Geotech Baracuda - REV-A - Coils

[ionut_mtb]

just an update on this project as its all working sweet, good depth with the mono coil as in image with graphite shielding. 9kt mens ring in the beach sand 220mm .Have a flat spiral coil @250mm dia that pick up the gold ring @390mm in beach sand, so happy days. thanks to all who helped along the way to build my first PI detector. Now to silver dog in the new year for my next kit or may be kits
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good job! Can you make tests with us 5cent coin? Air and sand? And what diference you have on gold ring air test/sand. Thanks

[Chiv]

I shall do the test for you as soon as I get a US 5cent coin, but aussie 5cent piece 140mm air test and beach sand non black sand 130mm ish wet, and heavy mineral beach sand (black sand) 90mm wet but that with playing around to find just 5cent coin when I set up reject for 5 cent coin then I loose depth for gold ring so that means I might be loosing of few coins down a bit deeper as I use a gold ring to set up on the beach as every beach here has different amount of mineral sands mixed in to it. one beach here I cant even pick up my test ring as barra just cant settle down but even the neo magnet will pick up the dirt, lucky it a national park or the mining companies would be mining it

[Chiv]

I think your going to find the figure 8 coil also cancels out target signal on a PI.

you where totally right with this, I made up a 320uh figure 8 coil and almost nil pick up on my targets, also had no emi or end swing noise from the earths magnetic field either.
I could detect a spoon sinker (1'3/4" in size) about 3 inches above coil, and a bean sinker number3, 3/4inch in length only by rubbing very close to coil, so scrapped that one and started on a new coil.
this coil was a lets see what happen type coil at random winding spots in a cheep housing.

It started out with two strands of .236mm enamelled wire 19meters in length and after wind is come out at 356uh, then added cable witch brought it up to 540uh 2 ohms, for shielding I use some type of sticky ally foil that found at local tip shop only on the edge of coil housing, some quick scope work and fitted a 550ohm resister in for damping and tested it on led sinkers, getting a good hit @12inches with bean sinker which for some reason is the hardest shape sinker to get a nice clean sig, and the spoon sinker best I can get is 17inches as in air test, and 9crt mens ring @ 9 inches good sig after that sig gets iffy.
So I filled it with model resin and to my surprise that inductance had risen a fair bit to 651uh and 825.1pf and still 2ohms and resonance frequency @541991hz if I got that measurement right.
with reasoned filled coil now hard as I have basically lost 2inches of depth on all my test targets and still no EMF effect with the swing of the coil, so on with next project and that's to make my own litz wire and build another coil with only edge shielding.
This barra is getting a good work out on different coils and has not let me down

[waltr]

Sounds like the resin changed the dielectric considerably which increase the inter-winding capacitance thus slowing the coil's decay.

Check the Coil articles here:
http://www.geotech1.com/cgi-bin/page...&file=info.dat
http://www.geotech1.com/cgi-bin/page...e=projects.dat

Also check my thread "my take on the HH2" in the HH II sub-forum about coils.

Note: The HH (Hammer Head) is also a PI detector and coils would be the same.

[Chiv]

Thanks Waltr I have been reading your posts with intrest

[waltr]

Thanks,
I currently am running non-shielded for several reasons:
1- I didn't have good shielding material
2- Read some that PI detector coils really do not need shield (been wondering if shielding is just a hold over from VLF coils). Tinker's threads are also very interesting.
3- In the KISS principle- start with the minimum needed then test. My coils are not glued so easy to pull apart to change (add shield, etc).

When weather is warm enough to get outside I'll be testing and reporting in my thread.

[green]

Thanks,
I currently am running non-shielded for several reasons:
1- I didn't have good shielding material
2- Read some that PI detector coils really do not need shield (been wondering if shielding is just a hold over from VLF coils). Tinker's threads are also very interesting.
3- In the KISS principle- start with the minimum needed then test. My coils are not glued so easy to pull apart to change (add shield, etc).

When weather is warm enough to get outside I'll be testing and reporting in my thread.

With no shield I get an indication when my hand comes close or touches the coil with my PI. Interested why yours doesn't.

[waltr]

With no shield I get an indication when my hand comes close or touches the coil with my PI. Interested why yours doesn't.

Yes, seems odd.
Even with a mono coil I didn't get a response to my hand but did get response to Ferrite, a Brick and a magnet.

Possibly it is when the two samples are taken? Or op-amp offset adjust or something else different in my PCB, I did run a full Ground (VB+) on the bottom.
Have you played with TX Pulse width and adding a series R (10 Ohm) between TX MOSFET and Coil? I found this really helped sensitivity and stability.

Just got more connectors for coils so will be getting the Mono coil wired. I'll double check this.

[green]

Yes, seems odd.
Even with a mono coil I didn't get a response to my hand but did get response to Ferrite, a Brick and a magnet.

Possibly it is when the two samples are taken? Or op-amp offset adjust or something else different in my PCB, I did run a full Ground (VB+) on the bottom.
Have you played with TX Pulse width and adding a series R (10 Ohm) between TX MOSFET and Coil? I found this really helped sensitivity and stability.

Just got more connectors for coils so will be getting the Mono coil wired. I'll double check this.

I've have played with different pulse width, delay and sample timings for other reasons. Don't know if they would help the hand indication, had the problem when I first started. Shielded the coil, problem went away. Haven't tried adding a 10 ohm resistor in series. Add a MUR460 in series to allow earlier sampling with mono coils. Haven't decided if the diode helps with separate Tx and Rx coils.

[waltr]

Just check the Baracuda schematics and instructions:

Check that the main sample and EFE sample times are very close to the same.
This is noted in the instructions (Other Points: #2) that they may not be exact. Try matching C12-C13 and R15-R21 if the sample times are not the same.

Also look with your scope at the integrator output (U7a pin 1). Look for the two samples, the first should show as a drop , the second as a raise.
Are the R's and C's (R26-R27, R19-R20, C10-C11) in the integrator stage matched?
Is the second (EFE) sample working?

Really just taking Guesses as to why there is a 'hand' response. A 'hand' would be just an electrostatic field (which is what the shielding blocks) unless you have a ring on the hand.
So my guess is that the EFE sample should remove this effect.

[ionut_mtb]

With no shield I get an indication when my hand comes close or touches the coil with my PI. Interested why yours doesn't.

in my projects hand become detected when delay is at minimum (don't know exactly, around 10us).

[green]

in my projects hand become detected when delay is at minimum (don't know exactly, around 10us).

I've been using a 6usec delay, maybe that is the reason I need shield. Next time I make a coil I'll test if a longer delay doesn't need a shield.

[waltr]

6us sounds too short for any mono coil.
The first sample (delay) should start after the pre-amp's output signal is raising and is fine starting the sample when the signal it nearing the rolling off to level.
Have you looked at the pre-amp out and first sample on a scope?

The mono coils I've tried needed at least a 15us delay.
With nulled DD coil sampling delay can be 8us and maybe a little sooner.

Try increasing the delay a bit.

Check Figure 26 & 29 of the Hammer Head article. This is about as short a delay as can be used. Note that the sampling takes place on the rising wave form (pre-amp output).
If there are noise, stability of other issue increase the delay.
http://www.geotech1.com/pages/metdet...ead/HHv1p5.pdf

[Chiv]

Possibly it is when the two samples are taken? Or op-amp offset adjust or something else different in my PCB, I did run a full Ground (VB+) on the bottom.
Have you played with TX Pulse width and adding a series R (10 Ohm) between TX MOSFET and Coil? I found this really helped sensitivity and stability.

tried this 10ohm in series added it in coil head, gave me a real nice square pulse, also cleaned up some of the emi I was seeing too. Thanks for that Waltr, I'm still reading the Inside of the detector book so still lots to learn and try and make it stick.

[waltr]

tried this 10ohm in series added it in coil head, gave me a real nice square pulse, also cleaned up some of the emi I was seeing too. Thanks for that Waltr, I'm still reading the Inside of the detector book so still lots to learn and try and make it stick.

That's great news.
I was still running on proto-boards when I tried a series resistor. This does two things:
1- it decreases the Coils Tau (L/R) allowing the magnet field in the coil to reach a peak much sooner.
It seems that allowing the coil current to level out before switching off works better.
2- It decreases to maximum coil current and also decreases to fly-back Voltage preventing the MOSFET going into Avalanche.
A lower coil current can also help maintain the battery Voltage preventing Voltage drops that can affect other circuits.

I got to adding a connector to my 10 inch Mono coil last night. I am not getting a response to my hand.
Cold (0°C) again today but planning to get out in my yard for some 'real' testing.

ITMD book is pretty good but does not go into lots of details on the PI detector. Much more info within the threads here.