IGSL

[satdaveuk]

I'm afraid I have only Croatian Kuna which is rich in nickel, and i noticed that cracky sound quality related to them.
My shaft is almost ready. I'll have to decide on how to attach a device to it. Batteries will not be a problem as I have a stash of 9V cubes that are beyond expiration date, but still pumping juice. Maybe I'll just hang it losely - I don't know. I did not think that far. When I do - it is movie time.

For the time being, I'll just attach a retouched photo of the current state of the device. Funny thing is - I did not bring any LM358 to my li'l island, and I had to improvise - I've put NE5532 insteadstick:
In this place this is actually a bad choice because of the protective circuitry in NE5532's input - but it works. TL072 didn't.

So here is a photo of the upper side in a box. There are only two shorts extra on the lower side so it is mostly WYSIWYG:

You have been busy Davor
Think a new pcb layout is in much need there by the looks of things

[satdaveuk]

An unshielded coil is practically useless in the field work. I use shielding to both coils, TX and RX. I prefer the aluminum adhesive tape, because it handles very well and does not usually cause any problems in the field.

Theres nothing wrong with alloy tape if you can get it thin enough, I used to use lead and alloy tape for years, the stuff i used for shop fronts when installing security sytems, infact still got some of each, although dont know how good the sticky is now, there years old.

Course it all phased out for me when started using break glass detectors which was about 25-30 years ago.
I like blasts from the past Eduardo, new ideas are not always the best option, im a firm belever if it works for you why in the hell change.
All the best
Regards

[Davor]

I'm afraid I have only Croatian Kuna which is rich in nickel, and i noticed that cracky sound quality related to them.
My shaft is almost ready. I'll have to decide on how to attach a device to it. Batteries will not be a problem as I have a stash of 9V cubes that are beyond expiration date, but still pumping juice. Maybe I'll just hang it losely - I don't know. I did not think that far. When I do - it is movie time.

For the time being, I'll just attach a retouched photo of the current state of the device. Funny thing is - I did not bring any LM358 to my li'l island, and I had to improvise - I've put NE5532 insteadstick:
In this place this is actually a bad choice because of the protective circuitry in NE5532's input - but it works. TL072 didn't.

So here is a photo of the upper side in a box. There are only two shorts extra on the lower side so it is mostly WYSIWYG:

[ivconic]

Davor, before you shot the video, please do watch this:

http://www.youtube.com/watch?v=7tfBHvLMr68

and try to find similar items to test you IGSL with.
Ok, it is impossible to find exactly the same items with same alloy, but please do try to find similar and perform same tests in front of camera.
Simply because that would be closest way to compare behavior of original IGSL and your modified version.
Also use small ferrite piece to test GEB.
I would like to see and hear the differences you are talking about.
Items on my video are 2 coins; one with increased zinc percentage (cracky audio) and another with increased nickel percentage, than two long nails, piece of Al tape and black iron pliers.
Thanks in advance!

[Jerry]

Crackles? Try adjusting your GEB properly by waving pieces of ferrite in front of your coil. With GEB properly adjusted most of the noises simply vanish.

I have a 25k ten turn pot for fine tuning the GEB. Tuning it to a piece of ferrite gives an approximate ground balance, the final setting to completely quiet the ground is fairly critical and it does calm the detector down a lot. However it will still crackle when running high sensitivity. Turning the SENS down a bit quiets the detector but that isn't the issue. What I have been looking for is a calm, steady background tone to listen to instead of running completely silent. I am sure this was not the intent of the designers, but still something I am working towards.

Jerry

[Davor]

Yeah - a broomstick. Model "Raskov 2000"

I realised that all those plastic looking shafts for various brooms, mops, etc. are actually thin walled iron tube with plastic on :yuck:
So what remains is a classic wooden stick Ø22mm. Yeah, broomsticks will fly.

I have two options to fix my shaft. I have 2 crutches where upper portion is Ø22mm and lower is Ø19mm (3/4"). In case I find Ø19mm stick my shaft will be extendable, otherwise I'll stick a wooden one Ø22mm all the way.

The existing broom will not part from its stick ... yet.

On my li'l island there are not many options for supplies, but I'll manage ... something.

...

When you say tone, is it the Low, High or a mixture of the two. Also when you said Nice Continuous tone, I take that to mean with no, or at least very few crackles and pops. Nice!

I am convinced that having a threshold settable tone will help wring out the best in the TGSL/IGSL detectors.

With the ikebana mod where the two comparator chips have swapped input pins to allow proper "disc sens" (actually threshold) setting happen on both channels, yes, I have single tone for targets such as iron or copper, but some yodel on targets of combined origin, and overlapping tone for tin foil. In original setup the overlapping tone was a commonplace for everything other than copper.

Crackles? Try adjusting your GEB properly by waving pieces of ferrite in front of your coil. With GEB properly adjusted most of the noises simply vanish.

Regarding the sound qualities - yeah, I'm not too happy about it. It is still a bit on a binary side. The resistors R36 and R56 at 330k are pumping too much juice for a proportional tone to ever happen, and I'll replace them with 680k (if I find them in my resistor stash). Next, I'll replace R67 and R68 with 10k. So far I'm using 33k with headphones, but I'll fix the headphones attenuation by some other means instead.

I'm not too happy with the oscillator. It produces a lot of 2nd harmonic when pushed hard, or it loses amplitude otherwise. At some settings it creates some periodic instabilities in relation with the binary tone. I'd say it is the PWM problem of these designs materialising in front of me. Chatter? My lower back! Just instabilities.

I think I know the perfect candidate oscillator to replace the original one. More about it later.

[Jerry]

It works

Impressions? I can sweep very slowly, and still have a nice continuous tone - pinpointer style. If mistuned - it is a *****. I'll have to learn to tune it properly. I did not observe any delays in responses. It is dead on. I'd say those gain stage mods did good......................
I also have an odd problem. I am missing a non-metallic rod for a shaft. Today I was swinging a coil on a broomstick - upside down. It was silly

Congratulations on getting your detector working! I do have a question on the fact that you are able to get a nice continuous tone. This is what I have been trying to accomplish with my threshold controls and past conversations with you. Am close but still not there. When you say tone, is it the Low, High or a mixture of the two. Also when you said Nice Continuous tone, I take that to mean with no, or at least very few crackles and pops. Nice!

I am convinced that having a threshold settable tone will help wring out the best in the TGSL/IGSL detectors.

Funny you mentioned using a broomstick for a shaft. I just turned the end of a short piece of broomstick for use as the lower shaft between the coil and first metal piece. Not finished yet, but will write that one up when it is.

Jerry

[Davor]

Thanks, I'll take a day or two to decide on final touches and then you'll see it all.
Let me just gather some thoughts on differences against the original IGSL.
- I joined GEB of both Cu and Fe channels together, and also ported it to a 10k potentiometer for fine tuning. It is less confusing this way. Now that I think of it - there is a spare channel doing ... exactly the same job as the other GEB channel, and it could work as an "all metal" (GEB) with time constants improved for pinpointing ... just a thought.
- discrimination potentiometers are as they are in original.
- I joined "Disc sens" (in reality low signal threshold) to a single pot. Ikebana mod allows me to do that, now that both channels work by the same principle. Again, this is less confusing this way.
- "threshold" controls are now ported to a stereo potentiometer and provide some volume control. I'll have to put a few kohm in a series because the binary sound sucks.

About the guts...
- frontend is a true differential amplifier with input impedance at 6k8 - I found resistors combination from E6 that do that nicely. It can work happily with regular coils, but I arranged my DD coil Rx as a center tapped bifilar. So far I did not notice any problems with static, and I guess it is fine. On LM833 output it produces 40mVpp so this seem to be OK.
- I joined both GEB controls to one by connecting pins 1 and 13 of the phasing comparators, and disabling the other one by forcing it high.
- LF351 is not used, yet by inertia of populating a board first - decide on mods later, I left there a few components that are not used.
- gain stages were set with both diodes in a second stage, and with feedback capacitors removed from the second stage. I can write about it whole day and more, but the consequence of this setup will be more obvious in a video.
- I removed all unnecessary bits from the comparators section, so there are only 100k resistor, and a 22n capacitor. The diode is replaced with a short. Together with modded gain stages this stands for full duration of beep over a target. Disc sens controls are merged to one.

About the coil...
It is balanced by hand-squeezing it here and there, from outside to the inner overlap, fixing it with a crazy glue to a plexiglass form. At the end only a small portion of it remained unglued + a small curl for finetuning. I noticed that the curl was not of essence, and that most probably the coil would have been nulled even lower without it - it breaks coil symmetry. Now I have less than 1mV of unbalanced signal, and I just can't make it go lower. I'll not put any shielding on.

More to come...

[der_fisherman]

Davor, congratulations.
I hope you are able to get the final adjustments completed and to produce a schematic...
rhanks and regards from
Andy

[ivconic]

"... I'm not too happy with headphones output, it is too loud. Maybe some other audio stage would do better, but I'll just apply some quick and dirty fix for it...."

Just add resistor in series with earphones on jack. Or use "PC headphones" with included potentiometer. I am using those.

Rest of your post: promising!
IGSL is a bit tricky in adjustments and understanding. There are several ways to combine those two Disc&GEB potentiometers.
Video clip would be good. Will tell us more than words. Use ordinary camera and shot short video, than register on Youtube and upload it there.
Anyway: congratulations!

[Davor]

It works

Now just a bit of fine tuning and final mods. I already found some "relics" in back yard, including a jar lid (aluminum). It confused me completely. It sounded as non-Fe and I started digging, found nothing and checked again. More digging, checked again... finally at depth of, well, too much (~25cm) there it is.

Anyway, I gave up cleaning my back yard - far too much "relics" everywhere. Soon it would look like bad case of mole infestation. Not to mention there are no moles living in this area - too much gravel in soil. Difficult to dig.

Impressions? Ikebana mod to fix Fe channel works. I tried original first and it sucks. Big time. However, this way of modifying chips is not very efficient, and I'd recommend ... anything else. Took me several tries to make it work - first tries were using too thin wire and I was missing a contact ... somewhere. At one point I even thought chips are done for. Then I replaced the ikebana wires, and hey presto! it works. Now Fe channel gives a single beep for a single pass, and I can hear only Fe sound, not a mix, and ... "Disc sens" works exactly the same as for Cu channel. I joined these two controls to a single pot now and I like it this way.

I finally calibrated my Visual analyser, and checked coil at LM833 output. It is 40mVpp. Guess it is OK.

I have 5 pots: GEB fine tune (single turn 10k), Disc. Cu, Disc. Fe, "Disc sens", and volume (stereo pot). I'm not too happy with headphones output, it is too loud. Maybe some other audio stage would do better, but I'll just apply some quick and dirty fix for it.

At first I had some stability issues, but these can be fixed with "Disc sens" control. Later on I realised that these can be wiped by properly adjusting GEB.

Impressions? I can sweep very slowly, and still have a nice continuous tone - pinpointer style. If mistuned - it is a *****. I'll have to learn to tune it properly. I did not observe any delays in responses. It is dead on. I'd say those gain stage mods did good.

I know I am supposed to show it all in a video, but I'm not sure how will that work. When I learn how - you'll see.

I also have an odd problem. I am missing a non-metallic rod for a shaft. Today I was swinging a coil on a broomstick - upside down. It was silly

Hey people! I got my first metal detector

[Davor]

Everything is welded and in its place. Now comes fine tuning of coil, and reviving the detector. Nice

[Davor]

Shield makes sense ONLY if connected to common ground.
So, how it works?
Suppose we have some electric field in which a detector is immersed. In case it is a stationary field - nothing happens. In case the field changes, some of that will be picked up, just like an electret microphone does. Such microphone is more efficient if impedances are high, so one way of fighting E field is lowering impedances - wherever practical.
E field when presented to some coil will produce voltage that is in average ~ half of the peak voltage achieved by E field value reduced by the coil and Rx low impedance. In case E field is screened by some shielding, no E field is seen from the coil, and half of zero is zero. In case such E field is observed from a bifilar center tapped coil, the E field reduced by low impedance and symmetry happens between center tap and both coil ends, but Rx input being differential cancels these.

Symmetry is great for fighting E field, but not too practical. You usually wish to transform it to a more practical asymmetric path. But when you do, just try hard to have a common ground point as close as possible to every high impedance and low signal device you have. In case of shielding, yes, you want it to be connected to a common ground point. Otherwise it is not a shield but an antenna.

[der_fisherman]

It is funny but making a true differential input requires just adjusting a few resistors values - nothing else. It goes like this... The inverting side resistors R1 and R2 set the gain, and the noninverting side resistors just divide the input voltage so that R1/R2=R3/R4. It is a common practice to use R1=R3 and R2=R4 because in ancient times of lousy opamps it was the only way to keep the output offset at bay. Even today with bipolar opamps you get minimum offset this way.

In case of AC coupling to the next stage (✓) you can freely play with values R3 and R4, and the worthy goal is to achieve a symmetric impedance to both inputs against the ground.

By virtue of feedback the inverting input impedance of symmetrically fed opamp is ~R1/2, and at noninverting side it is R3+R4. Matching these gives you a true differential preamp. With a little offset, of course.

I am using 6k8 330k 68ohm 3k3 values to get amplification of 48.5 and impedance mismatch of ~2% (which is within the component tolerances anyway). My total input impedance is ~6k8, while the original IGSL is over 220k. I expect some noise improvement this way, but also somewhat more demanding GEB tuning range - for which I know the cure.

I do believe that Don B. made an unshielded coil for some special testing once, an IB coil if I remember correctly. So it might be a good idea to have Don in the conversational loop......(assuming he is not, but of course he may be fully up to date with this blog already!)
I am also of the belief (maybe wrongly) that an IB Detector needs to have a shielded coil....I followed slavishly all the discussions about the best shield type....
I am also following the conversations on this blog very avidly....I am sure that the end result will be that our knowledge will be improved durch Davor and Ivconic, plus others, but my thanks to all concerned.
regards
Andy

[eduardo1979]

Can you please show an example?
I have two types and they are both too thick and wide for coil shielding. The stuff I have works good for inside plastic housings. Speaking of which, do you think it would be any benefit to shield the inside of my Cibola? I was thinking that extending a shield on the top end wouldn't help unless it was true earth grounded. No?

Protect the interior of Cibola do not know. For detectors made by hand, I always use metal boxes.