Arduino Nano VLF Metal Detector Project - Main Discussion

[Olly]

Thanks guys, just a thought... ;-)

[Qiaozhi]

Hi George,

The depth results you achieve with this design are particularly impressive (Especially with the Fisher F75 coil). This led me to wondering whether a forced TX signal has an inherent sensitivity advantage over an Oscillator based TX.

The coil inductance will change (however slightly) as it passes over a target, and with an oscillator based TX, this in turn would alter the TX frequency and possibly its phase as well (also very slightly). In turn, it is possible then, that the perceived phase shift between the TX and RX signal may be less than the case where the TX signal is forced and there is no alteration of the TX frequency and phase as the coil passes over a target.

Any thoughts?

As Carl said ...

[Carl-NC]

I doubt this will matter much. You are right that a target will slightly alter the TX frequency of a free-running oscillator but probably not enough to make a difference. And the driven coil has a similar issue; a target will still alter the inductance, but since the frequency is fixed it will instead distort the TX waveform. Again, probably not enough to make a difference.

[Olly]

Hi George,

The depth results you achieve with this design are particularly impressive (Especially with the Fisher F75 coil). This led me to wondering whether a forced TX signal has an inherent sensitivity advantage over an Oscillator based TX.

The coil inductance will change (however slightly) as it passes over a target, and with an oscillator based TX, this in turn would alter the TX frequency and possibly its phase as well (also very slightly). In turn, it is possible then, that the perceived phase shift between the TX and RX signal may be less than the case where the TX signal is forced and there is no alteration of the TX frequency and phase as the coil passes over a target.

Any thoughts?

[Qiaozhi]

Another minor "Oops" on P 135 of Parts List-

SENS external pot (1k) Connects to J5 (J3 is the coil)

AM/DISC switch Connects to J7 and J12 (Not J9 which is the Speaker/Headphones output)
2-pin plug Add J12 8

PS Hope I got this re-entered correctly as I had to re-type it after accidentally powering down the UPS with my foot and losing everything

Thanks. Errata updated.

[Goaty]

Another minor "Oops" on P 135 of Parts List-

SENS external pot (1k) Connects to J5 (J3 is the coil)

AM/DISC switch Connects to J7 and J12 (Not J9 which is the Speaker/Headphones output)
2-pin plug Add J12 8

PS Hope I got this re-entered correctly as I had to re-type it after accidentally powering down the UPS with my foot and losing everything

[Qiaozhi]

Also, 2 X LT1054 (U2 & U3) missing from IC s on p133

Thanks ... added to errata list -> Arduino Nano VLF Metal Detector Project - Published January 2024
It doesn't matter how many times you read through the text, there's always some pesky errors that slip through.

[Goaty]

Also, 2 X LT1054 (U2 & U3) missing from IC s on p133

[Qiaozhi]

There is a little mistake on page 134. In the 10k listing the last resistor should be R70 (not R79).

Thanks.
I've add that to the Errata section -> Arduino Nano VLF Metal Detector Project - Published January 2024

[turboaleks]

There is a little mistake on page 134. In the 10k listing the last resistor should be R70 (not R79).

[ivconic]

One of the ideas that comes to my mind is the use of a mux circuit in sine wave generation.
Of course, that part of the code will undergo major changes.
With just 5 random pins you expand to 16 channels.

[Qiaozhi]

Got my book this morning, very nice indeed.
I particularly like the flexibility achieved by using the R-2R DAC driver for the TX.

Thanks Olly.
Glad you like it.

[Olly]

Got my book this morning, very nice indeed.
I particularly like the flexibility achieved by using the R-2R DAC driver for the TX.

[ivconic]

[Qiaozhi]

Very interesting, neat, likeable!
I am interested in one thing; why did you decide to generate the sine signal from the MCU directly?
In this way, you took even 8 pins!
Among them is a UART port as well as some very useful PWM pins.
Some are directly tied and controlled by timers.
Too many resources wasted. Hence the two Nanos.
I'm not saying it's bad, it can be done that way too, why not.
I'm just wondering why you didn't just use one PWM pin and still shape it with a discrete solution out of MCU.
Even more; wouldn't it be interesting to drive the TX directly with a square signal?
So much for now, I didn't get to analyze everything.
But you certainly gave a very good starting point for upgrades!
Great idea!

I actually discuss those points in the book.