My Home Built PI Detector

[Smitty II]

Greetings Everyone,

I have been detecting since the late 60s and probably have designed and built every type metal detector out there. That may be a bit of an over statement but at least the major types. My latest project is a PI machine. I thought at least some of you might be interested in it and how well it performs for me. Here are the basic specs.

It has a Pulse Frequency of 1250 Hz and can be adjusted about 10%.

The Coil Pulse Width is 70us

It has a Pulse Delay that is adjustable form 5us to 85us and has an Iron Test switch.

It has a Target Sample width of 10us and 30us, 10us for tiny gold targets and 30us for normal coin and jewelry.

It has a Gain (sensitivity) control with a range of 1 to 3.

It has a SAT (self adjusting threshold) with a range of 1.5sec. to about 5 sec.

Naturally it has a Threshold control.

I have two coils for it, a 8" and a 10".

I have built two audio versions of the detector, a VFO version and PVA (proportional volume audio) version.

It runs on 8-AA batteries that give it about 8 hours of operation time. It does have a battery test.

Enough specs, now does it actually detect anything. Air Testing it indoors using the 8" coil, the Gain at max, the Pulse Delay and minimum (5us) and using the Narrow Target sample width, it will detect my 3 grain gold test nugget (more of a flake) at about 4+ inches using headphones, and my gold wedding ring at 13+ inches. I have been using it in the field now for about a year now. It has really opened the door to finding tiny gold jewelry targets like ear ring studs, tiny gold baby rings and thin gold chains. It is not a prospecting machine since it does not have a Ground Balance channel but it still does pretty well here in the Colorado gold country mineralized ground using the SAT and Pulse Delay. I have also had it down to the Gulf of Mexico beaches. I recently found a nice ear ring stud with a 1/2k diamond in it about 5" deep in the sand. The detector does an excellent job for me at detecting coins, Jewelry and relics. It has really added to my play money account over the last year.

I have had a lot of fun learning, designing and playing with this project and a pleased with my results.

Have a great time detecting and HH.

Smitty II

 

[ewtaylor]

Sounds great. Do you have any pics or videos?

 

[ionut_mtb@yahoo.com]

At 5us without gb not detecting the sand?

 

[Smitty II]

Greetings, Yes! at 5us the detector will detect ground mineralization and when you get within about 2 to 3 inches of the mineralized sand or ground you will start picking it up. I have found that by adjusting the SAT speed and using a smooth sweep speed you can work highly mineralized ground reasonably well. You can increase the pulse delay and eliminate the ground signal but you will loose the ability to detect tiny gold targets. As I indicated, I did not intend the detector to be a prospecting machine. I just wanted it to have better capabilities at detecting small gold jewelry targets and it does do that. At the time I started this project, I owned three commercial built machines and none of them would detect my 3 grain nugget (flake).

 

[Smitty II]

Greetings, Actually I have tons of pictures but I am still learning how to get around in this forum and I need to reduce the size of about all the pictures that I have. I will work at getting a few pictures on here someway. The only one that I have been able to reduce the size is a picture of the detector. I sort of over did it but I will attach it anyway.

 

[Smitty II]

Here is a few additional pictures of some of its finds. A better picture of the detector, some finds from the Gulf of Mexico, The test targets that I used and some other finds including some chains and small gold jewelry.

HH

Smitty II

 

[Smitty II]

Here is another picture of some of the gold jewelry that I have recovered using the detector.

HH

Smitty II

 

[Smitty II]

Here is a picture of the loaded PC Board that I had made for those that are interested in seeing it.

HH

Smitty II

 

[mschmahl]

Great work. Thank´s for the pictures. Can you post something about the coil´s?

 

[Smitty II]

Greetings,

I have two coils for my detector, an 8" and a 10" coil. A friend of mine has built a DD coil and said that it worked fine and handled mineralized ground a bit better. It is a big coil so it was not all that good for small gold targets. I have attached some pictures of my coil building process.

HH

Smitty II

 

[hecbom]

Hello Smitty
what gauge wire did you use and how many strands? is it Teflon coated? How about the coil capacitance? I have several coils I made for my "home made" Pi using Litz wire 175/46 but I finally settled for a 6" Mono from Miner John. I get the best depth on a 3 gram nugget @ 3 to 4" The Miner coils is a 310uH @ 1.3 ohms.

Hector

 

[Smitty II]

Greetings,

I use #24 gauge stranded wire with Teflon insulation. It comes in 7 strands and 12 strands. I have used both and I don't think I can tell any difference in the coils performance. I have not ever used Litz wire. I know that several people have used it in their coils. The combination that I have been using works very well with my detector so I have not tried any other combinations. Using my 8" coil I can detect my 3 grain gold flake at 4+ inches using headphones when bench testing. My coil inductance is 270uh at the connector on the end of my 4 foot cable. I don't know the coils capacitance but I think I can measure it. I do know that the self resonance frequency of the coil is just a bit under 3MHz.

 

[Idefix]

Hello ,

Very nice job.
As as see, you have fully shielded your coil with a copper tape.
I think , because the shielding is close on itself, that you may have eddy currents in it. So, it is better to open , near the ground connection. as it does not make a full circle. So , the shielding makes a C, and not a full ring (O) .
(Difficult to explain without a drawiang )

If you try this, let us know your results. The noise will be lower I think.

 

[ionut_mtb@yahoo.com]

Hello ,

Very nice job.
As as see, you have fully shielded your coil with a copper tape.
I think , because the shielding is close on itself, that you may have eddy currents in it. So, it is better to open , near the ground connection. as it does not make a full circle. So , the shielding makes a C, and not a full ring (O) .
(Difficult to explain without a drawiang )

If you try this, let us know your results. The noise will be lower I think.

You can see the gap on the opposite side of the coil connection.

Smitty, one thing i don't understand. does this coil in picture goes as low as 5us? with that shield? and the spiral wrap is pe or pvc?

Thanks

 

[Idefix]

You can see the gap on the opposite side of the coil connection.

It is not a gap, but a piece of tape , I think.
It is what I see while zooming.
Put before potting with resin.

 

[kt315]

again foil shielding? who suggest you to use the foil? do you need a protection from EMI? why?

 

[Smitty II]

Idefix,

Actually the coil shield is not a complete circle. On the opposite side of the coil from the connection to the cable there is an break in the copper shield. The break is insulated with black electrical tape and the ending of the shield winding wrapped up on to the tape insulation. You are right if the shield is a complete circle it becomes the same as a shorted turn and lowers the coil "Q" drastically and the coil will not work.

Smitty II

 

[Smitty II]

Kt315,

The shielding is NOT an EMI shield. It is a Faraday shield that shield the coil against ground capacitance. Without the shield the ground capacitance and grass and bushes will cause the detector to sound off. EMI is still an issue where there is a lot of electrical and electro magnetic noise.

Smitty II

 

[kt315]

i see THE FOIL on your coil. FOIL! its meant EMI PROTECTION. for Faraday one you have to use GRAPHITE powder.

 

[Smitty II]

kt315

The foil shield is a Faraday shield that provides capacitance shielding for the coil. The foil is not continuous as there is a break under the electrical tape on the opposite side from the cable connection. Believe me it does not shield out EMI. It doesn't shield the detector from computer noise and various EMI noise in town and around the house. It will even detect solar flares at max gain. You can call it whatever you want but without the copper shield, the coil is unusable, as it sounds off from everything that the coil come close to or touches. It will even sound off if you bring your hand close to the coil. I sure wish it did shield out EMI because that would sure make it run a lot less noisy here in town. If it shielded out EMI you could not detect any targets as the detector generates an Electro Magnetic field pulse and then detects the Collapsing of that field in the metal targets.

Smitty II