A surf PI question

[Capt Slog]

I bought the components for this and built it as a kit some time ago. Picked it up again this year and decided to get it put on a stick and working properly.

I wound a 'basket' type coil, using 24awg plastic covered wire, on a former that's about 21cm OD and 17cm ID. The coil works quite nicely, it finds my wedding ring at about 25 to 30cm when the coil is hanging away from anything else.

The problems start when I try to use it on the ground, then it seems to pick up everything. I'm certain I don't have that much metal under my lawn, and I'm not about to start digging up to find out as it would like like the surface of the moon if I did. I think it was even tracking tree roots.

I think I read somewhere that the PI type of metal detector is best for the beach, and that is primarily what it will be used for, is this the case?

[eclipse]

Have you use shield for the coil ?

[kt315]

you are not first who tell about the trouble here. probably the board needs being re-designed IMO.
Daverave told about
http://www.findmall.com/read.php?34,...98#msg-2241698

[FatBob]

You have a lot of gain on the front end and minimal filtering. You could reduce the gain or add some EMI filtering.
Shielding the electronics and or coil may help. You soil may have lots of minerals in it. Did you try reducing the Sens?

I just put together a Surf Pro board but am still waiting for a FET and 4093 to come in. I am building it for strictly on the beach.

[Capt Slog]

Answers to the above.

No, I didn't shield the coil, because I didn't think it was necessary. What I built was a version of the Chance PI coil I'd seen on here, sometimes referred to as SS3D, (Self Shielding 3 dimensional). I was under the impression that the last few winds of this coil (to the ground side of the circuit) acted as the shield.
Perhaps a layer of shielding will help, I'll give it a go. And although the board is in a metal case, but I've just realised that this is not grounded to the circuit in any way when i came to think about it.

It might be the soil, and as I said, I'd read this unit would be better on the beach, but I wasn't expecting it to be that bad in my garden. Perhaps the sensitivity was too high. I made a bit of a mistake with the sens potentiometer, I thought it would be cool to have a ten-turn wire-wound pot in that place, mainly because I happened to have one! In hindsight this wasn't a good idea, too many turns between working and not.

[godigit1]

Hey Capt,
It should not be that bad on normal soil. literally your yard could have tons of iron in it , especially if their has been a couple shingle changes.
I tested a 4x4 foot area in a 100 year old yard and in the first 4 inches I removed 125 pieces of varying iron and 6 non ferrous items before giving up.
One thing you might want to check is coil polarity try turning your coil upside down and or switching the + and - at the connector or coil itself.

The other thing is it sounds as though the coil is not sealed. It simply may be falseing signals due to moisture in the grass. even some pro coil have problems with this.

Good Luck
digit

[Capt Slog]

You could be right about the soil under the lawn. I think my late father made the lawn over some right old rubbish, maybe even furnace slag! Now before you say "well then, that should have been obvious", I should tell you that this isn't the first metal detector that's been over this little bit of garden. I've owned a couple of shop bought ones in the past, and also made one as a teenager. I remember finding a few targets then, but not being able to dig them out.

So, either my new build is really really good because I'm getting all the stuff i missed before, or it's faulty..

I know that I have the ends correct from the coil. The start of the winding on this should be the '+' and the last few turns go to the ground. I hadn't considered that the coil might work better upside down from how it is now, and to be honest, I can't get my head around that, but everyday's a school day.

[Qiaozhi]

If it detects metal targets in an air test, then obviously target detection is working.
If you can wave the coil around in the air, away from any metal and the ground, the Earth Field elimination is working.
If it doesn't detect ferrite in an air test, it will ignore ground mineralization.

Then the only conclusion is that your soil is full of metal trash.

You should try increasing the sample delay so that it rejects foil and small nails.

[godigit1]

I'm a beginner here and still have tones to learn as far as building detectors. I've used them for 25 +years.
That being said I welcome correction and constructive criticism.
I thought I had read here that as far as the coil being upside down or not that the electrons should flow in a clockwise direction. That the flow also gives the coil sides a + or - side. That having the wrong side down could cause erratic behavior in some coils.
Of coarse I cannot find that post as this time.
Please any Gurus out there that can enlighten me on this.
Thanks

[baum7154]

I'm a beginner here and still have tones to learn as far as building detectors. I've used them for 25 +years.
That being said I welcome correction and constructive criticism.
I thought I had read here that as far as the coil being upside down or not that the electrons should flow in a clockwise direction. That the flow also gives the coil sides a + or - side. That having the wrong side down could cause erratic behavior in some coils.
Of coarse I cannot find that post as this time.
Please any Gurus out there that can enlighten me on this.
Thanks

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I have witnessed this on the oscilloscope just this past week. To do this test put a spare coil on the o'scope with a damping resistor and turn on your PI detector facing the spare coil from about two or three feet away. Observe the polarity of the flyback on the scope and then turn the spare coil around facing the detector coil again. You will see the polarity of the flyback reversed from the polarity of opposite side.

Regards,

Dan

[kt315]

I'm certain I don't have that much metal under my lawn

your lawn may be so infected by ferrous garbage massively but you do not aacept the fact. PI detector very sensitive
to all metal and all size of metal targets.
1 point. you have to find really clear lawn at your placement.
2 point. try use 2-4" coil for testing if you are not at beach.
3 point. low sensitivity by FIRST/MAIN DELAY pot setting it in right position, on larger value. that has to eliminate small targets
like the nails from horseshoes.

[Qiaozhi]

I'm a beginner here and still have tones to learn as far as building detectors. I've used them for 25 +years.
That being said I welcome correction and constructive criticism.
I thought I had read here that as far as the coil being upside down or not that the electrons should flow in a clockwise direction. That the flow also gives the coil sides a + or - side. That having the wrong side down could cause erratic behavior in some coils.
Of coarse I cannot find that post as this time.
Please any Gurus out there that can enlighten me on this.
Thanks

That's an old wives tale.

---------------------------------------------------
I have witnessed this on the oscilloscope just this past week. To do this test put a spare coil on the o'scope with a damping resistor and turn on your PI detector facing the spare coil from about two or three feet away. Observe the polarity of the flyback on the scope and then turn the spare coil around facing the detector coil again. You will see the polarity of the flyback reversed from the polarity of opposite side.

Regards,

Dan

This is correct, but is not a cause of erratic behaviour.
Remember that In a PI detector with bipolar pulsing, the polarity will change on alternate pulses.

I suspect the root of this question is associated with VLF detectors, and the polarity of the TX wiring in a concentric coil. Depending on which way round the TX + nulling coil is connected to the TX oscillator (if I remember correctly) the TX coil should be connected to 0V, and the nulling coil to the TX oscillator.