как раз в последнее время
делаю металлодетектор на основе части схемы Classic III (режим пин поинт).
Есть интересные результаты :

Если подать сигнал RX со сдвигом 90 градусов относительно сигнала TX
то режим пин поинт работает не так,как в оригинальном Classic III. 
Оригинальный режим - на все металлы увеличивается громкость сигнала.
Мой режим - на цветняк увеличивается ,на черный уменьшается.
Чувствительность очень хорошая.

 For a start you can look up White's patent # 4293816 on the US Patent Office web site, http://www.uspto.gov 

This may not exactly match any of the coils they are selling today but it gives you some basic information that would be helpful in building your own.

There is a capacitor across the transmit winding that tunes it to 6592 Hz.

Robert
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what connector type White's uses
It's a SWITCHCRAFT SL405F

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Fisher 1210
Rx 6,48mH / 15,3 Ohm
Tx (1,64mH / 7,7Ohm) + (1,62mH / 7,5Ohm)

Fisher 1266
Rx 10-10,4mH / 38,2-39,3 Ohm
Tx 2,14-2,2mH / 4,7-4,9 Ohm

Fisher Quick Silver
Rx 6,69-6,7mH / 31-32Ohm
Tx 0,945-0,960mH / 3-3,1Ohm

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Winding, followed by pins (pos,neg), followed by inductance, and finally DC resistance.

White's Spectrum XLT 9.5"
Rx 1,2 36mH 123 ohms
Tx 4,5 540uH 1.4 ohms

White's 6B
Rx 1,2 16mH 35 ohms
Tx 4,5 1.7mH 7 ohms

White's DFX concentric
Rx 1,2 13.8mH 36 ohms
Tx 4,5 540uH 2.7 ohms

White's DFX DD
Rx 1,2 14.2mH 55 ohms
Tx 4,5 680uH 3.5 ohms

Tesoro concentric
Rx 3,4 6.2mH 22 ohms
Tx 1,5 5.7mH 22 ohms

Tesoro early Lobo DD
Rx 1,3 1.35mH 12 ohms
Tx 2,4 10mH 55 ohms

Fisher CZ
Rx 1,2 6.8mH 30 ohms
Tx 3,4 1mH 2.8 ohms

Most numbers are based on several coil measurements, including different size coils for a given coil type.

!!! - The White's 6B coils are really easy to find, and come in a huge range of sizes.

silver sabre - I found in a dutch book of metaldetectors : tx 100 turn of 0.5mm copperwire and a diameter of 200 mm and rx 170 turns of 0.5mm copperwire, diameter 80mm


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Re: Silver Sabre circuit ***IMPROVEMENTS***
Posted by Sean Goddard 
Wednesday 10 April 2002, 11:33:00
In Reply to Re: silver saber circuit - Posted by Carl on 4/04/02

You could try different Op-Amps in the SS circuit, but if you look LONG and HARD at the existing ones, you wont find any which are better them in the areas which matter most.

You COULD try ones like the National Semiconductor LF353N (MUST be the N, as it's an "improved" version of the standard LF353.

I believe there's a newer version of the '442 as well which has better a current noise figure.

The "bottom" signal path has a constant retune and Isuspect is the "motion" path. Changing the time constants of the two "auto balancing" circuits i.e. the LF442 and the LM358 blocks, simply involves changing BOTH the 330K resistors.

You could try starting at 470K. This will "theoretically" give you better sensitivity to small items. Too high a value, and you will "smear" the motion signal.

Also, changing the value of the two 150K resistors on the "front end" LF353 to 330K will give a marginal improvement in depth and sensitivity, but too much will make the GB setting a LOT more critical, as well as making the detector VERY unstable.

Replacing the TIS75 VERY CRUDE synchronous demodulators with REAL "full time" demodulator circuits which also sample the negative portion of the target signal, will SIGNIFICANTLY reduce the system noise levels and drift, and make a much more stable machine.

Other methods are to amplify the incoming signal, and phase shift the ground and discriminate signals DIRECTLY, this means that you could dispense with the "wandering" Colpitts type TX oscillator in the top lefthand corner, and drive the thing with a crystal (VERY STABLE) one. Using a buffered take off of the coild drive signal for the Demodulator switching signal also reduces noise as it eliminates clock "jitter".

Finally, use "metal oxide" resistors THROUGHOUT, they're a LOT quieter than the cheap carbon ones used by Tesoro, and use OFC cable to do ALL the link-up. Keep ALL cables as SHORT as possible, and use screened cable if you can, only connect ONE END of the screen when running a single wire to controls, this will avoid creating multiple paths for any noise picked up on the screening, also connecting the screens at ONE COMMON point in a "star" ground will also help.

I could list over 30 simple and immediate improvements which took me just over two hours to come up with, but Tesoro aren't going to pay me for them.

Hope someone finds this info useful. ;)
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Sorry I don't have time right now to go into all the details of the improvements which have been suggested. Most of them will not improve anything, or will screw up things that are already good compromises.

The Silver Sabre circuit is one of the most successful beeper platforms in this industry. I think it dates to about 1986, and products which are very nearly the same thing are being manufactured today. Admittedly it is not particularly sensitive, but it is easy to build with readily available parts, and has an excellent discriminator. 

I recommend building it just the way you see it on the schematic, using a purchased Tesoro searchcoil. If you happen to know how, you might want to substitute CMOS switches for the JFETS, which however is not likely to improve performance. If you're using a Tesoro coil of the specified type, I would discourage you from attempting to operate it at a different frequency or using a crystal controlled transmitter: although conceptually easy, there are many potential pitfalls in doing such things, which are not obvious. 

Attempts to increase the sensitivity of this circuit are likely to either fail, result in only slight improvement, or cause other problems (which may not be obvious at first). If a person is striving for high sensitivity, the Silver Sabre is not a very good foundation for such a quest.

If, once you've gotten the circuit to work satisfactorily, you want to play around with it, the easiest thing to do is add simple "features", for instance an external ground balance knob and a switch to disable discrimination.

--Dave J.





