CHANCE Pcb's Finally Here

[baum7154]

Nimh batteries

Never mind, I just read several articles on charging Nimh batteries and a stack of 10 is best in the auto charge application.

[baum7154]

Is 13.2 Volts OK?

I have a set of 11 1.2 volt nickel metal hydride batteries that I am considering using in my Chance PI. What problems do any of you see is using this higher voltage? It is a good voltage from the standpoint of charging directly from a car cigarette lighter. I think it would pretty hard to overcharge this pack in a car. Also I think the extra voltage may help performance a bit, but is it too much?

[baum7154]

I took my unit outside yesterday and zeroed it with the coil laying on the grass as I still do not have it integrated into a package with a boom or electronics housing. The unit does detect my vial of gold flakes totaling the size/volume of a book match head. It also detects my gold wedding band very well from about 4-5". It appears to detect the gold flakes from various distances with the closest being about 3-4". There are steps of detection of the gold flakes with decreasingly smaller areas of sensitivity at about 7" and one at about 12". The 12 " area is really small I estimate it to be about 3/4" in diameter at 12". The unit also detected my iron fireplace poker at about 30".

I noticed the need for somewhat frequent resets using the ZERO button as the battery voltage was depleted below 10.8 volts. I have been running this unit on the same batteries for many hours now and I estimate the 8 AA batteries in the pack to have run the detector for at least 12 hours now.

Does anyone have any good ideas for a telescoping boom for the detector? I'm looking at the Seymour Superhandle II paint roller extenders. They are fiberglass construction with 2 twist locks for the boom extension. They come in 2'-to 4' and 4' to 8' lengths. I wonder if an aluminum boom on one of these will even work...any ideas?

I'm also wondering if the coil should be Faraday shielded at least on the top and was considering using the old black conductive plastic from old static protection bags. An ohm meter test on these gives resistance in the range of 50K ohms per inch.

Thanks

Dan

[Tepco]

Small update on this. I put together my (old) chance, and programmed new software (8.04), and i have to admit it is much better now. More stability, better response, less prone to interference. I cannot claim anything about range (workbench, noisy conditions, high barrier setting etc) but some improvement is obvious. VDI also have more sense now. Looks quite good so far. Still useless as a beach machine (never intended to be), issue whit fast targets (jewelry) is still present, otherwise seems to be quite decent detector. And considering it is not especially fast maybe can accommodate larger coils as well. I have completed 12.5" coil and shaft for another detector, but i think will fit timing requirements for this one, will be interesting to see test results, i will put details here if it works.

[baum7154]

My software is version 0.8.4.

I am using coax but have way too much lead length (6")on the coil before it connects to the coax. Will shorten this ASAP. Also am considering using 300 ohm twin lead instead of coax.

Test outside today in the air were somewhat unimpressive. Unit did detect a roll of aluminum foil at about 24". Also detected my brass keys on a steel ring at about 8". Seemed to detect the vial of gold flakes at about 2-3" with only a very weak signal strength, but would not detect my gold ring.

I know it is said that the unit sensitivity is better when fully assembled so will pursue that end.

Regards,

Dan

[Tepco]

I have completed the circuit board,switches, and an exact replica of Andy's coil using #24 AWG silver plated teflon wire on a basket weave HDPE plastic form (18 turns + 24 turns and 1.5 ohms due to the silver plated wire. It does detect and discriminate iron from gold nuggets in an air test on the bench.

I have some questions about operations and the buttons:

1. I assume that the BARRIER settings are essentially sensitivity thresholds...correct?

2. What are the purposes of the four MASK functions?

3. Is there a way to make the display backlight to stay on?

4. Is the ZERO button used after all other parameters are set?

5. How does Andy provide shielding for his coil? I am thinking of using polyetheylene anti static bag material for this purpose.

6. Has anyone else built this same coil and if so was the 1 watt 390 ohm resistor the best choice for it or was a different value better?

My unit draws between 100ma and 160 ma in operation and all the functions seem to work as expected.

I did test detection of a small vial of gold flakes with the largest being about the size of a match head and it seemed to detect it at about four inches in an air test. It also appeared to detect a long nose pliers at about three feet in an air test.

I appreciate your help.

Dan

Lovely, it is time to put together my Chance once again, i didn't even noticed new software versions (mine was built in 2009, when only oldest version was available, don't remember which one, and at the end i scrapped this project). So, first question is: did anyone tried to compare different software versions, and what is changed? (What is your version?) And few answers i can give:

1.Barrier is threshold control, keep it as low as you can, depending on interference etc.

2.In old software it was DISCRIMINATION 0-4 setting

3.Connect backlight directly (in one modified version of old software, it turns on when target is detected, battery saving or something, otherwise just at a press of the button)

4.ZERO detector on soil (and do this often)

5. Original coil is unshielded. Antistatic material is not nearly enough conductive for effective shielding. And if you shield this coil by other means (graphite etc.) this will slow it down and then it wont work at all. This is probably intended to be used whit unshielded cable, not coax.

6. 390R seems to be optimal value for original coil. (actually "must have" value).Coil is somewhat underdamped whit this value, don't attempt to speed it up, machine expects exactly that part of waveform when it takes sample, leave everything as close as possible to original specifications.

Current consumption is OK. Only fully assembled, battery powered and outdoors it will show full sensitivity, not bad considering coil is in essence 8''. What is very strange behavior of this detector is that it can see very small nudges, but some large (almost 30g) specific shaped jewelry can get completely undetected, in fact you can put it in a middle of searchcoil, ZERO and detector will work quite normally detecting everything else!!! Obviously some software crap whit "fast decay" targets or something, i hope fixed in newer releases. Fortunately, my PCB and coil is not completely cannibalized, i will give another chance to Chance and report results ASAP.

[baum7154]

Silverdog,
Thanks for the response. Any idea on the 390 ohm resistor value. I haven't had a chance to put it on my scope to optimize it.

I did set the coil up on my slate pool table last night and the sensitivity has really gone down from that . I'm not sure why. It just about will not detect a gold sample when it is lying on that slate surface. In addition it will only detect a long nose pliers at about 8 - 10 inches on the slate. I'm going to take it outside my house today to get it away from other influences.

I'm also going to get it into a metal case ASAP so it will be better shielded.

Thanks again!

[silverdog]

As far as I have worked out;

Barrier is sensitivity
Mask is discrimination - off,1,2,3,4 - 4 seems best for iron rejection
I tend to use zero as a reset if it becomes a bit noisey - which it does on my bench !
I haven't shielded my coil
390 works ok for given coil, not tested others yet
my lcd has no backlight, from what I have read light only comes on with each press, to save power I guess
install google toolbar with the translator and head here http://www.md4u.ru/forum/viewtopic.php?t=3033

78 pages of discussion on chance - andy_f the author is actively involved

[baum7154]

Chance PI is built and working?

I have completed the circuit board,switches, and an exact replica of Andy's coil using #24 AWG silver plated teflon wire on a basket weave HDPE plastic form (18 turns + 24 turns and 1.5 ohms due to the silver plated wire. It does detect and discriminate iron from gold nuggets in an air test on the bench.

I have some questions about operations and the buttons:

1. I assume that the BARRIER settings are essentially sensitivity thresholds...correct?

2. What are the purposes of the four MASK functions?

3. Is there a way to make the display backlight to stay on?

4. Is the ZERO button used after all other parameters are set?

5. How does Andy provide shielding for his coil? I am thinking of using polyetheylene anti static bag material for this purpose.

6. Has anyone else built this same coil and if so was the 1 watt 390 ohm resistor the best choice for it or was a different value better?

My unit draws between 100ma and 160 ma in operation and all the functions seem to work as expected.

I did test detection of a small vial of gold flakes with the largest being about the size of a match head and it seemed to detect it at about four inches in an air test. It also appeared to detect a long nose pliers at about three feet in an air test.

I appreciate your help.

Dan

[mollax]

in fact such

http://www.md4u.ru/forum/viewtopic.php?f=5&t=3033&start=1350

[Tepco]

Very good simulation, excellent job, works exactly like this in real world, observed on scope. I had some pics of different w-forms, but it was while ago, i will post if i manage to find them.

[Altra]

I simulated the code, here are some screen shots of the timming. Sorry for the quality

I think because the sample timming is fixed. The coil must be well matched and be exactly the same as Andy's when he set the timmings in code. For decent results.

I hope somebody will post some scope pictures in the future with different targets

Regards Mark

[Tepco]

Chance is completely useless as a beach machine, it can be actually good for any other type of search, except this one. In short, good for anything else except jewelry. Even Andy himself mentioned this. After all, give it a chance and try it.

[mario]

All your gold are belong to us, but whit this thing, not much we will get. I have built this thing something over year ago, (on different PCB) and i was not " enlightened" whit this detector at all. It works, but...

First of all, sampling time is fixed in software, 17uS, and way too slow for any criteria. So you don't need to use exotic fast MOSFETS, drive circuit for them is slow anyway and if you modify it, no change in performance will appear. Detector uses Minelab multipulse technology, but how well it is implemented, whit monocoil, im not so shure. Coil output is alternative combination of "normal" 120uS PI pulse and "burst" of 20+ asymmetrical short pulses, whit total coil charging time being the same (MOSFETS short the coil between pulses), at repetition rate of abouth 200Hz. Discrimination is based on taking 5 subsequent samples, two after first (normal) pulse, first one starting at 17uS, another 20uS later, and 3 samples after "burst", now delayed, so totally 5 samples are taken, 10uS apart, starting from 17uS. This is latter used for calculated software guess of "discrimination", whit questionable effect. Coil have to be fast enough for this 17uS, faster coils will work too, slower ones will mess up everything, use original coil for reference. Main disadvantage of this relatively slow sampling is very poor sensitivity to small conductive targets (jewelry, small nudges etc), but for coin-relics hunting may be good, this is coin shooter basically. So, if it sees any gold, will be on correct VDI position, if not, sorry. And detector is not especially deep, normal analog PI will kill it, also audio response is very interesting (but may be not something you can use to it). Really good thing is that this "multipulse" trick seems to work very well on very bad ground, thing can probably find its target in pure ferrite, well, if it can see it at all. So far, most advanced DIY PI detector, simple and cheap, most of stuff done in software, build and test it anyway. Just we may have to wait for some other "incarnation" of similar concept.

I don't think Andy intended to design a gold unit. On the gold fields, using discrimination means passing up those really small or deep nuggets. Depending on the mass of gold, it can behave as a very low or high conductor, so discrimination is not a very useful feature. Mind you, I have not used the $4k+ Minelabs, so I don't know how reliably Minelab implements discrimination in those units.

Still, if true, what he has done is nothing short of amazing. With this unit, I see a chance at fighting those guys who litter our beaches with cheap pennys. One day, when vacationing in California, I remember seeing a guy hunting for jewelry. I found it odd that every once in a while he would drop a penny or two. It's not until afterward that I caught on.

Those pennys must have frustrated the PI guys hunting for deep jewelery.

It's the coin and relic hunters that considerably use discrimination. I think the nugget guys consider sensitivity much more important. I don't mind digging every target if it means occasionally finding a small nugget.

Also, even though this unit may not be very deep, you can always build a bigger coil. And If you want sensitivity, you may easily experiment with a faster crystal to increase sampling -albeit, you may lose LCD and/or discrimination features.

On a side note, have you taken a look at the timings of the gpx? How is the sampling handled? It's a matter of time before some one emulates the timings of a modern unit.

[Tepco]

Has any one built a working unit yet?

All your gold are belong to us, but whit this thing, not much we will get. I have built this thing something over year ago, (on different PCB) and i was not " enlightened" whit this detector at all. It works, but...

First of all, sampling time is fixed in software, 17uS, and way too slow for any criteria. So you don't need to use exotic fast MOSFETS, drive circuit for them is slow anyway and if you modify it, no change in performance will appear. Detector uses Minelab multipulse technology, but how well it is implemented, whit monocoil, im not so shure. Coil output is alternative combination of "normal" 120uS PI pulse and "burst" of 20+ asymmetrical short pulses, whit total coil charging time being the same (MOSFETS short the coil between pulses), at repetition rate of abouth 200Hz. Discrimination is based on taking 5 subsequent samples, two after first (normal) pulse, first one starting at 17uS, another 20uS later, and 3 samples after "burst", now delayed, so totally 5 samples are taken, 10uS apart, starting from 17uS. This is latter used for calculated software guess of "discrimination", whit questionable effect. Coil have to be fast enough for this 17uS, faster coils will work too, slower ones will mess up everything, use original coil for reference. Main disadvantage of this relatively slow sampling is very poor sensitivity to small conductive targets (jewelry, small nudges etc), but for coin-relics hunting may be good, this is coin shooter basically. So, if it sees any gold, will be on correct VDI position, if not, sorry. And detector is not especially deep, normal analog PI will kill it, also audio response is very interesting (but may be not something you can use to it). Really good thing is that this "multipulse" trick seems to work very well on very bad ground, thing can probably find its target in pure ferrite, well, if it can see it at all. So far, most advanced DIY PI detector, simple and cheap, most of stuff done in software, build and test it anyway. Just we may have to wait for some other "incarnation" of similar concept.