Air tests with Hammerhead ver C

[nick_f]

Hi All,
I've done some tests with my new HH board after tuning. I am interested if other people obtained better results.
The coil is not properly made (29 turns 0.5mm enamel copper wire, coil diameter 200mm), shielded with aluminium tape straight over the wire).
I use 1.5 meters of coax cable, 50 ohm impedance.
Here are the results:
1 eurocent copper coin (11mm diameter) is detected clearly from 17cm
50 cent Australian coin (32mm, white metal) is detected from 25cm
$1 Australian coin from 20cm, $2 Australian coin from 17cm
EUR1 from 18cm. These numbers are conservative, the results could be actually better.
I shielded the board into a kitchen aluminium pot put to the ground, lid included
Large metals are detected from about 1 meter.
I am not happy yet about the circuit because it still comes up with some noise every few seconds, although it wasn't too bad while shielded.
I took the contraption for a test outside, in the yard. I couldn't find any place where to put the coil on the ground without beeping. Either the ground affects it, either there are plenty of junk metals all over the place (it can be possible). It was dark outside, I will do some digging tommorrow.

Regards,
Nicolae

[Unregistered]

How about a .2 gram gold nugget

[nick_f]

Unfortunately I don't have any gold nuggets to test. And I found out that my copper coin is actually attracted by a magnet, therefore is not copper, only copper-plated.
When I will get some real copper coins, I will post the real result. For the gold nugget test, I will have to find one before

[Tinkerer]

Testing standards

Unfortunately I don't have any gold nuggets to test. And I found out that my copper coin is actually attracted by a magnet, therefore is not copper, only copper-plated.
When I will get some real copper coins, I will post the real result. For the gold nugget test, I will have to find one before

It is not very easy to get universal testing standards. Coins of the same dimensions vary greatly as to their alloys in different countries.
The same applies to gold jewelry. 18 carat gold can come in a very great variation of alloys.

One standard that is fairly universal is a steel bottle top or crown cork as it is sometimes called.

Another good standard is aluminum foil. Take the thin kind that is used to wrap chocolate etc.
Cut out a square of 1" x 1". Apply a piece of tape to the back, otherwise it crumples up easy.
This target has a TC of about 10uS. A sensitive detector should be able to detect it at 30cm in air.
Now cut this test target in halve.
You have now 1" x 1/2". This has a TC of about 5 uS. You should be able to detect this at 15cm.
If your detector can accomplish that, you have indeed a very good detector for finding gold nuggets.

Tinkerer

[nick_f]

Hi Tinkerer,
Thank you very much for this info. I've done the test with the 1 square inch and with the beer cap. At the moment the sensitivity of my detector is slightly lower compared with last time. It can detect the beer bottle from about 20 cm. The 1 square inch aluminium kitchen foil can be detected from about 1 cm (need to make the coil faster).
Is there anybody else who built the Hammerhead and can provide some comparative results? Otherwise I start to believe that I am the only one who completed this project, at least on the work bench

Regards,
Nicolae

[Tinkerer]

TESTING STANDARDS

Hi Tinkerer,
Thank you very much for this info. I've done the test with the 1 square inch and with the beer cap. At the moment the sensitivity of my detector is slightly lower compared with last time. It can detect the beer bottle from about 20 cm. The 1 square inch aluminium kitchen foil can be detected from about 1 cm (need to make the coil faster).
Is there anybody else who built the Hammerhead and can provide some comparative results? Otherwise I start to believe that I am the only one who completed this project, at least on the work bench

Regards,
Nicolae

Hi Nicolae,

you are doing very well and have a good understanding of the PI. The alu foil has a very short time constant and at 20uS after switch OFF, its signal amplitude is only about 10% of the initial signal amplitude.
What is the TX pulse length that you are using?
What TX voltage?
If you reduce the TX pulse length, you will be able to sample earlier.
What kind of shielding do you use on the coil? Do you have a spacer between the coil windings and the shield?
What kind of wire do you use for the coil winding?

All the best

Tinkerer

[Ray-NM]

Tinkerer
Looking at his coil I see a couple of problems. Interwinding capacitance for enameled wire is high. He is using aluminum foil for the sheilding. So it seems to me that both of those will prevent him from having a fast coil. Also he is using a 9v battery meaning less power to the coil. Could the gap for the shield be a little wide also? Just my thoughts.
RayNM

[nick_f]

Thinkerer,
Thanks again for explaining how the alu foil acts, your pointed what I have to focus on. I will post another email with heaps of CRO shots, because I am not very happy with the high noise level at TP4 and about the idle audio chattering that I can't get rid of.
In order to detect the alu foil, I already decreased the pulse width close to the minimum. At the moment, I have a Tx Pulse Width of 29us and the Pulse Amplitude is only 175V (I can easily get over 350V if I increase the pulse width - I set up this lower voltage just to decrease the delay time, to measure the aluminium foil).

Ray-NM had a look at the photo of my device and noticed how the coil was built, and he is right about all the problems noticed. I am also aware of these matters. I am actually using a 12V regulated power supply, the battery was for some testing I've done outside.
My coil has a diameter of 20cm, I think it has 29 turns with 0.5mm diam of enamelled wire. The sticky aluminium foil is placed straight over the wire, with no spacer in between. It has 0.492mH and the capacitance measured between the shield and the coil is 407pF (which I think is very high - but expectable, considering there is no spacer). I will read again the article about coils of bbsailor and try to make a better coil. I have to find some suitable spacer and apparently aluminium foil is not suitable for a fast coil. I have the chance to measure the self-resonance of a coil at work, which I will do later.

Ray, the gap between the end of the shielding is about 14mm. Do you know how does this distance affect the coil and why should I try with a larger gap?

Thank you both for your feedback, it is much appreciated.

Nicolae

[nick_f]

This target has a TC of about 10uS. A sensitive detector should be able to detect it at 30cm in air.
Now cut this test target in halve.
You have now 1" x 1/2". This has a TC of about 5 uS. You should be able to detect this at 15cm.
If your detector can accomplish that, you have indeed a very good detector for finding gold nuggets.

Tinkerer

Tinkerer,
Such performance belongs to professional, high quality detectors or is it a very good standard for detectors built by amateurs?

Regards,
Nicolae

[hobbes_lives]

Nicolae,

You may want to check the frequency of the noise on TP4 and make sure it's not power supply noise coming from the 7660's oscillation. Also, the gap in the shielding should be small as possible without touching.

[nick_f]

Nicolae,

You may want to check the frequency of the noise on TP4 and make sure it's not power supply noise coming from the 7660's oscillation. Also, the gap in the shielding should be small as possible without touching.

Hello Hobes,

The noise frequency is much higher than the frequency of 7660 (which is also synchronised with the circuit). The noise has a period of 5us, which means a frequency of 200kHz.
Just to remove any doubts, I will replace 7660 with a 9V battery.

About the gap in the shielding, I decreased the gap distance to just a few mm. Not much change.

Regards,
Nicolae

[nick_f]

Just to confirm the noise is not caused by 7660, I removed it, I raised D2 off board and applied 9V to pin 3 of IC2.
The noise is still present, it didn't change at all.
To me the noise is more a self-oscillation of the NE5534. I will remove the socket of IC6 and see how the noise is affected.

Regards,
Nicolae

[nick_f]

Hi All,
It looks like I found a convenient way to reduce the noise, increase the sensitivity to varios targets, including the 1 square inch aluminium foil, maintaining the same setup (the same bad coil I was using).
The solution was pretty simple: I added a 100k resistor on top of R13 and now the circuit is much more stable (I don't get all kind of random screaming noises when no target is nearby, as before). Maybe Carl should have added a pot to adjust the gain of IC6 as well.
Interestingly, I can adjust the circuit for even greater sensitivity (beer cap detected at 23cm and 1" square alu foil at around 2cm).
You can notice that also the pulse at TP4 is about 8us width, compared to about 12us before.
I don't know why the pulse now has that distortion on it, I will investigate (it is not due to pulse at TP6). I found out with big surprise that lowering the value of the coil resistor R11 increases dramatically the width of the pulse (I would have expected rather the opposite, or no change), so there are multiple advantages in using a resistor as high as possible.

Regards,
Nicolae

PS - I also posted this message on the thread "TP4 very noisy", where it was my intention to post it in the first place...

[Tinkerer]

Testing standard

Tinkerer,
Such performance belongs to professional, high quality detectors or is it a very good standard for detectors built by amateurs?

Regards,
Nicolae

Nicolae,

The HH is a very good detector design. It is capable of high performance.
However, a lot depends on the execution of the various parts. The coil is of primary importance.
I believe that the GQ5 is capable of the performance indicated. I would very much like to hear test results from the owners of other high sensitivity detectors to compare with.

I can do quite a bit better than that and after all I am only a "Tinkerer"

If you strive for perfection, I will be glad to help you getting there.

Tinkerer

[nick_f]

Nicolae,

1. I believe that the GQ5 is capable of the performance indicated.

2. I can do quite a bit better than that and after all I am only a "Tinkerer"

Tinkerer

Hi Tinkerer,

1. What's GQ5?
2. Can you describe what are your results and with what detector? I also posted another thread about results with the new teflon coil, I get better results. Don't be modest, you seem to have quite some experience with MD

Regards,
Nicolae