Arduino Nano PI Main Discussion

[ugocapeto]

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.

[Qiaozhi]

I ended up using a epoxy resin for doing molds and the result is very good, at least, esthetically.

About the audio tone, it's a bit chirpy at rest.

When an object gets close to the coil, the tone become continuous (and louder).

Is the tone at rest supposed to be a bit chirpy, that is not continuous (I am testing indoors)???

PI detectors tend to be sensitive to external noise, especially when tested indoors. This can cause the input signal to jump around so that it keeps crossing the audio threshold.

[ugocapeto]

I ended up using a epoxy resin for doing molds and the result is very good, at least, esthetically.

About the audio tone, it's a bit chirpy at rest.

When an object gets close to the coil, the tone become continuous (and louder).

Is the tone at rest supposed to be a bit chirpy, that is not continuous (I am testing indoors)???

[Carl-NC]

It can matter. Usually you want low shrinkage and low dielectric constant. Low shrinkage is more critical in a DD coil and the dielectric constant is important for high performance. For some coils you can get away the cheap stuff, you'll have to try & see.

[ugocapeto]

Hi all:

About the coil, does the type of epoxy resin matter much?

Is it ok to use a cheap epoxy resin commonly used in craft projects?

Or do you really need an electronics grade epoxy?

[ugocapeto]

Thanks for your suggestions. Really appreciated.

I have just realized that there is video in youtube land that shows what the traces are supposed to look like:

https://www.youtube.com/watch?v=G1gUYHVw20k

I do get about the same thing, so that's good.

[Qiaozhi]

Thanks a lot for your replies, well appreciated.

Yes, at some point, i will put a damping resistor that makes it closer to the critical value. Was just wondering if a little bit of underdamping was ok.

I am trying to get to the finish line and was wondering if you could tell me if those scope traces look ok, in particular, the signal at TP10. I am at step 13 in the book. I don't understand why the sample pulse is so large, time wise.

See here the traces i am getting at TP6. TP5, and TP10: https://imgur.com/a/1jamqw8

The book says I should get 700mV DC at TP10, but i am nowhere near that with this sample delay as it's actually negative.

Not too sure what I am doing wrong here. Again, thanks for your patience.

TP6 is the preamp output. This looks OK.

TP5 is the main sample pulse. This should be 50us wide, as defined in the Arduino sketch. If your timebase is set to 8us (which it looks like it is) then it seems to be OK. Measure it to make sure.

TP10 is the output of the 1st integrator. The 700mV is a guide in case someone attempts to build the detector without access to an oscilloscope. In the first image it looks to me like the start of the sample pulse is higher than halfway up the decay curve. Try reducing the sample delay with the pot connected to PL4. The 1st integrator stage inverts the signal from the main sample gate (TP7), which I suspect is currently slightly positive.

[ugocapeto]

Thanks a lot for your replies, well appreciated.

Yes, at some point, i will put a damping resistor that makes it closer to the critical value. Was just wondering if a little bit of underdamping was ok.

I am trying to get to the finish line and was wondering if you could tell me if those scope traces look ok, in particular, the signal at TP10. I am at step 13 in the book. I don't understand why the sample pulse is so large, time wise.

See here the traces i am getting at TP6. TP5, and TP10: https://imgur.com/a/1jamqw8

The book says I should get 700mV DC at TP10, but i am nowhere near that with this sample delay as it's actually negative.

Not too sure what I am doing wrong here. Again, thanks for your patience.

[GeoMax]

Why do you not place two resistors in series to get your desired resistor value? For example take maybe 390R + 56R in series to get 446R.

[Qiaozhi]

As Altra says, I would go with 470R.

[Altra]

Use the 470 ohm. It is better to be a little under damped than over. The scope shot for the 470 looks fine

[ugocapeto]

Hi all, I am not completely sure what to do regarding damping resistor.

Using the tool, it looks like I need 446 ohms to get critical damping.

If I use a 330 ohms, i am at -V for 12.0 us and the time it takes to go to 0v is about 8.8 us. Not sure if that info is relevant.

If I use a 470 ohms, I am at -V for 8 us and the time it takes to go to 0V is about 11.20 us. Note that it's underdamped so it goes above 0V for a tiny bit before coming down.

Now, the question is:

Do I need to use 446 ohms exactly or very close to it or is it ok to use either 330 or 470?

I am thinking 470 ohms should be just fine but not sure tbh.

I have uploaded what the scope shows at TP6 for both resistor values. Hope that helps.

https://imgur.com/a/AZWxpMJ

[Qiaozhi]

The ANPI is designed for use on the beach so the main sample pulse probably doesn't need to start before approximately 10us, otherwise you'll begin to get false signals if you search in the surf.
In this case you don't need to be ultra cautious about the shielding. A lot of discussions (and videos) about coil building are concerned with getting the shortest sample delay possible. However, this is only really necessary if you're searching for gold nuggets. Also, you would need a detector that includes ground balancing, which is not the case with the ANPI.

I have successfully built PI coils that are shielded with aluminium foil (not thick kitchen foil) that was 20um thick. In that case I used plastic wire wrap to space out the shield.
Eric Foster always used copper mesh tape in his coils, and I have followed his advice as it gives good results.

[ugocapeto]

Hi all:

About the coil construction described in the book, I am particularly puzzled by the shielding.

Is it ok to use plain kitchen aluminum foil instead of copper mesh tape?

Can the copper mesh tape (or whatever else) be put on directly on the wire or should there be some kind of air gap between the two?

In a video I saw on youtube about the nano PI detector build, a plastic sheathing is used between the copper wire and the shield. Is that necessary?

Thanks in advance.

[Qiaozhi]

Is there an "official" BOM list I can use and try and work out which replacement components I need to order?

The Component Parts List is in Appendix A of the book.