TGSL troubleshooting questions

[der_fisherman]

Loops in shield wiring - preventing them!

Andy, I can't believe your are from Germany. Your English is too good! I have thought about using microphone cable or audio cable for some time as others have. Can you do a quick schematic on how you run your wires? Maybe you already posted elsewhere.

I think that coil wiring sometimes comes down to availability and cost. USB cables can be bought almost anywhere and they are cheap! I only figured out how to get it to work with the TGSL. I would be using Belden otherwise for sure.

I like your idea and may build my next ISGL that way.

Thanks.

Don

It depends upon the cable type used to a great degree, but a shield may only have one connection to the ground plane.......many talk about "star" connecting.
Although I have never used USB cables personally, I have nothing against them, except that until one cuts it open, you don't know exactly how much was saved in very thin wiring.....I personally hate thin wiring as it presents an extra resistance.

I am sure that a good quality USB cable would not suffer from that......

I have added a couple of simple drawings showing the connection with twinCore cable and with Dual Microphone cable where each core is shielded, my preference....no loops....

I tend to bring all connections separately into the electronics box as I can both best measure and see that all is ok.

A typical mistake is to cause a short between the Rx and Tx Faraday's shield where they cross one another, make sure a good piece of insulation is placed in these 2 areas......(in most VLF designs that I have seen....). To stop a loop, you need to not use one of the shields, us just one (assuming its a low Ohm short....)

Regards

Andy

PS. If anyone has a sensitivity or ANY problem with any MD they built or bought and they know that the coil's cables Faraday shields or cable shields may form a loop somewhere, it would be interesting to hear from you.......and if you can fix all problems......finding the loop after building is difficult or impossible......unless you can separate everything at the electronics again.....

[dfbowers]

Don's advice is good to follow, but he neglected to fully talk about forming loops with the shield and its wiring, he knows ll about connecting with USB cable, but many use other types.........so I would just like to add the following comments:-

If you are using shielded or coax type cables, it's important that you don't make any "loops"...sometimes referred to as "ground" loops.

I in fact always use twin core cable, each core with its own shield, it's quality microphone cable, e.g. for two coils, I have 4 cores and 4 screens, 2 cores and two screens as one cable, therefor two cables per search head.

I connect two screens from one cable (for one coil) together and also to the Faraday shield of one coil.

I do not connect both Tx & Rx coils screen's together in the search coil.

I connect ONE screen (not from both cores on one coil), from each coil together in the electronics box....not both as that would make a ground loop on each coil.......

Although the working frequencies for VLF Detectors are low, I personally would never risk a loop.

If you are using a cable with two cores and a single shield, do not connect the shields both at the bottom and top, I personally prefer at the top in the electronics box, but then they MUST be insulated from each other in the search head......

I have no proof that a (ground) loop is bad in a metal detector, but I wonder if anyone reading here has made a loop and had sensitivity problems.....its possible.....it's certainly not recommended when sending low level signals over wires......

Ripping apart a well made search coil assembly to separate/correct shielding errors (and finding out that was not the real problem maybe!) is downright disheartening.......

Check between the shield for one coil to the shield of the other coil with an Ohmmeter to make sure that no shorts are apparent between the Tx & Rx coils.....before "potting".......Check afterwards too!!!

I hope I wrote this clearly enough, if not, just complain and I will try and do better......Any help from anyone else who understands about such loops would be greatly appreciated.....

Regards

Andy

PS. Anyone using this info for the IGSL Search Coil Head needs to be most careful when attaching the shield to one side of the Tx coil, you may cause a bad problem depending upon how it finally ends up. If phasing is incorrect, swap only the RX leads to the PCB, not the Tx leads.......I personally will shortly connect my IGSL to the coils, and I will connect the Faraday shield to battery negative, not to either side of the coil.......be careful!!

Andy, I can't believe your are from Germany. Your English is too good! I have thought about using microphone cable or audio cable for some time as others have. Can you do a quick schematic on how you run your wires? Maybe you already posted elsewhere.

I think that coil wiring sometimes comes down to availability and cost. USB cables can be bought almost anywhere and they are cheap! I only figured out how to get it to work with the TGSL. I would be using Belden otherwise for sure.

I like your idea and may build my next ISGL that way.

Thanks.

Don

[der_fisherman]

Faraday Shield loops.....

HI every body

i don't understand about Faraday shield over coil

when finished the coil " Coil will have, than, 3 wires: start, end and Al shield (Faraday cage). "

the proplem is the other Al SHIELD !!

http://imageshack.us/photo/my-images/195/sansrejc.jpg/

Don's advice is good to follow, but he neglected to fully talk about forming loops with the shield and its wiring, he knows ll about connecting with USB cable, but many use other types.........so I would just like to add the following comments:-

If you are using shielded or coax type cables, it's important that you don't make any "loops"...sometimes referred to as "ground" loops.

I in fact always use twin core cable, each core with its own shield, it's quality microphone cable, e.g. for two coils, I have 4 cores and 4 screens, 2 cores and two screens as one cable, therefor two cables per search head.

I connect two screens from one cable (for one coil) together and also to the Faraday shield of one coil.

I do not connect both Tx & Rx coils screen's together in the search coil.

I connect ONE screen (not from both cores on one coil), from each coil together in the electronics box....not both as that would make a ground loop on each coil.......

Although the working frequencies for VLF Detectors are low, I personally would never risk a loop.

If you are using a cable with two cores and a single shield, do not connect the shields both at the bottom and top, I personally prefer at the top in the electronics box, but then they MUST be insulated from each other in the search head......

I have no proof that a (ground) loop is bad in a metal detector, but I wonder if anyone reading here has made a loop and had sensitivity problems.....its possible.....it's certainly not recommended when sending low level signals over wires......

Ripping apart a well made search coil assembly to separate/correct shielding errors (and finding out that was not the real problem maybe!) is downright disheartening.......

Check between the shield for one coil to the shield of the other coil with an Ohmmeter to make sure that no shorts are apparent between the Tx & Rx coils.....before "potting".......Check afterwards too!!!

I hope I wrote this clearly enough, if not, just complain and I will try and do better......Any help from anyone else who understands about such loops would be greatly appreciated.....

Regards

Andy

PS. Anyone using this info for the IGSL Search Coil Head needs to be most careful when attaching the shield to one side of the Tx coil, you may cause a bad problem depending upon how it finally ends up. If phasing is incorrect, swap only the RX leads to the PCB, not the Tx leads.......I personally will shortly connect my IGSL to the coils, and I will connect the Faraday shield to battery negative, not to either side of the coil.......be careful!!

[dfbowers]

HI every body

i don't understand about Faraday shield over coil

when finished the coil " Coil will have, than, 3 wires: start, end and Al shield (Faraday cage). "

the proplem is the other Al SHIELD !!

http://imageshack.us/photo/my-images/195/sansrejc.jpg/

When you are finished you will have 2 coils
Coil #1 - Tx start
- Tx finish
- Ground wire wound around the Tx shield with just one loose end
Coil #2 - Rx start
- Rx finish
- Ground wire wound around the Rx shield with just one loose end

Make sure that shields have a gap, not a complete circle!
Make sure that shields do not touch each other (insulated)!
Now you have a total of 6 wires.

Tie ground wires together. These get connected to the cable ground wire.
You have at least two choices on how to run your wires. I prefer the bottom arrangement.

Does this answer your question?

Don

[Merc]

If you are getting oscillations after a target is detected, you should check your power supply grounds carefully. Be sure that the negative side of your speaker or headphones is returning to ground very close to negative battery connection on PCB. The speaker and headphone ground path should be as short and direct to battery minus terminal as possible.

Merc

[M.T]

HI every body

i don't understand about Faraday shield over coil

when finished the coil " Coil will have, than, 3 wires: start, end and Al shield (Faraday cage). "

the proplem is the other Al SHIELD !!

http://imageshack.us/photo/my-images/195/sansrejc.jpg/

[simonbaker]

hi
for (TR3) BD140 i found BC141 are this ok ?????

and for (R45) 27K can i use 30K

tnx

I would say yes to both of those questions.

-SB

[M.T]

2n2907 is not critical probably, try any general purpose small signal PNP transistor.

You can try other N-Channel JFets instead of J107. They are just used as switches basically. Try to pick one with Base-source cutoff voltage approx around -3 to -4 volts.

For other parts, try combining caps, and resistors.

-SB

hi
for (TR3) BD140 i found BC141 are this ok ?????

and for (R45) 27K can i use 30K

tnx

[simonbaker]

HI EVERYBODY

i don't found this cuirciuts i want to know the remplacement

transistors : 2N2907 and BD140 AND J107

capasitor: 15NF/50V (film)

and this resestonce : 470 k , 1m , 27k (can i use 30k ?),

my regards

2n2907 is not critical probably, try any general purpose small signal PNP transistor.

You can try other N-Channel JFets instead of J107. They are just used as switches basically. Try to pick one with Base-source cutoff voltage approx around -3 to -4 volts.

For other parts, try combining caps, and resistors.

-SB

[M.T]

HI EVERYBODY

i don't found this cuirciuts i want to know the remplacement

transistors : 2N2907 and BD140 AND J107

capasitor: 15NF/50V (film)

and this resestonce : 470 k , 1m , 27k (can i use 30k ?),

my regards

[der_fisherman]

Do you use a 7808 or a 78L08?

I believe the latter, it can only supply a max of 100mA and will need a cooling tab to stop it overheating and starting to shut down.

If you have a 1amp 7808 (no "L" in the middle) it should never overheat with the current you tell us you are measuring, but still may need a cooling tab in a fully enclosed box with no ventilation holes......possibly anyway....

You must identify the device exactly.....use a magnifying glass.

I know you posted some time ago, but I did not see any other posts in a similar vein.....I hope this helps.

Regards

Andy

[Jerry]

Went I built the TGSL I got around the problem of speaker location by doing away with the speaker completely. Since I never hunt without headphones on, it is not missed and saves some weight at the same time. No circuit modifications, just cut the wires that went to the speaker and ran them to a headphone jack. For testing purposes, I plugged in a speaker but with a long enough cord so it was well away from the detector.

Jerry

[simonbaker]

It makes double beep only when the target is realy close to the coil.
The disc leads are ok.
The coils are still lose for testing.
I'm using usb 2 cable.
Circuit is TGLS EDU
Yes, the sensitivity affects the beeping, increasing the sensibility will triger the beeping, it is trigered and stoped also by how the coils are aranged one over the other.

I'm only using a speaker, is it about 5-10 cm away from the circuit bord.
I do have a power transformer near and a mobile phone antena at about 200 m.

Loose coils are very sensitive to vibrations, especially from the cable, so try to fix them down as best as possible.

It sounds like your circuit is actually working fairly well - it may require a scope to determine how to get more improvements and fix small problems.

If you have an AC voltmeter rated to about 20 kHz frequency, I would recommend you make a "glued, shielded, and potted" coil, first using the AC voltmeter to null it. There is a lot of advice on nulling the coils on this forum, in addition to Qiaozhi's suggestion.

(My advice is to put the voltmeter on the output of LF353 pin 7, and move coils together until minimum voltage, then move a tiny more overlap so voltage rises 20% above minimum).

After making potted coils, take your circuit outdoors away from power lines (or deep in a basement) and test again. I would use headphones instead of speaker, or put speaker on longer wires far from PCB and coil as a test.

If you have "dfbowers" mod in your sensitivity circuit, then turning pot to max sensitivity can make beeping, that is normal.

-SB

[Qiaozhi]

It makes double beep only when the target is realy close to the coil.
The disc leads are ok.
The coils are still lose for testing.
I'm using usb 2 cable.
Circuit is TGLS EDU
Yes, the sensitivity affects the beeping, increasing the sensibility will triger the beeping, it is trigered and stoped also by how the coils are aranged one over the other.

I'm only using a speaker, is it about 5-10 cm away from the circuit bord.
I do have a power transformer near and a mobile phone antena at about 200 m.

I would suspect that your coil is not balanced correctly.
With DISC off, and GEB set to middle position, adjust the coil overlap until a piece of ferrite is rejected. This is probably the simplest way to achieve correct balance without a scope.

[soverey]

There are many things to check.

Do you have RX coil leads reversed? (makes double-beep with target if you do).

Do you have DISC pot leads reversed? (will work backwards, but OK).

Have you mounted and fixed your coils, or are they still loose for testing?

What are you using for coil cable? Sometimes certain cables very touchy, cause beeps easily, or connectors are noisy.

Which exact circuit did you build (there are several versions)?

Does your sensitivity pot affect the beeping?

Is the beeping the same with headphones and speaker?

Is the speaker or headphones far enough away from circuit board?

Do you have power lines nearby or other strong EMI sources nearby?

Regards,

-SB

It makes double beep only when the target is realy close to the coil.
The disc leads are ok.
The coils are still lose for testing.
I'm using usb 2 cable.
Circuit is TGLS EDU
Yes, the sensitivity affects the beeping, increasing the sensibility will triger the beeping, it is trigered and stoped also by how the coils are aranged one over the other.

I'm only using a speaker, is it about 5-10 cm away from the circuit bord.
I do have a power transformer near and a mobile phone antena at about 200 m.