my Dual field coil plus small mono

[satdaveuk]

Hi
A few pictures of interest the small 6" 3 ohms coil detects gents 9ct gold ring buried in pasture land at 6" loud and clear scale 3/4 way up, so with headphones looking at around 8" or so.
6" coil made with bog standard six core stranded alarm cable stript and wound as single core, faraday shielding graphite applied as usual, damping resistor 1.114k ohms.
Dug into my stock of 1w 1.2k carbon resistors and found one to fit the bill.

2nd one is dual field inner 5" outer 10", shell and coils graphite , same amount of the same cable as above twice as many 2:1 ratio inner/outer.
Not got a clue how good it is because as you can see graphite still needs to dry.

Now for a question? Can anyone see a problem by me installing the dampers into the search heads as I want to make various interchangable coils.
Had thought about wiring from pin6 u3 to a spare connection on the rs232 isp socket on the front panel and mount another pot to adjust damping for various coils, but of course without bringing a scope along with the detector it would be a waste of time.

Regards

[Qiaozhi]

Now for a question? Can anyone see a problem by me installing the dampers into the search heads as I want to make various interchangable coils.

Putting the damping resistor in the search head will work just fine. The advantage is that this allows you to swap coils easily.

However, it is more common to put the damping resistor on the PCB. The advantage here is that coils will be swappable between different machines.

A third alternative would be to place the damping resistor in the coil connector. Then, even is the coil is potted with epoxy, it is possible to change the resistor at a later time.

[satdaveuk]

Thats worth considering Qiaozhi thanks for the input, funny enough thats what I did with my second build TGSL for manufactured Tesoro coils, just made exstension lead with TX and RX components built in the inline socket, then when making my own coils put components into the shell and excluded exstension lead, so i could swop between shop coils and home brew by incl/discluding male to female lead.
Remember these pics earlier this year.

Many thanks

[satdaveuk]

Up to now the Mini pulse 3 seems to demand a higher inductance than the surf pi etc but is early days , seems to like the magic 3 ohms at around 1-1.2mh damping at around 1.2k although the designers were using 2k.
Im posting the circuits etc here incase someone can shed light on reasons why, or maybe im missing something.
The parts list 1 is not all correct ie, U12 which is a Atmega 8, not as listed , parts that are not listed are in parts list 2 which is now mostly correct, so basically I worked from parts list 1 and remaining components were taken from parts list 2, will all be sorted into one list and correct when I post the project in proper categery.
The Dual field coil posted here has not been tested yet as the graphite needs to be bone dry.

Many thanks in advance for expert advise as stated before im quite novis to PI format and want to learn more.

Regards

[Jose]

Hello Satdaveuk
I am experimenting with dual field coil for my homemade GS4.
This requires two coil damping resistor.
One for the larger coil and one for the smallest. If lacking in the coil damping resistor small, oscillations occur.
-Do both coils with CAT6, the two have two rounds of 8 conductors (8X2 = 16).
When measured separately one has 175 uH and 100 uH about the other.
Has added a total of 380 uH alrrededor and resistance of 2 ohms.
The improvement over the common coil is that it has a more intense signal for small objects.
I enclose a scheme, the resistor values are approximate and depend on the particular characteristics of the detector that is being used.
regards

[satdaveuk]

Hello Satdaveuk
I am experimenting with dual field coil for my homemade GS4.
This requires two coil damping resistor.
One for the larger coil and one for the smallest. If lacking in the coil damping resistor small, oscillations occur.
-Do both coils with CAT6, the two have two rounds of 8 conductors (8X2 = 16).
When measured separately one has 175 uH and 100 uH about the other.
Has added a total of 380 uH alrrededor and resistance of 2 ohms.
The improvement over the common coil is that it has a more intense signal for small objects.
I enclose a scheme, the resistor values are approximate and depend on the particular characteristics of the detector that is being used.
regards

Interesting Jose
Let me know how you get on.
Regards

[Jose]

Interesting Jose
Let me know how you get on.
Regards

I have not tried the coil in the field.
Perform tests in the air compared to other single coil of equal size, the results are a more intense signal for small items and an increase of about 2 or 3 cm. distance in the case of coins.
The delay is about 13 usec. May be lower if you limit the current during the TX pulse, or RX decreases gain in the first stage, so I think that this coil can easily reach 10 usec.
Tests with an RF generator, indicated resonance alrrededor of 1.5 Mz, including the cable.
I have not noticed a difference in the operation of GS4 (echo at home) with the coils described, compared with its similar single coil.
regards

[satdaveuk]

I have not tried the coil in the field.
Perform tests in the air compared to other single coil of equal size, the results are a more intense signal for small items and an increase of about 2 or 3 cm. distance in the case of coins.
The delay is about 13 usec. May be lower if you limit the current during the TX pulse, or RX decreases gain in the first stage, so I think that this coil can easily reach 10 usec.
Tests with an RF generator, indicated resonance alrrededor of 1.5 Mz, including the cable.
I have not noticed a difference in the operation of GS4 (echo at home) with the coils described, compared with its similar single coil.
regards

Hi Jose
Have you done any field tests yet?
The dual field coil i assembled was utter crap, noisy in and out the field, depth not as good as my 6" coil.
Im getting the impression that the minipulse 3 just dosent like large coils.
With the 6" coil which was just rushed together with alarm cable works like a dream, its stable and nice responce on targets, depth nothing to right home about unless your looking for large iron, coke can in my test bed is over 1 meter depth clear hit, scale 3/4 up with gain just over half way.
With a bottle caps and modern 5pence peace you need to be almost on top of it, coins in general 8-12" face on or edge on, no difference which really suprises me , steel 12" ruler 50cm edge on about the same face on, 9ct mans ring 8-9" same as coins.
Filled up a plastic candy floss container about 1 ltrs size with modern 2pence peaces weighing about 1 pound , and buried deeper and deeper, was getting very clear signals 50cm plus could of gone deeper, all tests done with internal speaker not headphones.
This PI stuff is so new to me, why is it not seeing certain metals yet others zoom in so well.
This detector works well with copper coins, but yet cant see stick back copper tape.
Can anyone here direct me to test points where I can measure sampling, delay etc and what/where to try and improve on this project to get maximum performance ?.
Ive now put the 2k damper back on pcb, taken from both ends one end series 390ohms 10k pot bridged with another resistor across pot cant remember value, and mounted on front panel.
Its standard size pot with slot for adjustment, gives me from 340 ohms to 1.6k.
How ive worked it so can be used in the field taken note of damping values of each coil which was calculated using scope and stamped on the back of detector.
So all i do when using in real situations is put a cheap multimeter in my pocket together with everything else , then when changing coils adjust damping pot to suit with multimeter connected across bnc coil socket before switch on.
Fucture plan will be to calibrated with markings on front facia around damper pot, then no meter required.
Thanks in advance
Dave

[Jose]

Hello Satdaveuk
I made some tests in the field with coins and can of coke of 333 cm3, buried in soil with low mineralization.
The bronze coin 1 "buried at 33 cm, the Coca buried 64 Cm, is detected well with the dual coil and single coil of equal size (30 cm), GS4 miss my home, with the GB and discrimination disconnected.
The dual coil I described earlier, has a slightly stronger signal and better defined, tests in air show better performance with small objects.
I have no experience with graffiti shield. All my coils to perform for the GS4 and delta pulse, cat5 or cat6 have cable and Scotch tape No. 24.
I know the Minipulse 3, but by what you say, this detector was designed to detect large objects deep. I think a roll of 6 "is small for this detector and work better with simple coils 30 to 45 cm, even without shielding.
The low sensitivity for You mention some items that are fast driving, such as bottle caps, they have to do with the least delay time can be set in the detector. And as I understand is normal.
I changed my Delta Pulse to work with shorter delay times, but I can not do it with the Minipulse 3.
Reg and others published a lot about the theory of PI detectors in this forum, and forum Eric Foster. I just begin to understand.
The damping resistance is critical, what I do is install a resistor of higher value than that required in the detector, such as 1K, then added a resistor in the coil to get the correct value buffer.
This way I can use multiple coils in my Delta Pulse, for example some have a resistance of 2.2 K, another 1.5 k etc., Depending on the size and characteristics of each coil.
Excuse my bad English.
Best wishes
Jose