Building the Geotech Barracuda Rev A (from silverdog)

[azigta]

you need an oscilloscope! i think your problem is not the coil, not the dumping. i think is the delay. check values for next resistors to be the same like in schematic: r10,r11,r12, r15,r17,r21. how many cm you get for a coke can? and how many you get for something bigger?

WOW! I think you solved the problem!! Thank you so much! R15 was incorrect, it was a 34ohm resistor instead of a 15k one, I don't know what happened there, can't explain it, I always measure everything before soldering. I guess I didn't do it with this one! Wow... I soldered the correct resistor and without adjusting the delay pots appropiately (Which I don't know how to do at the moment) I get this results, very different from the last ones.

American coins:
1 penny-> 13cm
5 cents-> For some reason It doesn't detect it, I guess it has to do with adjusting the delay properly
quarter-> 17cm

Euro coins:
1cent-> 13cm
10cent-> 10cm (surprised me a bit, since it's bigger that the 1cent one, but also the magnet doesn't almost get it, so I guess it has a really low metallic properties, no idea what this coins are made of really, but 1cm and 5cm ones, the magnet catches them hard, 10cm ones, not at all)
5cm coin-> 16cm

Wow I really think you helped me solve the problem. I can't believe I had a wrong value resistor! such a big facepalm...

Now I need to shield the coil, since I unshielded it when I added more turns, (which now I think It wasn't necessary but I won't mod it anymore, I will just shield it as it is. ) I didn't want to shield it jet because I'm going to shield it with copper tape and I didn't want to buy it if I didn't get some good results first.

Also I need to know how to adjust the delay (both the minimum delay and the 10k pot) and what is its function exactly, I don't quite understand it well.

Thank you very much, this differs greatly from the previous measurings, also the beeping is nicer, I don't know how to describe it, but now it isn't as high pìtched as before and is more consistent. I will do more measurings when I have more coins, like 2€ and 50cent, Now I don't have any.

[6666]

R15 was incorrect, it was a 34ohm resistor instead of a 15k one

so were does that 34 ohm go ?

[azigta]

so were does that 34 ohm go ?

Nowhere, I just have lots of resistors, must have mixed a 34ohm resistor inside a 15kOhm resistor box, so when I picked a resistor from the 15kOhm box, I really picked a 34ohm one and installed it on R15 position.

edit: Is really a 33ohm resistor, checked on the pdf and there should be only one 33ohm resistor in the circuit, and is installed in its place, r37

[ionut_mtb]

Good job!

5 cent is not detected maybe because of its composition. people test machine performance with this coin because it's close in response to gold. it means that your delay can be the problem. For the best performance it needs to be adjusted as low as possible. you can adj the min delay trimmer (not the pot) until you get the best performance for nickel coin. try to turn both direction to see the change in detection depth. For best detection depth keep the threshold level until you can hear verry little chattering (low volume). When adjusting the min delay you will start hear a sound in the speaker, then stops, turn again, the sound will be longer (1-2sec) then stops. you need to find the max sensibility for your coil, that will be the minimal delay. If rotate to much the sound will stop and detector will stop responding. This minim delay is influenced also by the coil capacitance. the lower the cap lower the min delay. shield will decrease the depth of detection but is good if you hunt highly mineralized ground or saltwater beach.

You got good result with he 1cent beacuse is made from copper or zink if it's older.

I will put a video later with my baracuda.

[azigta]

When you say "you can adj the min delay trimmer (not the pot) until you get the best performance for nickel coin. try to turn both direction to see the change in detection depth." do I disconnect the delay pot when doing this? or do I set it at some value? since you tell me not to adjust the pot but the trimmer, I don't know if I have to just disconnect the pot or put it at 0R. Thanks for the detailed answer, I will do more tests

[Qiaozhi]

When you say "you can adj the min delay trimmer (not the pot) until you get the best performance for nickel coin. try to turn both direction to see the change in detection depth." do I disconnect the delay pot when doing this? or do I set it at some value? since you tell me not to adjust the pot but the trimmer, I don't know if I have to just disconnect the pot or put it at 0R. Thanks for the detailed answer, I will do more tests

Turn the delay pot to zero, and then adjust the minimum delay trimmer.

[azigta]

ok thanks I'll do that

[kt315]

5 cent is not detected maybe because of its composition.

5 cent is not detected because of stainless steel. stainless steel is different on composition and take full discrimination range (in therminology of VLF disc) - from 'black' ferrous to non-ferrous. what you see outside on the coin is only tiny layer of copper - all
modern coins are fully sht absent copper. copper is blood of war. its hard fact. for example chinese strives to use a copper
in very small amount if they can. i did meet a wire to solder iron, chinese made, that magnetics! all world is prepared
to the war (global conflicts/war) and you see only Consequence - cooper absence in the coins.

copper is a metal that used in shells like a band around, it gives a rotation of the shell slicing gun barrels.
no different metal you can use in the case, only clear cooper.

[ionut_mtb]

meanwhile Qiaozhi answered your question.


And also you need to check the best dumping resistor for your coil:

It can give you extra depth. Install this instead of dumping resistor and check the best depth you can get, then take disconnect the "tool" measure the resistance and replace it with a normal resistor with that value.

[azigta]

kt315, very interesting, metal detecting makes you learn lots of things, I'm sure I will stick to this hobby for a long time.

ionut_mtb: Do any of the resistors in the diagram have to be more than 1/4w? thanks

[ionut_mtb]

ionut_mtb: Do any of the resistors in the diagram have to be more than 1/4w? thanks

normal resistor, normal pot. just try what you have.

[azigta]

normal resistor, normal pot. just try what you have.

I didn't have much time today but I built the damping resistor tool, tomorrow I'll test it, for now, the ohm test is ok. While adjusting with the tool, what position does the 10k pot has to have? Do I set it to 0 and test with the 5 cent coin like before?

By the way, the test that I did before with the 5 cent coin gave me a best min delay value of 5.3K, note that I'm using a pot instead of a trimmer and as far as I know, that doesn't affect the circuit, but probablly I will change that pot for a trimmer the next time I go to the electronics store and buy one so that it isn't that sensible to changes.

I still can't believe it's working

[azigta]

wow! I tested with the damping tool (Previously removed the resistor that I soldered on the board of course) and look at the results

With the original 470ohm resistor:

(american coins)
1 cent -> 13cm medium strength
5 cent -> 6cm kind of weak

With the damping resistor tool adjusted to the best value, that turned out to be 1.614Kohm (I noted the value as accurate as possible with all the digits xD)

1 cent-> 22cm strong
5 cent -> 13cm strong

More resistance on the damping tool made the tone more weak at the same distance, less resistance also made it more weak, it seems like the best value is around 1.6K

Wow, I'm shocked by the great result. I wasn't expecting to increase the detection depth that much, I just went from an almost non working metal detector to detect a small 1 cent coin at 22cm, I have to test it with the euro coins to see what I will get, but I'll do that once I put the fixed resistor. I will post more details when I have it done.

In the pdf it says that the damping resistor has to be 2 watts, but the tool was made of 1/4w resistors, and it worked... Could I place a fixed resistor of 1/4w? That will make it easier for me, as I don't have 2w resistors.

Thank you all for the great support!

[WM6]

Wow, I'm shocked by the great result. I wasn't expecting to increase the detection depth that much, I just went from an almost non working metal detector to detect a small 1 cent coin at 22cm, I have to test it with the euro coins to see what I will get, but I'll do that once I put the fixed resistor. I will post more details when I have it done.

Your test/settings was done in air, which can lead to somewhat deceptive results, by following this settings method.

For real test and proper damping R settings, testing target should be "harmonized" with soil environment:

Here proposal for very simple an practically to soil-tests adequate test, by boring
hole in soil, using some sort of metallic drill rod (p.e. from construction iron bars)
in diameter a little bigger than test coins diameter:



How to fix coin orientation on string:

[azigta]

WM6, thanks for the tip, I will do ground tests when I have the coil shielded and covered with silicone and mounted on the shaft etc. so that I can take it to the outside. Right now I don't have the posibility to do it, but I will do tests with the damping resistor tool doing ground tests too, now that I know this, but I don't think It will vary much, because other value than 1.6K in the damping tool made the air tests poor in comparison. I will perform ground tests too anyway. Thanks