Probe for Garrett Sea Hunter

[Carl-NC]

Try probing the leads of the coil, you should see a pulse that peaks at 100 volts or more. Try it without a damping resistor, you should see a lot of ringing right after the pulse. Try a 220-ohm damping resistor, the ringing should go away and you get a nice smooth decay. Go to 470-ohms and the decay should shorten, but still be smooth. At some point, as you increase the damping R, the post-pulse decay will start overshooting, and you've gone too far.

- Carl

[rgecy]

Carl,

OK. I now have my Oscilloscope and wanted to check out my probe. What exactly am I looking for? What kind of reading should I see?

How do I interpret what the Oscope is telling me? Damping resistor etc.?

By the way, thanks, I adjusted it and got a good reading on the 1V square wave.

Thanks again.

Robert in SC

[Qiaozhi]

Hi BB
If I knew the ratio of the metals I could probably make them, I have access to a small foundry. Is this a composition ceramic mix?

Buzz

Try here -> http://www.gredmann.com/div/divh.htm
Might be worth sending them an email.

[Buzz(ID)]

Here is a good link to learn about soft ferrites that are good for PI probes.
http://www.intalek.com/Index/Project.../MA012000a.pdf

I just wound a PI probe using a 8" long X .5" diameter ferrite material 43 from Surplus Sales of Nebraska. Save your money, it does not work for a PI probe. This rod is detected when I wave it near my 11" mono coil.... not good!

Ferrite material that that starts with the digit 3 indicated that the material is managanese zinc...good for a PI probe. Surplus Sales of Nebraska has a .281" diameter X 1.969" long rod, soft ferrite material 3C80 that works well.

Ferrite material that starts with the digit 4 indicated that the material is nickel zinc... not good for a PI probe.

Hopefully, this will save some curious PI probe experimenters from wasting their money.

Now, if I could find a 3C80 (or similar) ferrite rod .5" X 4", that would make a probe with a little more detecting range than the smaller rod mentioned above.

bbsailor

I guess I should have done a little research before I posted that last post.
You can buy manganese in rods, chips etc. As far a zinc I wonder if one can grind the two a make a compound with exopy as Carl was talking about?
Manganese is very falmeable so I guess the foundry bad idea is out unless you like rockets.


Buz

[Buzz(ID)]

Here is a good link to learn about soft ferrites that are good for PI probes.
http://www.intalek.com/Index/Project.../MA012000a.pdf

I just wound a PI probe using a 8" long X .5" diameter ferrite material 43 from Surplus Sales of Nebraska. Save your money, it does not work for a PI probe. This rod is detected when I wave it near my 11" mono coil.... not good!

Ferrite material that that starts with the digit 3 indicated that the material is managanese zinc...good for a PI probe. Surplus Sales of Nebraska has a .281" diameter X 1.969" long rod, soft ferrite material 3C80 that works well.

Ferrite material that starts with the digit 4 indicated that the material is nickel zinc... not good for a PI probe.

Hopefully, this will save some curious PI probe experimenters from wasting their money.

Now, if I could find a 3C80 (or similar) ferrite rod .5" X 4", that would make a probe with a little more detecting range than the smaller rod mentioned above.

bbsailor

Hi BB
If I knew the ratio of the metals I could probably make them, I have access to a small foundry. Is this a composition ceramic mix?

Buzz

[bbsailor]

PI Probe Material

Here is a good link to learn about soft ferrites that are good for PI probes.
http://www.intalek.com/Index/Project.../MA012000a.pdf

I just wound a PI probe using a 8" long X .5" diameter ferrite material 43 from Surplus Sales of Nebraska. Save your money, it does not work for a PI probe. This rod is detected when I wave it near my 11" mono coil.... not good!

Ferrite material that that starts with the digit 3 indicated that the material is managanese zinc...good for a PI probe. Surplus Sales of Nebraska has a .281" diameter X 1.969" long rod, soft ferrite material 3C80 that works well.

Ferrite material that starts with the digit 4 indicated that the material is nickel zinc... not good for a PI probe.

Hopefully, this will save some curious PI probe experimenters from wasting their money.

Now, if I could find a 3C80 (or similar) ferrite rod .5" X 4", that would make a probe with a little more detecting range than the smaller rod mentioned above.

bbsailor

[Carl-NC]

Yesterday I built a third probe with a 1/2" rod, about the same as Buzz. 64 turns for 300uH. Performance was about the same as my 3/16" probe.

"Also, is there a way to make the sides of the coil less sensitive and keep the tip sensitive?"

This has to do with the permeability of the ferrite rod. I suspect most ferrite rods are not particularly high quality. Also the length-to-diameter ratio affects field focus. So what you probably want are long skinny ferrite rods with high permeability. However, this might also reduce the end-field, I don't recall.

Which leads me to another thought... making the ferrite rod. Ferrite rods are just iron powder and epoxy. Would be nice to make the exact dimensions I want, namely to fit down the core of a fibreglass tent pole. Anyone know a source of powdered iron? Might be able to use black sand.

- Carl

[Buzz(ID)]

Thanks Robert , The ferrite rod 12.3 mm(approx 1/2 inch) only used one, the core was 2 1/2 inches long and double wraped the end with which for makes for a stronger field. The Goldquest. I need to get a LCR meter but on ohms it read .7 This was a mod on a probe I had . I was only getting 3 inches and now 5 inches.

Buzz

[rgecy]

Buzz,

Nice looking probe! What size ferrite cores did you use and how many? What detector is this for?

Robert in SC

[Buzz(ID)]

My Finished Probe

Here is my Finished Probe ,suspended in spray foam insulating sealant and wraped coil in rubber cable splicing form Radio Shack and used epoxy made by Super Glue called Plastic Fusion for the pipe end also heat shrink wires several times.The number of wire turns was 68 of 22 AWG and as for the pipe I used sch 1120 PVC 3/4"

Buzz

[rgecy]

The resistance across the TX and RX pins on the unit are 1k ohm. I dont have the ability to measure the pulse width yet.

Carl posted it as 750pps with a pulse width of 70us.

I have made several different coils, but didn't have the ability to measure the inductance until this weekend. All the coils I had wound were on the order of 450-580uH.

The latest one, I wound a coil over a single .281" ferrite core with 52 turns of AWG26 magnet wire. This gave me an inductance of 140uH with a resistance of .65 ohms. I then wound another coil that matched over the first. The results are the best yet, but still needs some work. I am picking up a quarter at 4", but I am getting a ringing noise when there is no object present.

Could this be a ground or shielding issue? I am just using twisted pair for bench testing.

Also, is there a way to make the sides of the coil less sensitive and keep the tip sensitive?

Robert in SC

[Carl-NC]

If a separate RX and TX coil are needed, I would try to make it like a stacked balanced coil.

For an ordinary PI, induction balance is not necessary. Just wrap the TX winding right on top of the RX.

- Carl

[bbsailor]

Probe Design Considerations

Robert, Carl,

Probes should be grounded to minimize noise. I use rubber grommets to slip over the desoldering brad which holds it tight and flat against the ferrite material. This makes a good ground connection. Just stretch a grommet to tightly wedge the ground and the start coil wire lead under the grommet.

Then, I use a piece of Scotch 24 wire mesh shield that I almost make one complete turn so as not to make a shorted turn. I solder the desoldering wick to one edge of the Scotch 24 wire mesh used as the outer shield, then take that lead back to the coax outer braid shield along with the one coil wire to make the ground connection. The other coil wire goes to the coax center conductor. When shielding a balanced coil probe coil, ground the outer RX and TX coil portions separately to the respectve coax grounds.

If a separate RX and TX coil are needed, I would try to make it like a stacked balanced coil. I would divide the ferrite rod into three winding sections by using a two thin nylon washers in the center sections with the rubber gromments on the ends. Drill a small hole in one nylon washer to hold the start winding lead. Then, I would try to make either a center TX coil with two out of phase RX coils or vica versa to see which one balances better. Just wind the same number of turns on both end coils that are out of phase. Add a few more turns on one outer coil and unwind a turn at a time to obtain a null balance. This can be done with a signal generator on the center TX coil and the scope looking at the outer RX coils to obtain the best null balance. AWG 30 Kynar wire wrap or Teflon insulated wire should work well for this application.

Here is where a larger .375" to .5 " diameter ferrite rod, 3" long or greater may be helpful. These larger ferrite rods should give you 5" to 6" range on coins. The smaller ferrite rods will typicall give 2" to 3" on coins, based on my early mono coil probe designs.

If someone measures the resistance of the damping resistors across the coil TX and RX coil connections and the TX pulse width, I can try to reverse engineer some recommended probe coil winding data.

bbsailor

[Carl-NC]

Yes, the SS coils are printed circuit coils. They might work with other detectors, but they generally don't perform as well as a regular wire coil. That's why I wanted to build some custom coils.

I've already swapped out the crystal in my SS to a higher frequency; this reduces the sample delay and improves performance on gold targets, about 2 inches on a man's gold band. I've suggested both of these improvements to Tesoro, but they don't seem to be interested.

One option on the Tesoro is to build a shaft-mount switch box... all the connectors on the switch box can be Bulgin or Switchcraft or whatever you like... then when you clip the connector off the coil, leave it a couple of feet and use that piece to feed the switch box... then add a new connector to the castrated coil. That way you don't lose the coil, and don't have to change the bulkhead connector.

Below is my latest probe winding... it uses a smaller ferrite rod, and has about 135 turns for 300uH. Diameter ended up larger because it has 5 layers of winding. What I would like to make is a skinny coil that fits in a hollow fibreglass rod, like is used in arrows. Then I could literally probe INTO the soil.

- Carl

[rgecy]

Yeah,

Darren is a Tesoro Dealer and they wouldn't even sell him one. He is willing to let me chop one for him. Is there any good data that can be gotten from the 8" coil?

Could it be adapted to another detector? I believe its a printed circuit board coil right?

Maybe someone could give Darren a few bucks for it to offset the cost.

Robert in SC