Arduino Nano PI Main Discussion

[Qiaozhi]

The work is finished. Everything works. You can dive underwater.

Nice job!
I see from the photos that the quality control inspectors were hard at work.

[Myntmaster]

The work is finished. Everything works. You can dive underwater.

[Myntmaster]

Front panel. The detector can be used underwater.
Aliexpress sells a special A4 film. She's like a sticker. You can print on a regular inkjet printer. Does not wash off or wash off later. We print what you need and stick it on.

[Qiaozhi]

Well done. Nice work.
Now go and find some treasure.

[Myntmaster]

Hi all.
While we are in winter, I decided to finish the postponed project.
With this coil in air in VCO mode, 1 euro - 38 cm. Gold ring 14K 3 gr. 30 cm. In sand the same ring is 25-27 cm.
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[kosacid]

just a pity the Arduino is not doing the sampling the only way i could see past that would be say use a atiny if you keeping the digital or back to a 555 to control the mosfet that would leave the Arduino dedicated to sampling using the timer as a trigger to sample, by experience i know you cant have it both ways i tried that lol

[Qiaozhi]

Ok, thanks, I kinda get it, you are interested in the DC variation, not the actual DC value.

As you said previously, simulating all that good stuff in LTspice would be useful in understanding how the whole thing works.

I just don't know enough about LTspice to be able to simulate going over metal and make the decay curve vary at the sampling points.

Any pointers on how to do that?

It could take a whole book to explain how to do that in detail.
If you're just trying to understand how certain sections of the design work, then you can simply apply the correct signals to the input to find out how it behaves.
Depending on the circuit, you can use dc, time, or frequency domain analysis as appropriate.

There is an LTSpice tutorial here: http://www.simonbramble.co.uk/lt_spi...e_lt_spice.htm

[ugocapeto]

Ok, thanks, I kinda get it, you are interested in the DC variation, not the actual DC value.

As you said previously, simulating all that good stuff in LTspice would be useful in understanding how the whole thing works.

I just don't know enough about LTspice to be able to simulate going over metal and make the decay curve vary at the sampling points.

Any pointers on how to do that?

[Qiaozhi]

What i don't understand is that high pass filter at TP10 going into the 2nd integrator. Isn't the output at TP10 pretty much a dc voltage? Shouldn't a high pass filter get rid of that dc voltage?

As it says on page 13 of the book ->

"C14 (470nF) and R25 (220k) form a passive high-pass filter that provides the detector
with a self-adjusting threshold (SAT). Therefore, if you hold a coin (for example) near the
coil, the audio tone will gradually reduce in volume. The idea of the SAT is to prevent the
dc level of the received signal from drifting over time, mainly due to temperature changes
and self-heating effects that would require the operator to constantly adjust a manual
threshold control to compensate."

[ugocapeto]

If I use 2v subdivision for TP11 instead of 1v (to match what's in the youtube video you linked to), the jumping is not as bad. I am indoors and i am using a battery pack. I think i am gonna let it be.

Now, I am still trying to understand that double integrator stuff.

What i don't understand is that high pass filter at TP10 going into the 2nd integrator. Isn't the output at TP10 pretty much a dc voltage? Shouldn't a high pass filter get rid of that dc voltage?

Thanks for your help in all this. Trying to learn as much as possible from that build.

[Qiaozhi]

This is what I see on my scope for TP6 (2v per subdivision on y-axis) and TP11 (1v per sub): https://imgur.com/a/L0wEAVc

This jumping around doesn't seem right to me. I would expect a little of bit jumping but not that much.

What do you think?

The output at TP11 will jump around a bit if you test it indoors. Running from a bench power supply can also make it noisier, so it's better to test with a battery pack.
However, your TP11 output does seem particularly noisy. As a comparison, have a look at this video.
https://www.youtube.com/watch?v=G1gUYHVw20k

[ugocapeto]

This is what I see on my scope for TP6 (2v per subdivision on y-axis) and TP11 (1v per sub): https://imgur.com/a/L0wEAVc

This jumping around doesn't seem right to me. I would expect a little of bit jumping but not that much.

What do you think?

[Qiaozhi]

Thanks for answering my newbish questions

I am a bit confused by the 2nd integrator to go from tp10 to tp11.

At tp10, I have a nice dc signal when at rest (no metal around) but at tp11, the signal is quite wavy (at rest) and the signal at the headphone is not continuous. Is it supposed to be like that? Do you get the same thing indoors?

I kinda understand the first integrator to get tp10 (not fully tbh) but i don't really understand why a 2nd integrator is used to get to tp11. If the input signal is dc, shouldn't the output signal of the 2nd integrator be a ramp?

The first stage is a sampling integrator, and the second stage boosts the gain and provides threshold adjustment.
The second integrator will not produce a ramp at the output because there is a resistor in the feedback loop. If there was no resistor present, then you would be correct.
Try putting the circuit in LTSpice to understand how it behaves.
https://www.electronics-tutorials.ws/opamp/opamp_6.html

[ugocapeto]

Thanks for answering my newbish questions

I am a bit confused by the 2nd integrator to go from tp10 to tp11.

At tp10, I have a nice dc signal when at rest (no metal around) but at tp11, the signal is quite wavy (at rest) and the signal at the headphone is not continuous. Is it supposed to be like that? Do you get the same thing indoors?

I kinda understand the first integrator to get tp10 (not fully tbh) but i don't really understand why a 2nd integrator is used to get to tp11. If the input signal is dc, shouldn't the output signal of the 2nd integrator be a ramp?

[Qiaozhi]

Thanks a lot for your reply.

Is the cabling from the coil socket to the pcb supposed to be shielded?

Also, in the book, you say that the delay can be set by waving a coin in front of the coil but isn't it better to set the delay with a scope so that it's at the middle of rising signal from the preamp and leave it there. Even if you would want to exclude pull caps and put more delay, wouldn't it make sens to have the pot directly on the pcb and not make it an outside pot that can be easily bumped into. Just wondering.

You could shield it if you find it makes a difference. Personally I didn't bother.
Ideally you could put the electronics in a metal enclosure, but this makes it heavier.

Waving a coin in front of the coil is just an alternative method of setting the delay. As you said, it could also be a fixed setting if that meets your requirements.