The coil

[dotniet]

Hi

I was looking up and down for information about the coil.
The information in the PDF for building the PCB is very very nice - but the coil was totally left out.

I have all wave forms exactly as described in the documentation, but my build only detects a big 2 Euro coin from 21cm. I think the problem is the coil.

So, what is the best type of coil for MPP detector?

More specifically:

• is a DD coil much better than a mono coil? how much better?
• how the coil diameter affects performance? (in other words, which is better a small diameter or big diameter coil?)
• what is the best wire (material cooper/aluminium. insulation: email/transformer wire, thick plastic/speaker wire, etc)?
• looks like a knitted coil (fast coil) is better because its quality factor. How much better?
• shielding - a must have?
• connection from coil to pcb? (I already found on this forum that the recommended option is 50ohm coax cable).

Many thanks.

[Qiaozhi]

There is no particular "recommended" coil for the MPP. The original MiniPulse design had a 500uH coil and a very slow pps TX. It was designed for searching for coins on the beach. The MiniPulse Plus can use various coils depending on your intended use. The two-stage preamp, and increased TX speed, allows searching for smaller low conductivity gold items with an appropriate coil. If you optimise the MPP to search for small jewelry or gold nuggets, then it will become less sensitive to larger items, such as a 2 Euro coin. If your intention is to search for coins, then go for a 500uH coil. You don't need a fast coil for that, and can get away with enamelled copper wire (I suggest 0.56mm width) and a 10" diameter coil.

[Darin]

There is no particular "recommended" coil for the MPP. The original MiniPulse design had a 500uH coil and a very slow pps TX. It was designed for searching for coins on the beach. The MiniPulse Plus can use various coils depending on your intended use. The two-stage preamp, and increased TX speed, allows searching for smaller low conductivity gold items with an appropriate coil. If you optimise the MPP to search for small jewelry or gold nuggets, then it will become less sensitive to larger items, such as a 2 Euro coin. If your intention is to search for coins, then go for a 500uH coil. You don't need a fast coil for that, and can get away with enamelled copper wire (I suggest 0.56mm width) and a 10" diameter coil.

What has to be changed other than the coil ?
Can you have a fast coil to go prospecting for gold nuggets then change it with a coil to look for coins on the same machine ?

[dotniet]

So, what I have found out until now, from misc sources:

* wire gauge doesn't matter
* wire insulation should be as thick as possible to decrease coil's capacity
* coil diameter doesn't matter (inductance matters)
* large inductance coil for large items

Can I edit the original post to put the conclusions I got (so far) so other users can have immediate access to them?

[Qiaozhi]

Can I edit the original post to put the conclusions I got (so far) so other users can have immediate access to them?

There is a 60 minute period during which you can make changes. After that it becomes permanent.

[Qiaozhi]

What has to be changed other than the coil ?
Can you have a fast coil to go prospecting for gold nuggets then change it with a coil to look for coins on the same machine ?

Adding an external control for the TX pulse width (either a pot, or a switch) will allow you to change the TX pulse width to suit both fast and slow coils. The Reject pot is already external, allowing you to change the main sample delay.

[Darin]

Adding an external control for the TX pulse width (either a pot, or a switch) will allow you to change the TX pulse width to suit both fast and slow coils. The Reject pot is already external, allowing you to change the main sample delay.

Can you explain more in depth please, as where to connect a pot or switch and what changes have to be made. And are there any negative effects to adding this?

This is very interesting to me as I am ready to go on to step 3 of my MPP build now.

[Qiaozhi]

Can you explain more in depth please, as where to connect a pot or switch and what changes have to be made. And are there any negative effects to adding this?

This is very interesting to me as I am ready to go on to step 3 of my MPP build now.

You will need to change the value of R7 (Fig. 4 in the Build Document) to alter the TX pulse width. The pulse width should ideally be adjusted to maintain the same energy in the coil, but in reality this is usually fixed and represents a compromise when different coils are used.

If you want a detailed (in depth) explanation of the relationship between the main sample width and the target decay, then read the following ->
http://www.geotech1.com/forums/showt...009#post162009
http://www.geotech1.com/forums/showt...056#post162056
and finally this ->
http://www.geotech1.com/forums/showt...228#post162228
Well, you did ask.

[dotniet]

Can you explain more in depth please, as where to connect a pot or switch and what changes have to be made. And are there any negative effects to adding this?

This is very interesting to me as I am ready to go on to step 3 of my MPP build now.

I intend to do this mod on my detector also, so if you wait a couple of days I will post the resume.
It is very very easy (trivial) to do actually.
You can proceed to step 3 and do this modification later/at the end.

[Darin]

You will need to change the value of R7 (Fig. 4 in the Build Document) to alter the TX pulse width. The pulse width should ideally be adjusted to maintain the same energy in the coil, but in reality this is usually fixed and represents a compromise when different coils are used.

If you want a detailed (in depth) explanation of the relationship between the main sample width and the target decay, then read the following ->
http://www.geotech1.com/forums/showt...009#post162009
http://www.geotech1.com/forums/showt...056#post162056
and finally this ->
http://www.geotech1.com/forums/showt...228#post162228
Well, you did ask.

Thanks Qiaozhi,
This is some great information, I do not understand all of it right now but I am finding as I learn a little more along about electronics things I have read on here slowly start to come together. Feels like I'm in system overload but I will get there.
Thanks for the help.

[Darin]

I intend to do this mod on my detector also, so if you wait a couple of days I will post the resume.
It is very very easy (trivial) to do actually.
You can proceed to step 3 and do this modification later/at the end.

Would like to know how it works out for you.
I am thinking about doing this or may just build another MPP set up different for fun. I can see how building detectors could become habit forming.

[Borisimo]

I intend to do this mod on my detector also, so if you wait a couple of days I will post the resume.
It is very very easy (trivial) to do actually.
You can proceed to step 3 and do this modification later/at the end.

Did you do this mod. How did it work out? I am at stage 4 in the MPP build and I'm thinking about the coil, so reading these 'coil' posts.

[dotniet]

Did you do this mod. How did it work out? I am at stage 4 in the MPP build and I'm thinking about the coil, so reading these 'coil' posts.

Sorry, my detector was just sitting abandoned on the table all these days.
I didn't had any free time. I will try to work today.

[morganton]

Adding an external control for the TX pulse width (either a pot, or a switch) will allow you to change the TX pulse width to suit both fast and slow coils. The Reject pot is already external, allowing you to change the main sample delay.

What value pot do you recommend? Is a multi-turn necessary?

[Qiaozhi]

What value pot do you recommend? Is a multi-turn necessary?

A multi-turn pot should not be necessary, but you'll have to experiment yourself as I've not tried this is practice. Also, you'll need to be careful to keep the MOSFET from avalanching.