Competition!

[ivconic]

Here you have a new coil idea for a PI. the picture shows the magnetic field lines. Note that the field lines "illuminate" the target from a different angle than usual.

What's that?
An "8" coil?

[Tinkerer]

Thanks Tony!
Finally something interesting!
What was the name of that detector?

At the time I was diving for treasure in Key West, Florida, USA. So I named the detector KW 1. I built a few dozen of them and sold them to other treasure divers.

[ivconic]

[Tinkerer]

What's that?
An "8" coil?

Any size coil really. An assembly of 2 TX coils with the windings such as to make the interaction of the coil fields "aiding".
Now, if you add an RX coil over the center of the 2 TX coils, the fields of the TX coils would be cancelled in the RX coils, thus "balanced".

I am sure you have observed that a coin seen in the "flat" position gives a strong response and a coin in the "vertical" position gives a weak response. Therefore, this coil assembly will enhance the response of a coin in the vertical position.
Now, you probably have also noticed that a steel "crown cork" as they call the beer bottle tops, shows a resistive response in the "flat" position and a magnetic response in the "vertical" position. It is probable that this coil assembly will enhance the difference between magnetic and resistive response.

You can also wind the 2 TX coils with the fields "opposing". The effects will be very different.

[ivconic]

I was thinking on "8" shape (twisted at center) and not on size like 8" (inches).
"8" shape coil has it advantages as more immune to noise and interferences.
But it is good that you explained.

[ivconic]

Tony I tried to redraw your schematic in a way more suitable for my way of undrstandings.
Though there are few things you can help me to understand.
I put marks (circles, elipses) on the parts I would like to clarify, their values.

[Tinkerer]

I was thinking on "8" shape (twisted at center) and not on size like 8" (inches).
"8" shape coil has it advantages as more immune to noise and interferences.
But it is good that you explained.

OK, now I understand what you mean. A "Figure 8 coil".
There are quite a few variations possible.
Which variation is the best?
The best variation is the one that best fits your purpose.
Before deciding on what coil, we must decide on the purpose of the detector.

The best electronics become useless with a bad or wrong coil.

[Tinkerer]

Tony I tried to redraw your schematic in a way more suitable for my way of undrstandings.
Though there are few things you can help me to understand.
I put marks (circles, elipses) on the parts I would like to clarify, their values.

I will look it up. I actually still have one or 2 assembled PCBs.

Tentatively:
C1 - clock timer 120 to 150pico F
C2,3,4,6, = pico F, the last number being the number of zeros = 104 = 100,000pf = 100nF or 0.1mF

? at the end = bone conductor clicker. Set at about 2 Hz for no target. Proportionally increases to about 120Hz with target saturation.

If I reember right, the detector was running at about 3000cycles per second.

[ivconic]

OK, now I understand what you mean. A "Figure 8 coil".
There are quite a few variations possible.
Which variation is the best?
The best variation is the one that best fits your purpose.
Before deciding on what coil, we must decide on the purpose of the detector.

The best electronics become useless with a bad or wrong coil.

Totally agree!

[ivconic]

Here is, I hope, finall schematic.
Guess what... I will make it.
I like such projects!
...
Just one more thing; bone conductor... kinda mechanical transducer right?
I would like there speaker instead it.
Would not be a problem to rearrange that part.
Thanks!

[GeoMax]

Hi Thinkerer,

thanks for posting this great PI. Would be interesting to see some pictures of the actual detector and/or assembled PCB as well.

[bbsailor]

Totally agree!

A few years ago, I read an article on Eric Foster’s web site that explored the optimum coil discharge time constant for stimulating a particular target time constant. After World War 2 many UXOs had to be found buried in the ground. After removing the metal parts from found UXOs they did research to build metal detectors that would best find the small metal parts. Here is the main point of their research related to this project. The military research found that the best stimulus of a metal target of a given time constant would be a coil discharge time constant five times faster than the target time constant. Since the coil discharge time constant is driven by the coil inductance divided by the damping resistor value, minimizing coil seen capacitance will allow higher damping resistor values and thus a steeper coil discharge slope to better stimulate smaller targets.

When building this metal detector for competition, someone needs to specify a low, medium and high target size, shape, weight and time constant so the coil design and fine tuning can accurately and honestly be compared in the design submissions. It would be normal for different size coils to be used to optimize the detection of each target size and time constant. Here is where fine tuning the damping resistor value is necessary when changing coils for each target.

Select commonly available metal parts as low, medium, and high target time constants for the design submissions to specify in their final report.

Creativity in circuit design also needs creativity in coil design to optimize the final performance.

Joseph J. Rogowski

[Tinkerer]

Hi Thinkerer,

thanks for posting this great PI. Would be interesting to see some pictures of the actual detector and/or assembled PCB as well.

It is amazing that these PCBs have survived all these years. I guess you could slap this circuit together on a breadboard and learn the basic functions of PI.
However, simple as it is, it was doing it's job at the time, so long ago.
After that, we can see how we can improve it.
Lots and lots of improvements possible.

[Tinkerer]

A few years ago, I read an article on Eric Foster’s web site that explored the optimum coil discharge time constant for stimulating a particular target time constant. After World War 2 many UXOs had to be found buried in the ground. After removing the metal parts from found UXOs they did research to build metal detectors that would best find the small metal parts. Here is the main point of their research related to this project. The military research found that the best stimulus of a metal target of a given time constant would be a coil discharge time constant five times faster than the target time constant. Since the coil discharge time constant is driven by the coil inductance divided by the damping resistor value, minimizing coil seen capacitance will allow higher damping resistor values and thus a steeper coil discharge slope to better stimulate smaller targets.

When building this metal detector for competition, someone needs to specify a low, medium and high target size, shape, weight and time constant so the coil design and fine tuning can accurately and honestly be compared in the design submissions. It would be normal for different size coils to be used to optimize the detection of each target size and time constant. Here is where fine tuning the damping resistor value is necessary when changing coils for each target.

Select commonly available metal parts as low, medium, and high target time constants for the design submissions to specify in their final report.

Creativity in circuit design also needs creativity in coil design to optimize the final performance.

Joseph J. Rogowski

specify a low, medium and high target size, shape, weight and time constant

Very well said

ivica, it is your turn to specify the precise purpose you desire for your detector. Without precise specifications, we are walking blind.

[ivconic]

A few years ago, I read an article on Eric Foster’s web site that explored the optimum coil discharge time constant for stimulating a particular target time constant. After World War 2 many UXOs had to be found buried in the ground. After removing the metal parts from found UXOs they did research to build metal detectors that would best find the small metal parts. Here is the main point of their research related to this project. The military research found that the best stimulus of a metal target of a given time constant would be a coil discharge time constant five times faster than the target time constant. Since the coil discharge time constant is driven by the coil inductance divided by the damping resistor value, minimizing coil seen capacitance will allow higher damping resistor values and thus a steeper coil discharge slope to better stimulate smaller targets.

When building this metal detector for competition, someone needs to specify a low, medium and high target size, shape, weight and time constant so the coil design and fine tuning can accurately and honestly be compared in the design submissions. It would be normal for different size coils to be used to optimize the detection of each target size and time constant. Here is where fine tuning the damping resistor value is necessary when changing coils for each target.

Select commonly available metal parts as low, medium, and high target time constants for the design submissions to specify in their final report.

Creativity in circuit design also needs creativity in coil design to optimize the final performance.

Joseph J. Rogowski

It will be easy for me to answer the question about the coil.
I prefer coils from 18cm to 22cm.
Target?
Very small silver and gold coins.
Very small pieces of silver and gold jewelry, from say 2-3mm to 10mm.
Irregular shape.
The coins, however, have a more regular shape.
These are thin pieces with a small mass.
Very difficult targets.
Time constant for such pieces?
Probably below 15uS and less.
I currently have no way of determining the exact time constant.
But from your post it is clear that such a detector and coil would have to be very "fast" and with the possibility of setting the delay below 10uS, or less.
We talk about the PI detector here now.
And at the same time, I'm also interested in what the chances are for VLF I/B non-motion with similar capabilities (to detect such targets).