HOME-MADE MD,S

[chris]

HI EVERYONE
HAS ANYBODY GOT A HIGH FEVER FOR HIS/HER SPECIFIC HOME-BUILT DETECTOR FOR NUGGET HUNTING AND HOW DOES IT COMPARE WITH MANUFACTURED DETECTORS LIKE SD2200?
I KNOW THAT HOME-MADE MACHINES ARE NORMALLY BETTER AS THE MAKER PUTS EVERYTHING INTO IT
THANX
CHRIS

[Reg]

Re: HOME-MADE MD,S

Hi Chris,

It is extremely difficult to build a home made PI to compete with the PI's such as the SD series. Can it be done? My answer is yes, but a whole lot of work has to be put into it.

I presently use a modified GQ clone which is an Eric Foster design. Does it compete with the ML's? In my opinion it does.

Now, I have recently built up another Hammerhead PI and modified it significantly to see just how well it will do. Initial testing indicates that it will not do as well as my GQ clone but will come close.

Will this HH compete with the ML's? I think so, but will lack any form of ground balance. This is a drawback. However, I have modified the PI to use a DD coil which does significantly reduce the ground signals. With the DD, I think it will be quite acceptable.

I might build a ground balance circuit later, but for now, it will be used without one.

I will use the Radio Shack mini amp mod with the HH which sort of helps for different reasons.

All in all, I am looking forward to using this PI in the field to see how it does. All tests I can run at home indicate it should work just fine.

Reg

[Stefano]

Re: HOME-MADE MD,S

Hi Reg,

I'm still having problems with my Carl's HH apparentely due to ground signal. I shielded the monoloop coil with a capacitor metallic film as I wrote in past which I think to be effective, but I still get a signal from ground as I lower the coil. The audio changes from 1 to about 10-20 Hz so it's impossible to detect any medium- small target. I've built some coils with different techiques, lorenz as well, leaving some mm beetween the wire and the shield but with no result. They all behave the same. I tried also to change the 2nd sample position from the shortest to the latest allowed with no result too.
I tried a very simple DD with no shielding at the moment and the ground signal seems effectively to be much weaker confirming what you say.
Since I think that out there most of you are using monoloop coils I can't understand why my don't work. I don't think the ground where I test to be mineralized. I'm wondering from what you say about HH if it depends on the circuit.
It's just my second PI but I hoped that it was able to detect something without all these troubles it's giving me.

Thanks
Stefano

[Reg]

Re: HOME-MADE MD,S

Hi Stefano,

What you are experiencing with a mono coil is normal if the delay is short eonough and you have the right ground conditions. You can start to get strong ground signals if the delay is less than about 40 usec or so and the ground signals are much stronger when you use a mono coil. At 15 usec to 20 usec delay, the ground response can be quite strong. Again, it all depends upon the ground conditions (compostion) as to how bad the problem becomes.

Shielding of the coil does help reduce some of the signal caused by the ground but will not eliminate the problem.

The DD coil should greatly reduces this ground noise signal even in bad ground.

So, by adding a DD coil and using a fast autotune, much of the ground signal can be eliminated, or at least greatly minimized.

Now, I will add that the ground response can vary significantly from almost no signal to a very strong signal. Nice clean sand may not produce much of a ground response. I have checked stream beds and had effectively no ground signal also. However, if I move up the hill where there is a clay based ground, suddenly, the ground signal has become quite strong. Again, it depends upon the composition of the soil. Generally, if the soil has a lot of clay or iron oxides in the form of maghemite then I have found the ground response to be greater.

I did not build option 1 so I am not sure just how sensitive the audio frequency changes with respect to the signal generated by an object or the ground. Based upon what you say and what another person has recently told me, I am wondering if the circuitry wasn't maximized for option 1, and as such, the senstivity is far greater than what would be experienced on option 2.

In other words, it might be possible that the sensitivity is too high, which would cause the slightest "target" change to cause a very large frequency change in the audio. This condition could cause the ground to generate a very strong response. I would have to build one using option 1 to see. Fortunately, I do have another pc board coming so I will try to build up option 1. That way I will have a better idea of what is going on with that circuitry. Until then, I can only guess.

Now, as for your coil shielding, do you see any significant difference when you shield the coil and not shield it? There should be quite a difference.

The shielding should be connected to the leg of the coil that attaches to the coax shield, and that shield should attach to the pc board ground trace. If any of this gets switched, then strange things can happen.

Reg