Replies to Minipulse Plus

[Qiaozhi]

Hi Guys. First time user of this forum though I have been looking here for a while. I recently completed the minipulse plus and I have been playing with coils that were leftover from my surf pi build.
I have a couple of questions.
First all my mpp spec's when viewed on a scope are as per the rev c document. I can however set my sample delay to about 7us before it drops out. I am using a coiltek 11 inch mono and a home made 8 inch mono coil. Coiltek is a commercial coil designed for use with minelab pi detectors.

Yes, that's correct. The MPP can sample at sub-10us delays. However, please be aware that (according to the datasheet) the monostables are capable of a minimum pulse width of 10us. But in general they can go lower than that before cutting out.

I changed my external sample delay pot to 100k in order to have a greater adjustment range. 7us to 32us.

That is as expected.

Does the on board trimmer need to be set to 22us delay as per the rev c document? or can I just leave it at 7us?

Setting the on-board trimmer to 22us is only for initial testing. After that you can set it to whatever minimum delay you prefer. If, for example, your coil cannot sample below 10us, then set the internal trimmer to that value. The external Reject pot simply allows the user to increase the sample delay to a higher value, and hence eliminate unwanted targets based on conductivity.

When testing the detector on green grass in my back yard I get a signal as the coil nears the ground. I get this even at the longest sampling delay and yes both coils are shielded. For test purposes I added extra shielding to my home made coil but even though it dropped the sensitivity a bit it still sounds off when the coil nears the ground. The SAT does bring the threshold back to normal if I leave the coil on the ground but of course it sounds off again if I rise the coil a bit. Is this normal behaviour for this type of detector?

If the detector is sounding off as the coil nears the ground when on its longest delay, then the soil is most likely contaminated with iron. I had a similar problem the first time I built a Hammerhead detector. Take a large ferrite core and wave it in front of the coil. If there is no detection, then the soil is iron-infested.

I haven't tested it on just soil without grass or out in the gold fields yet! Once I know the detector is working normally I want to try swapping out opamps. Currently getting a small gold ring (just detectable) at about 12 inches (30cm) and a can at about 100cm in air. (11 inch mono)

That's a good result.

Qiaozhi suggested emailing him some images of completed builds but I can see any way of doing that here.

Please send photos to [email protected]

Well done on a successful build, and doubly well done on being the second person destined for the Completed Projects thread.

[Qiaozhi]

If it was not done in Sprint Layout; it does not exist for me!
Nobody else did it; so i had to do it, again!
Recently i had to switch on Sprint Layout 6, so this was good exercise for me.
Personally i don't like the chosen size and spacing for 470nF capacitors... but that's just me!

Thanks Ivica,

I've added your files to the Minipulse REV-B thread for use by other Sprint Layout users.

Please have a look at the REV-C PCB. This contains 2 minor fixes (see MiniPulse REV-C thread) and the addition of TP10 for connecting a ground probe. If you make these changes to your Sprint Layout file, I will add that to the REV-C thread as well.

[josewashere]

Emailed you the pics

[josewashere]

Thanks for all the info. I tested it with a ferrite rod 8 inches long by 3/8 in diam. (200mm x 10mm diam) The type you would find in a large transistor radio and it will detect that at about 4 inches (100mm) at all delay settings though a bit less at the longer settings.
Using a ferrite tube about 1.25in long by 3/4 in diam with a 3/8 hole (30x18mm with 10mm hole) the type you use on cables for suppression it does not detect that or its just detectable when touching the coil.

thanks

[joop]

From what I know, the ferrite rod is composed of sintered MnZn or NiZn compositions and the RF suppression tube is made of iron powder, small iron particles glued together.

I might be able to help out in a couple of days. My detector is ready but can't take it outside yet... I am waiting for the coil cable to arrive.

[josewashere]

Hi Joop.
That would be good. I might take mine out on the weekend to see how it behaves in various types of soil. Both my surf pi's and my minipulse plus do the same thing on my lawn so it could well just be my backyard but I did the tests with the same coils. Ultimately I want to be able to use it on our gold fields here. Our gold fields are very mineralised and some have high iron levels so probably it wont work so well.

I'm still very happy with it though and will be happy if it works fine on beaches.

Regards to you from Australia

Jose

[ivconic]

Thanks Ivica,

I've added your files to the Minipulse REV-B thread for use by other Sprint Layout users.

Please have a look at the REV-C PCB. This contains 2 minor fixes (see MiniPulse REV-C thread) and the addition of TP10 for connecting a ground probe. If you make these changes to your Sprint Layout file, I will add that to the REV-C thread as well.

[bernte_one]

SL files look nice

[ivconic]

SL files look nice

Makes my life much easier in process when putting components on pcb.
I use to print such picture on paper and than work is much easier.
Would be even better if i had color laser printer, by now i have only b/w laser.

[Qiaozhi]

Thanks for all the info. I tested it with a ferrite rod 8 inches long by 3/8 in diam. (200mm x 10mm diam) The type you would find in a large transistor radio and it will detect that at about 4 inches (100mm) at all delay settings though a bit less at the longer settings.
Using a ferrite tube about 1.25in long by 3/4 in diam with a 3/8 hole (30x18mm with 10mm hole) the type you use on cables for suppression it does not detect that or its just detectable when touching the coil.

thanks

Both a ferrite rod and a ferrite ring will produce an audible response from the detector, but a ferrite pot core will not.

[Qiaozhi]

Hi Joop.
That would be good. I might take mine out on the weekend to see how it behaves in various types of soil. Both my surf pi's and my minipulse plus do the same thing on my lawn so it could well just be my backyard but I did the tests with the same coils. Ultimately I want to be able to use it on our gold fields here. Our gold fields are very mineralised and some have high iron levels so probably it wont work so well.

I'm still very happy with it though and will be happy if it works fine on beaches.

Regards to you from Australia

Jose

For the gold fields in Australia you will need to add a ground balance circuit, and even then you'll have problems if the ground matrix is non-homogeneous. Perhaps you'll be able to field test Davor's GB solution when he publishes the design.

[josewashere]

Ok thanks I see what you mean by pot core. I don't have one here but can get one easily enough. I will try it.

[Davor]

My lab under the stars is not producing - it is very rainy. And mosquitoes are a murder.
Soon.

[bbsailor]

Both a ferrite rod and a ferrite ring will produce an audible response from the detector, but a ferrite pot core will not.

Qiaozhi,

I believe you want to focus on using what are known as "soft ferrites" to prevent them from being detected when used as the core of a PI probe. I have used some ferrites made of manganese/zinc with good success on a PI probe. Most cores used on DC to DC converters use soft ferrites. This tip was initiated by Eric Foster many years ago. Look up "soft ferrites" on his web site. Here is the web link: http://www.findmall.com/read.php?34,...915#msg-135915

Here is a good web link: http://www.intl-magnetics.org/pdfs/SFG-98.pdf

I used ICH ROD7.5/50 with good results on my PI machine.http://www.surplussales.com/Inductor...s/FerRods.html

Joseph Rogowski

[Davor]

Actually, even with a proper material the problem remains with the core geometry, as most of the cores have some conductivity, and thus a ring core has a pronounced eddy current that is detected as a target. E-shaped cores are OK.