Surf PI - successful build

[Old cart]

Thanks Polymer. This may be a little to conductive. Sean, it is quiet but did it affect depth?

[Polymer]

A quick note for coil shielding, Try RS nicked spray. it works a treat! Don't forget to leave a small gap in the circle of you will get naff all depth.

THis stuff has 0.7Ohm/cm resistance and makes for a mind blowingly quiet coil on wet sand. You CAN do the top and bottom halves of the coil but you mush be VERY careful that the gaps line up so not to make a shorted turn.

Have fun folks!

WOW!
Although I have no experience in shielding yet, I have been looking into possible solutions finding a myriad of them, dreading the decision of where to begin.
Your experience with nickel shielding appears counterintuitive, because nickel is ferromagnetic and I thought that would be a no-no.
Perhaps the thin layer & low conductivity of nickel, like gold (metal detector wise) mitigates this? Just pondering ...
Anyway, I find this great.

I also found copper based sprays (which are more common) and am wondering if they have the same great effectivity/ease of handling. Have you tried them?

Thank You for sharing this info!

@Old cart: http://uk.rs-online.com/web/p/shield...osols/0568483/

[Old cart]

Sean, where do you get that spray?

[Sean_Goddard]

A quick note for coil shielding, Try RS nicked spray. it works a treat! Don't forget to leave a small gap in the circle of you will get naff all depth.

THis stuff has 0.7Ohm/cm resistance and makes for a mind blowingly quiet coil on wet sand. You CAN do the top and bottom halves of the coil but you mush be VERY careful that the gaps line up so not to make a shorted turn.

Have fun folks!

[chrisw]

I decided that if a resistor is too hot for my fingers then it is too hot to be sealed in my box.

So now I have replaced the 2W resistor with a 3W ceramic resistor. I could only get this locally if I had a 3.9 ohm instead of 3.3 ohm, but I dont think this would make much difference? Please correct me if im wrong.

Maybe this is a bit of overkill, but it is only mildly warm now, feels much better. The power transistor is lukewarm also. And performance wise I am still getting 19 inches on my £2 coin so I cant tell any difference.

[chrisw]

I did all this when I set it up but the resistor is still too hot to keep my finger on it. Maybe it shoudl get hot; it is a 2W resistor so should be able to manage the heat. I checked the pulsewidth again yesterday and it is bang on 120uS. I was just a little worried about this heat buiding up in the sealed box.

[Qiaozhi]

There should be less than 1W dissipated in the 3R3 resistor, but has been rated at 2W for reliability reasons.
I've found my lab book containing the Crossbow notes, and here's how I calibrated the detector:

1) Adjust TX frequency (R44) to 1kHz.

2) Adjust TX pulsewidth (R34) to give a quiescent current of 200mA. i.e. Connect the coil, and put an ammeter in line with the battery during the adjustment. This should result in a TX pulsewidth of approximately 120us.

3) Disconnect coil, and set the voltage at the preamp output to 0V by adjusting the null offset pot (R2.

4) Set Sensitivity preset (R6) to mid-position.

5) Set the minimum delay setting (R18 ) to 20us (or whatever is suitable for your fastest coil) with the Delay pot at minimum.

6) Set secondary sample pulse (R19) to 400us.

[chrisw]

Can I bother George again with another question;

On my crossbow board my R24 (3.3 ohm in series with the coil) is now changed to a 2W resistor, and it gets that hot I cannot keep my finger on it. Has it been designed this way or not?

I notice that the image of this resistor on the pcb looks bigger than all the other resistors on the board. So what size was it originally designed for? With all this heat I dont know if this is good to keep inside a sealed box. Or is there a problem with my build causing this?

Chris

[chrisw]

I had scraped it to see if there was copper underneath, and I coudnt see copper. But after you posted this message I tried again and scraped harder and it is tinned copper. This also has a layer of foil as well. I dont know what the impedence is. It is a linking tv aerial cable.
So I have a choice now, Im not sure which would be better.

[SVEN1]

Thanks George.
I have two types on is a bit thinner and is steel braided and steel cored, or I have some 75 ohm which is all copper, but the overall diameter is a bit bigger.
I was wondering if I could use the steel, but I thought connecting steel to copper wasnt very good as you get some electrolytic action because of the two dissimilar metals. So I will maybe give the 75 ohm a go.

Is the steel coax actually steel or just tinned copper?

[chrisw]

Thanks George.
I have two types on is a bit thinner and is steel braided and steel cored, or I have some 75 ohm which is all copper, but the overall diameter is a bit bigger.
I was wondering if I could use the steel, but I thought connecting steel to copper wasnt very good as you get some electrolytic action because of the two dissimilar metals. So I will maybe give the 75 ohm a go.

[Qiaozhi]

Hey George,
What coax did you use on the original crossbow?

It was nothing special: URM43 black PVC coaxial. Farnell (Element 14) 665-5858
It's basically 50ohm coax suitable for CATV/MATV, broadband, HDTV and satellite broadcasting transmission.
Probably not much good if you're trying to produce a fast coil.

[chrisw]

Hey George,
What coax did you use on the original crossbow?

[chrisw]

I will wait and see if Sven has the same problem. But I think I will just leave it out of the board. Im using a 12v Lithium battery anyway not the 10x1.2v as in the original.

[chrisw]

Yes I just checked it.