Probe for Garrett Sea Hunter

[Carl-NC]

Where can I find the connector for the SandShark? Darren wants me to make up a probe for his.

Ah, now comes the hard part. On the SS, the connector and coil are a molded assembly. The easiest way to get a connector for the SS is to buy a coil, cut the cable off, and throw the coil away. Now you have a connector/cable assembly.

You would think that ordering a cable assembly from Tesoro would be easier. But, for some bizarre reason, Tesoro refuses to sell the cable assembly. OK, you could also discover that the cable is manufactured by Methode. But it is a custom cable made only for Tesoro, and Methode will not sell any unless Tesoro approves. For some bizarre reason, Tesoro refuses to approve.

I really like the Methode connector, but I'm thinking of switching my SS over to a Bulgin connector. I want to make some custom coils for it anyway.

- Carl

[rgecy]

OK. After talking with Carl we decided that a two coil solution may be needed with the Garret Sea Hunter MkII since there appears to be two coils in the 8" mono coil. Basically a TX and an RX.

Following guidelines provided earlier of 98 turns of awg30 I wasn't able to get a stable reading just using a single coil over the ferrite core.

What I did, after talking to Carl, is wind a second coil of 98 turns over the first and connected them to the TX and RX pins on the unit. And bingo, I am getting a fairly good response.

But I guess the settling characteristics need improving since the tone will ring for a second or so after removing the coin. I am only getting two to three inches on a quarter, but I think once I get my LCR Meter and OScope, I should be able to resolve this and increase the distance.

I did not connect the desoldering braid yet. Should I connect it as a ground to the negative pins on both coils?

Thanks Carl and Joe for getting me on the right track. I am not done yet, so I am sure will still need you guys for additional input.

Robert in SC

[rgecy]

Carl,

Where can I find the connector for the SandShark? Darren wants me to make up a probe for his.

Thanks, and let me know how the coil works on the Garrett as well. I ordered the LCR Meter and should have it Friday.

Robert in SC

[Carl-NC]

I made a PI probe with a ferrite rod I had on hand. It was 3/16" x 1-3/8". I wound 102 turns of 28AWG, 2 layers, and got 300.3uH (my target was 300uH). Resistance ended up 7.1 ohms.

The PI circuit I happen to have on my bench is a very crude design that detects my gold ring at about 4-5 inches with an 8-inch coil. With the ferrite probe, it gets the ring at almost 3 inches, so that's not too bad. Next is to try it with my Sandshark. Actually, next is to get the Sandshark back together.

I kept track of inductance as I wound:

25T = 22.1uH
50T = 69.9uH
75T = 165.3uH
85T = 217.5uH
100T = 293.1uH

This comes out roughly to

L = 0.03 * T^2 (uH)

Your mileage will vary.

- Carl

[rgecy]

OK, I hooked the probe up at 1.9 ohms and it would hardly register an object and would only detect it after a second or two. And then would continue the tone for a few seconds after the object was removed. I removed about 5' of wire and got the resistance down to 1.3 ohms with coil and 6' RG8X cable. The same thing happens. It will only detect an object at about 1" and only after a second or two.

I added damping resistor and it just got worse.

I am going to rewind the coil. I think I may have rubbed some of the enamel coating off the wire and it could be grounding to the ferrite rod. I am going to insulate the rod this time and see if it makes a difference.

Any suggestions?

Robert in SC

[bbsailor]

PI Probe

RGECY,

Give it a try and see how it works as is. Then, if the low delay does not work just remove about 3 ft. This will make the inductance less and make the probe closer to the coil inductance. You would only need to put a series resistor on higher resistance coils like the Sand Shark.

To see how a damping resistor is connected to a coil, print out the Hammerhead schematic and look at R11 (the damping resistor). You probe is essentially another L1. R11 is on the circuit board. Your external switch box is just replacing the coil with the probe when you flip the switch.

Probes design is not too critical. My only problem came when I tried a probe with very high inductance on my PI that samples at minimum of 7.5uS. At the low end of the delay, the probe locked up. See my article about the coil locking up or stopping to respond to targets. Too much inductance would couse this but I think you are close enough to have it work as it is now. The simple fix is to remove coil turns until the probe works through the full range of delay.

I'm out for a week on vacation tonight.

Good luck.

bbsailor

[rgecy]

After widing the coil for the SeaHunter, using 14' of AWG30 magnet wire and 98 turns, I measured the resistance of the coil without the resistor and got a reading of 1.9 ohms. This is significantly higher than expected. Should I unwind some of the turns to get it down to say 1.3ohms.

With this, should I still use the 10 ohm 1 watt resistor? How will I be able to tell if I need the damping resistor? Do you have a diagram of how the two should be wired? One in series the other in parrallel?

Thanks,

Robert in SC

[bbsailor]

More PI Probe

RGECY,

Since the Tesoro coil is 11 ohms, you should plan to use the 10 ohm resistor in series with that probe coil.

Since the Garrett Sea Hunter coils are bewteen 1.18 ohms and 1.38 ohms, you should plan on making the probe have between 1.2 and 1.4 ohms of resistance. AWG 30 wire (purchase a 50 ft spool of Kynar insulated wire wrap from Radio Shack) has .1 ohms per foot resistance so your coil should contain about 15 ft of wire, 14 ft for the coil with 6" for each coil lead. Start out with this amount of wire and see if the detection range is acceptable. You should be able to make three probes with one spool of the Kynar insulated wire.

If you use thicker wire, you will be putting on many more turns to match the coil resistance but the inductance will probably be too high. So stay with AWG 30 wire to keep the probe small and thin. The AWG 30 Kynar wire is .019 Outside Diameter. You will be making a single layer of wire about 2" long in the center of the core to obtain a 1.4 ohm coil (107 turns on a .5" diameter rod core). You can go as low as 1.2 ohms with 90 turns.

There is a posibility that you may need to add a small damping resistor inside the probe housing (in parallel with the probe coil where the coil wires are soldered to the coaxial wire) if it does not work at the lower end of the delay range. (Assuming that your PI has an adjustable delay.) This is not the same as the series resistor mentioned before. This is because the probe inductance may be higher than the damping resistor in the PI control box is set for. So try adding values of about 5000 ohms, 4000 ohms, 3000 ohms, 2000 ohms to see which value allows you to operate over the full delay range. Pick the highest value that allows operation over the full delay range.

bbsailor

[bbsailor]

PI Probe

RGECY,

You do not need to buy a MOSFET. Go to RS and only purchase a 10 ohm metal film, 1 watt resistor. The resistor comes in a two pack. Where you solder the coil leads to the coax wire, just put a resistor in between the coax center conductor and one coil wire. This will raise the resistance of the probe to approximately match the resistance of the coil. You could even put it in the switch box.

The MOSFET is inside the PI control box and is what drives the coil. It will sometimes get hot when the coil resistance is too low. You do not need to open the control box and monitor the MOSFET heat if you just put the resistor in series with the probe coil. I was suggesting that you might not need the resistor if the MOSFET (that looks like the RS IRF510) is not getting hot without it. The MOSFET would be located near the coil connection on the circuit board and have three leads and a metal tab on top.

bbsailor

[rgecy]

Will this one at Radio Shack do?

http://www.radioshack.com/product/in...&tab=techSpecs

How is the MOSFET wired in?

Robert in SC

[rgecy]

BB,

Now you threw me another curve, whats a coil driver MOSFET and where do I get one?

You didn't mention this in your first post.

Robert in SC

[Buzz(ID)]

Probe

Hi BB
I rewired my probe for my GQ, I was only getting 3 inches before and now I get 5 inches. I used 22 AWG w/ insulation stranded wire 68 turns on 12.3 mm ferite rod 2 1/2 long .4 ohms and .7 ohms with the cord on it. Works a lot better, thanks BB for getting me started The probe was not made by Eric, it was some other brand I did mod on.

Buzz

[Carl-NC]

Robert,

My Infinium has the DD coil; it measures 310uH and 520uH. I suspect the 310uH is the TX coil and also should be the mono coil inductance. The SS is 360uH.

For a switchbox, I would go with one of those really small Radio Shack black plastic project boxes. Seal the lid with RTV before screwing it down. Maybe even fill it with RTV to weight it down, then it won't matter if the lid leaks.

- Carl

[bbsailor]

Coil Probe

Rgecy

Go to the link below and click on "coil parameters". Make your coil the same inductance which is usually more coil windings but less resistance than an air core coil . Then, add a resistor in series with the coil so the total probe resistance is about equal to the air core coil resistance. In most cases a 10 ohm 1 watt metal film resistor from Radio Shack will work fine. You can try it without the series resistor or a lower value, but monitor the heat on the coil driver MOSFET. If it is too hot to keep your finger on, you need to add or increase the series resistance.

http://geotech.thunting.com/cgi-bin/...&file=info.dat

Let us know if this helps. You should get a minimum of about 5" on the probe I described.

bbsailor

[rgecy]

Thanks guys for the replys. I ordered enough to build 3 probes since there was a minimum order on the ferrite probes. I finished my winding my probe last night and hope to have it working tomorrow. I hope it performs as well as you guys say it will. I have used the Fisher Pulse 8 probe and got very little distance on coins; maybe 2" at best. With that, two buddies of mine decided they would also like to have a probe for their unit if it performs as stated.

Carl, I think you know Darren in NC. He has a Tessoro Sand Shark.

Brad is over in Wilmington and he has a Garrett Infinium LS.

So what I need to know is what the induction of the coil would be for these units.

Also does anyone have any suggestions on building a waterproof switchbox?

Thanks in advance,

Robert in SC