Is the Minipulse Plus the right detector for me?

[barnett25]

Hi,

Since this is my first post I want to say hello to everyone, and thank you for the wealth of information I have found on this site. I have been on several forums trying to learn everything I can about building metal detectors, and I am sure you would not be surprised to hear that this forum has been the most useful that I have found. I hope I am posting my question in the right place, and that it is not too elementary a topic for this location.

I am looking to build a waterproof metal detector for a beach trip this summer. I have a couple of White's Classic series VLFs, and an ACE 250 currently. However I would like to try detecting in the shallow salt water, with the hope of finding gold jewelry as a primary goal. I had considered modding and waterproofing one of my White's, but from what I have read I don't know that I could expect very good performance, even with ground balancing and threshold controls.

My research so far has lead me to think that a PI detector should be able to do the job, so it just comes down to what specific model to build. It seems that the MPP is one of the most favored projects at this time, but much of the emphasis I have seen seems to be centered around gold prospecting. I would expect that the jewelry targets I want to find would be somewhat larger than some of the small gold flakes many are using this machine for. Is the MPP still the best way to go, even for my purposes? Or would I be better off with the Surf PI, or another project?

Also, are there any parts of the build I should modify for my purposes? Is there a certain coil type and size that I should consider?

Thanks,
Justin Barnett

[adenyer]

Hi Justin
I have made several of the metal detectors on the website (bought from silverdogs site), the easiest to build and the one with the smallest size ( longboard) is the surf pi i am currently trying to waterproof mine, i then made the baracuda, i did get better depth from the baracuda (maybe the coil i made was better ) but i did use an oscilloscope to tune the circuit to perform its best, i have also built the minipulse, but am still trying to tune it for better depth, so i would say the surf pi is the easiest , simple build and setup, if you have access to an oscilloscope the baracuda next and then the minipulse (just my preference).

Regards
Alan

[sinclairuser]

apart from the fact that the MPP is an excellent design, george has made building it as easy as possible both with and without a scope, the comprehensive build document is just that.
while the surf and barracuda are spread over a couple of threads, for that reason the MPP would be better for you.
someone will probably tell you to get "inside the metal detector", but if you are only going to build one beach machine dont bother, just buy the full kit from silverdogs shop, download the build document, then follow it completely.

[barnett25]

Thank you for the advice and suggestions.

In reading about PI detectors I see a lot of emphasis on short sample delay (and fast coils to allow it). Is that only for small gold flakes, or am I going to want to make sure I have that for gold rings and such as well? I ask both to make sure I choose the correct circuit that allows for good performance, but also to help decide what kind of coil to build. Is a 7-8" mono coil within the range of 300-500uH going to work pretty well, or should I be looking at more highly tuned balanced coils of a very specific inductance for my purposes?

I suppose what I am really asking, is this; is hunting for gold rings in the surf going to require a machine tuned precisely for that purpose, or is just about any PI going to perform pretty similarly for these targets (give or a take a couple inches of depth of course)?

Edit: I do have an oscilloscope, which although long overdue for calibration, seems to at least be in the ballpark and might do for my needs.

[sinclairuser]

the other bonus of the MPP is it can use simple mono coils, or twin balanced ones with the use of an extra resistor in the coil plug.
the fact you have a scope is a further bonus, you can build the machine using a mono and then concentrate on building a balanced coil later, actually the hard part for you is going to be waterproofing it.
thats not as easy as you might imagine.

[Qiaozhi]

Thank you for the advice and suggestions.

In reading about PI detectors I see a lot of emphasis on short sample delay (and fast coils to allow it). Is that only for small gold flakes, or am I going to want to make sure I have that for gold rings and such as well? I ask both to make sure I choose the correct circuit that allows for good performance, but also to help decide what kind of coil to build. Is a 7-8" mono coil within the range of 300-500uH going to work pretty well, or should I be looking at more highly tuned balanced coils of a very specific inductance for my purposes?

I suppose what I am really asking, is this; is hunting for gold rings in the surf going to require a machine tuned precisely for that purpose, or is just about any PI going to perform pretty similarly for these targets (give or a take a couple inches of depth of course)?

Edit: I do have an oscilloscope, which although long overdue for calibration, seems to at least be in the ballpark and might do for my needs.

Although the Surf-PI is a good machine, many members have had problems getting it working, either due to inexperience, a lack of understanding of the design, and/or fitting components in the wrong place or the wrong way round.

The Minipulse Plus has superior documentation, and a much higher chance of working first time. It has a 2-stage preamp to allow for earlier sampling, which helps to detect smaller targets. For your purposes, you can simply adjust the sample delay to 15us (which is good for beach work) and off you go. The requirements of the coil are not demanding, and you can easily construct your own (300uH is recommended), or purchase a commercial coil. Both mono and IB coils are supported by the MPP. If you want more depth, then it's a simple matter to increase either the width of the TX pulse, the sample pulse, or both.

[barnett25]

Minipulse Plus it is then! Thanks for the helpful tips as well.

Waterproofing is something I am a bit concerned about. I had planned to use a commercial waterproof box like I have seen many use in the past. Assuming the box is good, my only problems are obviously going to be because of any holes I have to make. One I can't get away from is for the coax for the coil. I figure I will use a bulkhead gasket of some kind and some silicone caulk. I would like to have external controls, but that may not be absolutely essential. However I have found some potentiometer boots that *may* be good enough for at least splashproof, and maybe shallow submersion.
The only thing left is audio. Maybe a hardwired 1/4" phono extension through a bulkhead gasket and caulk like the coax.
I had planned to embed the coil in epoxy, so water shouldn't be a problem there.

I have been surprised how little information I have found online for building waterproof metal detectors actually. Does anyone know of any good guides or forum posts I may have overlooked?