that makes that center point much "deeper", but of course a weak send/receive circuit, will make it shallower.

I had a colleague, who was/is an aerial expert, and he could not find anything wrong with my thoughts on that subject! Though I have not discussed that with any other designers of Metal Detectors for many years now.....But then at least, they were of a similar opinion.

Your depth results would tend to support that, 7 to 10 cm you quoted....with a send coil of 7 to 8.5 cm wide.....

This point is also probably the main reason that most Pulse Metal Detectors use one large coil, very large for when going deep, to maximize search depth on both send and receive.

If using a two coil system system in the manner you have made, may I suggest that you might want to swap the coils around, that is the smaller coil as the send and the larger as the receive, which might gain you a few cm more depth, but just a thought.....not actually tested by myself! As I feel that the receiving "antenna" is the most important one!

With the valuable knowledge and experience you have obviously made here with this very unusual design, may I make a suggestion that possibly a simpler 2 coil design, might increase search depth quite considerably, especially with regard to your expertise on the computing and electronics! Plus it would be even simpler to design and build for the rest of us here.

One problem that I had many years ago, was that some designs of PI Detectors, tended to pick up radio stations as well, did that happen to you at all? I do believe it was mainly Long Wave stations, which are getting fewer as we speak, but a method to "tune out" such interference would maybe needed by some builders....

Two Brothers, designed and published a twin coil system maybe 30 years ago, that ignored any sorts of signals that were the "same" on both coils, only amplifying "difference" signals, but both coils were used together in both sending and receiving! I have the design somewhere still if you would like a copy, as it was published and I did make one, which worked quite well!

Many thanks for your excellent "instructable", I really liked it and please do not see any of my comment as being in any way negative, hopefully it was just "food for thought" for you.

regards

Andy

