Shielding an IB metal detector's coil

[Hash]

Hello all, I'm relatively new to metal detecting and that type of stuff so I wanted to start by building a simple metal detector. I've found an induction balance design in a magazine (everyday practical electronics, 10/2002) and it seems quite simple to make but I'm stuck with the coils, the writer suggests a 'Faraday shield' made from a long strip of aluminium tape wrapped around each of the coils, keeping 1cm between the ends.
Then comes the part that makes no sense: I'm supposed to solder a 10cm bared wire to the transmitting coil's end and wrap it "AROUND THE INSULATING TAPE (that keeps the coil wrapped-up thightly, beneath the layer of shielding), WHICH PROVIDES AN ELECTRICAL CONTACT FOR THE FARADAY SHIELD" - is this a typo or am I really supposed to wrap the wire around the 1cm unshielded area? Also, this procedure is repeated for the receiving coil's beginning.
Hope you guys can clear it out for me... thanks!

[Reg]

Hi Hash,

I haven't read the article but I will try to answer your question. The use of aluminum foil is somewhat common as a faraday shield. Unfortunately, one can't solder to the aluminum foil, so a different means of electrical connection is needed. So, the idea of using a piece of bare copper wire to make the connection makes sense. Now, if you decide to use aluminum foil, then wrap the foil around each winding, leaving a small gap in the foil around the circumference. In other words, foil will be wrapped around the winding almost all the way around the circumference, but not quite. Just leave a small gap, maybe 1/4" to 1/2". Next, take the bare wire you have soldered to the coil lead and wrap it around the aluminum foil tightly so it makes a contact to the foil. Then tape this combination tightly. Do not wrap this wire such that it spans the open gap. Just wrap the wire around one side of the gap to make the connection.

As for the foil, you might want to look at the pic I posted on another thread showing a basic pic of the gap. Here is a link to that pic.

http://thunting.com/geotech/forums/s...ad.php?t=11530

I displayed a little more than the desired gap in the pic but that was for explanation purposes so it would make sense.

Now, aluminum foil is not my favorite choice for shielding but it is cheap and readily available. Other options are much harder to obtain. I am hoping to find some reflective mylar that is conductive on one side. That would work very well also. The trick is to find some that is strong enough to use.

I prefer certain copper or silver plated tapes, but they are expensive and hard to find in small quantities.

Reg

[Richard Greswell]

Soldering the aluminion strap.

Yes you can tin solder a wire to the aluminium strap. Solder iron must be very hot. Use a bit ofd solder paste and go do some trying. Youl´ll find that this is possible.
Cheers
Richard

[Carl-NC]

Yes you can tin solder a wire to the aluminium strap. Solder iron must be very hot. Use a bit ofd solder paste and go do some trying. Youl´ll find that this is possible.
Cheers
Richard

I've done this, and even describe it in my HH article, but I found that after a while the solder joint comes loose from the aluminum.

[diminute]

Un método que uso

Disculpen el español... talvés alguno de ustedes pueda traducir el mensaje.

Cuando deseo soldar algo que no coge estaño, pues uso una barra del estaño más fino que tenga... un polo de la fuente estabilizada lo pongo a la pieza... (~ 20 V y 2 0 3 Amperes) luego sujeto una pequeña porción del estaño fino con la boca de caiman del otro polo de la fuente y hago pequeños corto circuitos con el estaño y el material que deseo soldar... esto crea ciertas zonas donde el estaño penetra y luego se puede soldar sobre ellas... he usado este método para soldar baterias (AA) con terminales de niquel con buenos resultados, pienso que en el aluminio puede dar resultados también. Descubrí este método simulando una soldadura por arco electrico, pero con una barra fina de estaño como electrodo.

Otra método es usar Ácido Fosfórico para limpiar la superficie... esto crea un oxido que coge estaño... lo he usado para soldar alambres de nicron para hacer resistores de pequeña impedancia (0.25 a 1.0 ohms).

Saludos

diminute

[Qiaozhi]

I've done this, and even describe it in my HH article, but I found that after a while the solder joint comes loose from the aluminum.

You need to use aluminium (English spelling) solder. My 12" Garrett coil recently became unstable and there was a similar problem. The outer TX coil is made of aluminium wire, but the receiver and bucking coils are made of copper. It's just impossible to get a good connection without the correct solder.