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Homemade PI Detector 2
Posted by: Smitty II
Date: August 28, 2011 09:15PM
Greetings again,

Since 6666 expressed an interest in the circuit that I ended up with for my homemade PI I will what I came up with. I will also pass along some of the thoughts behind what I did. This was a learning process for me and some of my thinking may be off base and some of you might be able to show me the error of my ways. I would appreciate any inputs to that effect. Since I have been field testing my detector every chance I get, I had it out for a bit this weekend. I didn't have a lot of time but I did spend about 45 minutes hunting in an old church play area in a small mountain town. I have attached a photo of the finds. It is only of the good stuff and does not include the wire, nails, bobby pins, paper clips and other iron treasures. The small gold ring has a little diamond in it at the center. It was found about 3 1/2 to 4 inch's in the ground. I was using the 8" coil and it had a nice clear signal. The ground up there has a fair amount of mineralization in it. Both my Xterra 705 and my AT Pro bare this out. The AT Pro read the ground at 92 out of a max of 99. My front yard is not a whole lot better, it reads 85 ot 87. The detector performed very well under that type of ground condition. I also included a closer shot of the control box so you can see the controls that ended up on the front panel. I have attached the schematic of the power supply for everybodys review. There is nothing earth shaking about the design.

I used three supplies in the machine, a -12V to -15V battery supply, a +5V and a -5V supply. The 12V comes from 8 AA batteries, and I have a 14.8V, 4 cell Lipo battery that can also be used. To generate the +5V supply, is used a bipolar NE555 timer running at about 20 KHz. The 20KHz frequency was selected for 2 reasons, less ripple and since bipolar NE555s can generate fairly large current spikes on the power supply, if there is any left after decoupling and filtering the 20 KHz will help to keep it from being heard in the output audio. The output of the NE555 drives a voltage doubler circuit to make sure that there is plenty of voltage for the LM7805 +5V regulator. The -5V is taken directly off the -12V to -15V battery supply using a LM7905 -5V regulator. Diode across the 3.3k resistor in the timing circuit on the NE555 is to separate the charge path and the discharge path and give a 50/50 duty cycle square wave output to the voltage doubler. The NE555 timer must be a bipolar device. The Cmos version does not have enough current drive to drive the voltage doubler.

Well there is my thoughts on the power supply, I will get into the rest of it next time.

God Bless and HH!

Smitty II




Re: Homemade PI Detector 2
Posted by: 6666
Date: August 28, 2011 11:48PM
Great work Smitty, there are some good finds there

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Re: Homemade PI Detector 2
Posted by: Sven
Date: August 29, 2011 05:13AM
Nice job. I myself just built 3 Surfmaster Pro Pi's. They mirror the White's Surf PI Pro in performance bench testing.
I would be interested in seeing your schematic as I would like to build another PI that goes deeper and hotter on small gold.
I haven't finished fully assembling a search coil.
Here's a picture of mine.



AKA Berkut 5 (Golden Eagle), Detectorpro Wader, Tek Turbo
Self built PI's --- they work great!!
Free Metal Detector Catalog Downloads etc.:
www.treasurelinx.com




Re: Homemade PI Detector 2
Posted by: Smitty II
Date: August 29, 2011 08:47AM
Good Job Sven,

I too have several PI machines. Right now I have an Infinium LS, a Detector Pro PI and the the one I built. I have also had, in the past, a Surfmaster PI and a Sea Hunter Mark II. I will be passing along the schematic that I ended up with and will also pass along a little of the thought process whether right or wrong.

It sounds like you enjoy building like I do. Keep it up.

God Bless!!

Smitty II

Re: Homemade PI Detector 2
Posted by: Smitty II
Date: September 04, 2011 02:57PM
Sven,

Here is a peek at the inside of my control box. You can see that the board is not as small as it could be. Remember that this machine went through several stages of development and a lot of changes.

Keep building and God Bless!!

Smitty II




Re: Homemade PI Detector 2
Posted by: 6666
Date: September 04, 2011 05:58PM
G'day Smitty
you have presented some very good info over your series of posts well done
some people say that you should not build PI detectors on vero board
but you have proved them wrong it can be done, I have built lots
of projects on vero board, and they seem to work.

Sven, nice project.

Re: Homemade PI Detector 2
Posted by: Smitty II
Date: September 05, 2011 05:56PM
6666,

For sure a properly laid out PC board is the best way to go. However, My detector works very well and has very good sensitivity to fairly tiny gold. It is pretty noisy in town at max gain but at minimum gain to functions reasonably well with only occasional bursts of noise coming through. Definitely well enough to hunt park play areas, volley ball courts and local lake beaches. Even a minimum gain, air testing, it will detect my 3 grain gold target sample at around 3" with the 8" coil and my 1 1/2 grain sample at about an inch. I don't really expect to find gold nuggets in the field that small but as you can see from my posts it found a 7 grain nugget with no trouble, so I would suggest that realistically it should be able to find nuggets in the 5 grain range reasonably well with the 8" coil using the lower gain range. This also assumes that it can handle the ground conditions. It does not have a ground balance channel. The ground in my yard is pretty bad, I have attached a picture of my magnet after I rubbed it in the dirt. If I am out away from the city it runs reasonably smooth at max gain. The thing about running at max gain is that it will detect solar energy bursts and noise. Fortunately we are in a low sun spot cycle and even at max gain it is not to bad. It is a 10 year cycle so it will be a while before I see how well it handles the peak sun spot season.

I am working on a PC board lay out and one of these day I might get it done.

God Bless and HH!!

Smitty II




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Re: Homemade PI Detector 2
Posted by: Sven
Date: September 06, 2011 05:05AM
I have the same magnetite sand (black sand) issue up here. I saved a vial of it that attached itself to the magnet in my water scoop.



AKA Berkut 5 (Golden Eagle), Detectorpro Wader, Tek Turbo
Self built PI's --- they work great!!
Free Metal Detector Catalog Downloads etc.:
www.treasurelinx.com

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