Beachmaster Digital

[jeep6789]

hi，Where are we? Do we have a screen

[jeep6789]

If we supply power to the coil at 24V, 36V or even 100V, can we increase the detection range? Has anyone done the experiment?

[Olly]

hi，Where are we? Do we have a screen

Patience my friend, I'm busy with the long process of trying to characterise decay curves of all sorts of coins and other items with various compositions, orientations and distances from the coil. I expect to be busy with this for quite a while before I even think about writing display driver stuff. It's progressing well though...

[Olly]

If we supply power to the coil at 24V, 36V or even 100V, can we increase the detection range? Has anyone done the experiment?

This is very much a law of diminishing returns as explained very well on page 197 of ITMD 2nd edition.
Doubling the output power will increase detection depth by only 8%, so you very quickly run out of battery power without any significant gain in detection depth.

[jeep6789]

Thank you

[Olly]

Having some hiccoughs....

Seems there is more to these LT6200 opamps than meets the eye.
I'm getting some very strange offset control and drift problems, I think this is down to their very high input bias current and the amount it changes depending on the common-mode voltage
Looks like I'll need to pause a while and dig a bit deeper, perhaps the slightly slower LT6202 will be better for this application.

Bear with me folks, this could take some time - but then, it is a fun research project....

Regards

[green]

I tried LT6234 and LT6237 when I started playing with mono coils, both seemed to work fine. They have a lower input bias current.

[Olly]

I tried LT6234 and LT6237 when I started playing with mono coils, both seemed to work fine. They have a lower input bias current.

Thanks, I'll try those, they may be more suitable. They also draw a lot less current than the Lt6200 which uses 23mA per amplifier....

It's a shame that LT chose to change the polarity of the enable/shutdown pin between the LT6200 and LT6233/LT6236 in the TSOT-23 package.

[Olly]

LT6200 problems solved

I decided to perservere with the LT6200's and see if I could track down the problems I was having.
Went back to controlling the offset via the non-inverting input as this gives better noise performance, but I reduced the pin 3 input resistor down to 47R to reduce the effects of bias current variation. At the same time I reduced the pin 3 bypass capacitor to 1.5nF and this seemed to solve a strange instability issue that I was having.

The preamp is now working as expected, which leaves me with the issue of bandwidth limiting via the feedback capacitor.
As I'm sampling at 1MHz, I thought I'd try limiting to around 500kHz with a 10pF feedback cap but this didn't eliminate much of the coil noise.
Tried 150pF which limits bandwidth to around 32kHz and although this cleans up the signal dramatically, the response is way too slow for fast targets.

I'm thinking that somewhere in the region of 80kHz to 160kHz may be a better compromise (30pF to 60pF). Will do some more experimenting and see if I can come up with a happy compromise.

I'm fairly pleased with the progress to date but am going to close up shop now for a few months while I enjoy the Summer. Lots of boating, fishing and hanging out at the beach to do while we have the weather for it.

I'll be back on the project when the weather starts getting gloomy again - In the mean time, enjoy the Summer and thanks for all the feedback so far....

Regards

[MartyJ1963]

Nice one Graham! Great to hear it's progressing nicely for you. Enjoy your time at the beach, hopefully there'll be loads of nice warm sunshine soon to go with the dry weather we're having! Regards, Marty.

[Olly]

Nice one Graham! Great to hear it's progressing nicely for you. Enjoy your time at the beach, hopefully there'll be loads of nice warm sunshine soon to go with the dry weather we're having! Regards, Marty.

Thanks Marty, time to enjoy some of the fruits of retirement

[jeep6789]

[jeep6789]

Come back

[jeep6789]

hi

[Olly]

Back from holidays

Well, I'm back from my nice long holiday but still have lots of mini-projects around the home to keep me busy. That being said I'm still hoping to get the opportunity to work on this project from time to time.

This detector needs a 5-cell power supply so thought I'd start out with printing a nice little LiIon battery pack which mounts under the cuff.


I've included a switch and connector for balance charging. The wires reach the control box by running through the stem pipe.