chance pi build

[baum7154]

Do you have a better picture? That ones a bit fuzzy...

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Sorry for the bad picture. Having trouble since the mobile app for this site has been jacked and all my pics are taken on my IPOD, so transfer got more difficult. Will try to get a better picture with a real camera tomorrow and post it.

Dan

[baum7154]

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Sorry for the bad picture. Having trouble since the mobile app for this site has been jacked and all my pics are taken on my IPOD, so transfer got more difficult. Will try to get a better picture with a real camera tomorrow and post it.

Dan

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Attached here is a much better picture of the schematic. Sorry for the delay!

[6666]

This post is out of sequence and can be deleted.

[baum7154]

I don't understand. Are you saying post #197 is 'out of sequence'?


Regards,

Dan

[6666]

Hi Dan
no my post #198 is out of sequence, I replied to another earlier post but did not use the reply with quote feature .

[baum7154]

UPDATE ON CHANCE PI 2 STAGE PRE AMP

I did a bit more testing on the 2 stage preamp today.. After first visiting the Russian forum to check schematic details on the interface between the amp out and the A/D in I noticed another mod that is not in place on my detector. A 220uf 16 volt electrolytic is shown at the VREF out that I was unaware of this as a mod. Presumably this is to further stabilize the VREF voltage. I will be making this mod soon and post the results here.

As I mentioned in the original design description for the 2 stage amp the 14 diodes in the feedback loop of the second stage allow output voltage to go to about 10 volts without going into saturation. I mentioned that I would lower this voltage if it proved unnecessary to have the 10 volt output and my testing today indeed shows that it is unnecessary. I characterized the effect of different inputs to the A/D and it's saturation tone, a raspy buzz, occurs at 4.5 volts above ground. Knowing this I decided to eliminate 6 of the series diodes in the feedback loop. As it is now, the detector operates normally with 8 diodes, giving about 5.5 volts max signal at the amp out. A bottle cap will still trigger the saturation tone at about 1.75" and I think that is enough. A larger target will trigger the saturation even deeper than the bottle cap. This change has no effect on the small/large target sensitivity. In this configuration full scale signal strength of 11 units is achieved without the saturation buzz with the bottle cap at 2".

Setting the amp output offset I found that the best operation is between .6 and .7 volts above ground. Will try to refine this further. LP filters have been modified in the prototype to pass 1.25us and longer pulses but the jury is still out on the benefit of that with a minimum 8us sample delay. Probably not worth doing.


Regards,

Dan

[6666]

Dan
does your chance make this melody of tones like in this video ?
http://www.youtube.com/watch?v=0qhxWWa9SVQ

[baum7154]

A very nicely built detector.

Does mine make similar melodies?
Yes it does and with DISCRIMINATION SCALE indications similar to that as well. It is difficult to determine just how close he is to the coil with his targets but it looks like he is within 2 or 3 inches and that is not like mine. If you watch the SIGNAL STRENGTH indicator it never goes above 7 units. Mine goes full scale and into saturation buzz with that large of targets at that distance. There is no indication of how it is set up but it looks like BARRIER is set to about '4' as there is visual indication of 'idle state chatter' with no audio chatter, no indication of GUARD INTERVAL or MASK or any other settings.

I almost never use such large targets as I am optimizing for small gold so I use 1/4", 3/8", 1/2" square pieces of aluminum can and sub gram 14 Kt gold and dental gold pieces in my testing. For iron discrimination I use a 'crown' bottle cap, the sawn off head of a 7/16" hex head bolt, and a 3/8" square nut. I also use a sterling silver ring, a US copper penny, a US silver dime, a US nickel, and 1/8" solid aluminum cylinders calculated to simulate small gold nuggets in a very compact volumes.

When I am testing gold and aluminum targets it is low tones. When testing a bottle cap with my 8" coil it starts out a with a low tone at 10" and goes to a melodious tone with high pitched components at somewhere around 6 to 7 inches and that is the audio clue that it is not gold or aluminum. At the same time as the high pitch tones begin the VDI scale begins to jump from the left end to the right end, another clue that it is not gold/aluminum.

Hope this answers your question.

Regards,

Dan

[smity]

Please make a video of your Chance (with new, double preamp). Particularly interested in the range of copper and gold.

[baum7154]

Once I get the latest tweaks in my CHANCE I will be happy to but I may need some assistance as I have never posted a video of anything on line. I'd like to avoid the poor sound quality so many videos exhibit and have meaningful air tests of a standardized target or targets. What standard sizes of copper and gold/aluminum targets are you interested in?

Regards,

Dan

[smity]

Interest tests on copper, silver coins. Well as on small gold (no ring), earring, cross pendant (weight about 1 gram). Gold chain, probably sees chance?

[baum7154]

I have never tried to detect a gold chain with CHANCE. I know it to be a very difficult target unless there is a large piece of gold on it. I think the a small gold chain acts much like small gold placer flakes in a vial in that they all act as individual small pieces. Will try it though just to see what happens. I presume a 14kt gold chain will do.

Dan

[baum7154]

Just ran a 20" long, 1 gram, 14kt gold chain and got only sporadic beeps from the detector using my 8" fast coil with the detector set 'wide open'. While could always get beeps with the passage of the necklace it was never enough signal for true tone lockup. This was tested at a distance of 1 to 3".

Dan

[baum7154]

2 more gold chains tested, a 4 gram 1.1mm box chain and a 9 gram braided wire chain, both 14 kt gold. I changed the LP filter response to pass 1.88us and longer signals and both chains gave correlated beeps with the passage of the chain through the center of the coil 3" out. The clasps of both chains were kept out of the field. While there are always beeps with target passage there was not tone lockup. Another test with a 4.5 grain (<1/3 gram)14kt gold head meant for mounting a 4mm stone with no post or any other attachment gave repeated first indication of detection at 6". I think the 677khz LP filter I originally designed in the 2 stage amp is too high as there was noticeably more chatter…so I lowered it to about 530khz and it behaves much better. By the way the 220uf cap mod at the VREF out I have found to be unnecessary as it only degraded performance. More silver and copper coin tests later.

Regards,

Dan

[smity]

Dan, try to shoot a video. It will interest many.