PICKINI - an easy to build PI detector

[Dixix]

Hello Bernard,
I have tested several coils on your PICKINIs in recent days. I have found that it is very important to use a good cable to the coil.
Shielded data cable: bad. Microphone Cable - Bad.

I have the basket - coil on the version 2 with RG58 coaxial cable for best results. That is strange. The shield (Graphite on hospital - tape) I simply clamped to the coil. Not on Ground (-). Because coaxial cable has only 2 conductors. But I will not make further tests.
Dirk

PS: I have now found a different place in my apartment where I can test the coils. A place with less interference. I put the coils on a large exercise - ball

[Qiaozhi]

The shield (Graphite on hospital - tape) I simply clamped to the coil. Not on Ground (-). Because coaxial cable has only 2 conductors. But I will not make further tests.

The shield must be connected to ground in order to be useful. If it is just floating, you might as well remove it.

[Dixix]

Please excuse my stupidity. I have not understood it. In the detector book I'm reading, I should preferably be as RG58 coax cable coil use. (Page 191) RG58 but has only two conductors. Where should the ground be connected?
Did I read something wrong?
Thank you for your patience!
Dirk

[bernte_one]

hello dirk
connect the shield from coil to the outer conductor of the rg58 koax cable (shield from cable)
the two conductors are called in german seele (center wire) und schirm (outer)
the try to use kos 3n connecter or iec koax (in german sat stecker und buchse) bnc was not goog some say

regards

[Dixix]

Now I understand it. I was confused. I have compared the schematics of various PI circuits. Coax cable can be used with other PI detectors, such as the circuits of the detector book. But not at the PICKINI. Here is a short circuit would occur. Therefore 3x connections for the coil. (see schematic). Here are 3 wires in the cable must be. For other detectors 2 wires (lines) sufficient-- so coax. PICKINI no coax cable.
Dirk

[Dixix]

Here the difference between the circuits

PI from Detector-book


PICKINI


I hope you understand why I was so irritated ....with coax-cableand shielding.......and so on....

[F117]

A couple of weeks ago I built a coil for a PICKINI detector that was the best so far. The detector remains stable even when sensitivity is cranked up to the maximum ( in a field away from any noise ).
Step by step instructions:

1. Wind a bundled coil with enameled copper wire around a circle of nails - diameter 21 cm - 27 turns.
Wrap the bundle tightly with e.g. kite string and add some super glue to the knots in the string.






2. Wrap the coil with 2 layers of electrical insulation tape.



3. Wrap the coil with a layer of aluminum foil. I used normal kitchen foil cut in narrow ( 2 cm ) stripes.
Do not wrap it completely around - leave a gap open of about 1 - 2 cm.






4. Add a ground wire over the aluminum foil -- wrap it around the foil a couple of turns so that it makes good contact.
Add another layer of insulation tape over the alu foil:



I then put my coil in a silicone baking mould, added 2 fixation brackets + the cable and cable strain relief.
Next, poured 2 component resin into the mould. It settles within 20 minutes and turns from transparent into an ugly yellow color:
The cable is microphone cable.



Fresh from the mould after about 30 minutes:




This coil gives good results and is a classic recipe for a shielded coil. I am in no way a coil specialist, but just picked up this information here on the forum.
The major advantage of this coil is that it doesn't trigger false signals when you touch the ground or grass when waving it over the ground. I had this problem with an identical unshielded coil.

Best regards,
- Bernard

[Qiaozhi]

I hope you understand why I was so irritated ....with coax-cableand shielding.......and so on....

In that case, connect the shield to the +12V line, but (if you're using a metal enclosure) make sure you don't connect the case to the 0V line, otherwise you'll short out the power supply.

Ask F117 where he connects the shield.

[Dixix]

Hello Qioazhi
F117 used microphone cable. (See last Picture) Two wires and a shield. Therefore, he has three wires in its cable and the shield can on the ground (-) connect . But I prefer to use coax cable (RG5.
Microphone cable I've tried. The coil damps extreme. Much less power.
Thanks for the note of connection to (+). So I can use coax!

Hello F117
Thank you for detail showing of your coil construction!
Regards
Dirk

PS: Thanks to all for the patience to answer my stupid questions. Here in Germany we say "I had a long line in the head"
I am very pleased that I understand the PI's much better.
Great forum!

[F117]

Dirk,

this is the microphone cable I used: http://www.conrad.be/ce/nl/product/609289/
It was the cable with the biggest wire diameter in their range of microphone cable products : 2x 0.34 mm² = 22 / 21 AWG.
Resistance: < 55 Ohms per kilometer => 1 meter < 0.055 Ohm... So I don't really understand your remark about 'the coil damps extreme'. Maybe the microphone cable you used is really thin ?

As Qioazhi says: if you insist on using coax cable: connect the shield to the +12V terminal.

- Bernard

[Dixix]

Hello Bernard
I bought at conrad this microphone cable.



If I have it connected to the coil, then walked down the performance. I've tried it. When I read the article by J. Rogowski (bbsailor) understand correctly, does this relate with the capazitance, not with the resistance of the cable. Both wires are close together.
A Oscope brings more certain.
At the moment I am still trying my oscope. I can not even cope with the oscope. I think that the wave does not look right.



all the best,
Dirk

[F117]

Hi Dirk,

nice oscilloscope picture !
At first sight, it looks identical to what I am seeing on the scope.
When you trigger on the falling edge of the top trace, you may be able to investigate the rising flyback slope in more detail.
I am not a coil / cable specialist so I will not judge/comment on the use of a particular type of cable.
There are people with far more experience on this forum that maybe want to add comments on this ?

Regards,
- Bernard

[F117]

Checked the microphone cable specifications again http://www.conrad.be/ce/nl/product/609289/
The capacitance of this particular cable is 58 pF/m between the 2 conductors and 115 pF/m between one conductor and shield.

From the coil forum - Static coil shielding topic:

A good rule of thumb is that each 100pf of coil circuit capacitance reduction equals about 1 microsecond of performance improvement in reducing the delay.

Taking the delay introduced by the cable into account, this would add an extra 1-2 microseconds overall.

Would this make any difference compared to other (unshielded ) cables ?

[bbsailor]

Checked the microphone cable specifications again http://www.conrad.be/ce/nl/product/609289/
The capacitance of this particular cable is 58 pF/m between the 2 conductors and 115 pF/m between one conductor and shield.

From the coil forum - Static coil shielding topic:


Taking the delay introduced by the cable into account, this would add an extra 1-2 microseconds overall.

Would this make any difference compared to other (unshielded ) cables ?

The total amount of capacitance seen by the coil when the current is switched off determines how fast the mono coil can switch from being a transmit coil to being a receive coil. More capacitance requires a lower damping resistance to damp that higher capacitance and extends the time it takes for the current to drop to 0.

Typically, coax cables rated at higher impedances 75 ohms, 90 ohms etc. have lower capacitance per foot. Thicker inner wire typically has higher capacitance. A coax with a thinner center conductor and a lower dielectric of the insulation (like Teflon) and more space (foam spacer) between the center conductor and the shield will have lower capacitance.

Keep the coax short. Only use a long enough length to go from the control box to the coil with minimal excess.

Here is mental picture to help visualize what is going on here. The absolute fastest that the current can switch off is a 90 degree vertical line going from max current at the end of the TX pulse down to 0 current, however that cannot happen due to the capacitance in the TX circuit and the damping resistor required to damp the oscillations that occur during the flyback period right after the TX pulse. Higher values of damping resistor allow the current to fall from max to 0 current faster, closer to vertical but not perfectly vertical. The coil discharge time constant is calculated by dividing the coil inductance by the value of the damping resistor. (Read the fast coil article to see more details about this as a little more is involved with MOSFET voltage rating and clamping diodes.) Lower coil discharge time constants (acheived with higher value damping resistors) allow smaller targets with lower time constants to be detected easier.

Unfortunately, there is no single thing you can do to make a fast coil but you must look to all the things that add capacitance to the coil circuit or cause using a lower damping resistor value.

1. Use shortest coax
2. Use coax with lowest capacitance per foot
3. Use low dielectric constant insulation on coil winding
4. Use a low dielectric spacer between the coil and the coil shield
5. Use a coil shield made of a thin wire mesh rather than a solid surface to minimize shield area and reduce coil to shield capacitance.
6. Use a TX MOSFET with a lower COSS rating
7. Only use a TX pulse width long enough to stimulate the primary targets you seek as longer TX pulses cause a higher flayback voltage and require a lower damping resistor value to properly damp the coil.

I hope this helps.

bbsailor

[F117]

Thank You, bbsailor for this detailed explanation and advice

- Bernard