Arduino Nano PI Main Discussion

[waltr]

Tried some other harmonics. Odd seemed to be higher amplitude. Is there some reason odd is higher or just what it happened to be today?

Short answer: Yes, typically the Odd Harmonics are higher due to half-wave symmetry in power systems.
The long answer is very long and requires much study and math.

[6666]

To continue this subject ...

I spent some considerable time today investigating this idea, and here are my findings:

First I tested the detector outside near a power line using the original setup, and there was some crackling noise evident in the audio as I got closer to the power lines. Then I modified the sketch to change the TX pulse rate to 1225pps by setting TXperiod to 816.3125E-6, and tested outside a second time, but there was still no detectable difference between 1000pps and 1225pps.

Next, I modified the sketch further by hijacking the Boost switch to allow me to switch between the two TX pulse rates. Again I went outside to do the testing, but still could not detect any reduction in external noise.

Finally I replaced C15 with a 1uF capacitor to set the cut-off frequency close to 1.5Hz. The main difference I noticed with this change was the increased stability of the audio threshold. Unfortunately this also had the side-effect of crippling the recovery speed. By waving a coin near the coil and then moving it away at the same time, the signal would completely disappear after being moved only a couple of inches. Continuing to then wave the coin at the new position would allow the audio sound to recover. Testing outside gave a very smeared out sound when passing over buried targets, and not the usual crisp beep with C15 set to 100nF. Also, there was no apparent reduction in power line noise as far as I could tell.

I have now reverted the detector back to the original hardware and software settings.

Question: Is the frequency adjustment control we see on some pulse induction metal detectors really intended more for eliminating general external RFI, rather than specifically noise generated by power lines?

Good experiment George, thanks for feedback.

[green]

Just guessing. Adjustment control is for power line harmonics or maybe some other continuous frequency. Don't know if crackling noise is aliasing.

Some scope pictures of differentiator out, about 1500 pps(25 harmonic). Amplitude not the same at each harmonic, maybe do to EF sample not centered?

12V PS orientation makes a difference with amplitude

Tried some other harmonics. Odd seemed to be higher amplitude. Is there some reason odd is higher or just what it happened to be today?

[green]

To continue this subject ...

I spent some considerable time today investigating this idea, and here are my findings:

First I tested the detector outside near a power line using the original setup, and there was some crackling noise evident in the audio as I got closer to the power lines. Then I modified the sketch to change the TX pulse rate to 1225pps by setting TXperiod to 816.3125E-6, and tested outside a second time, but there was still no detectable difference between 1000pps and 1225pps.

Next, I modified the sketch further by hijacking the Boost switch to allow me to switch between the two TX pulse rates. Again I went outside to do the testing, but still could not detect any reduction in external noise.

Finally I replaced C15 with a 1uF capacitor to set the cut-off frequency close to 1.5Hz. The main difference I noticed with this change was the increased stability of the audio threshold. Unfortunately this also had the side-effect of crippling the recovery speed. By waving a coin near the coil and then moving it away at the same time, the signal would completely disappear after being moved only a couple of inches. Continuing to then wave the coin at the new position would allow the audio sound to recover. Testing outside gave a very smeared out sound when passing over buried targets, and not the usual crisp beep with C15 set to 100nF. Also, there was no apparent reduction in power line noise as far as I could tell.

I have now reverted the detector back to the original hardware and software settings.

Question: Is the frequency adjustment control we see on some pulse induction metal detectors really intended more for eliminating general external RFI, rather than specifically noise generated by power lines?

Just guessing. Adjustment control is for power line harmonics or maybe some other continuous frequency. Don't know if crackling noise is aliasing.

Some scope pictures of differentiator out, about 1500 pps(25 harmonic). Amplitude not the same at each harmonic, maybe do to EF sample not centered?

12V PS orientation makes a difference with amplitude

[Qiaozhi]

In a handheld detector the alternative is to make sure the mains frequency aliases outside the target filter bandwidth. This is usually set by the demod and the Nano has a BW = 1.5Hz (which is pretty low). Assuming 50Hz mains, if you set the detector to 1200Hz then the alias is synchronous and becomes a DC offset. But if the mains drifts by 0.1% then the alias is at 1201.2Hz and you hear a 1.2Hz beat frequency. However, if you set the detector to 1225 Hz then the 50Hz alias is at 25Hz which is suppressed by the target filter BW. And any minor mains variation remains suppressed. BTW, the demod is often followed by additional filtering that roughly matches the demod BW to offer better mains suppression. In the Nano, this would be R26-C15... setting them to 1.5Hz will help (they are currently 16Hz).

Interesting ...
The values used by the Nano are the same as those for Surf-PI. I hadn't noticed the difference in cut-off frequency between the two opamp stages, so I'll try changing C15 to 1uF, and setting the detector to 1225pps. Then I'll report back here with the results.

To continue this subject ...

I spent some considerable time today investigating this idea, and here are my findings:

First I tested the detector outside near a power line using the original setup, and there was some crackling noise evident in the audio as I got closer to the power lines. Then I modified the sketch to change the TX pulse rate to 1225pps by setting TXperiod to 816.3125E-6, and tested outside a second time, but there was still no detectable difference between 1000pps and 1225pps.

Next, I modified the sketch further by hijacking the Boost switch to allow me to switch between the two TX pulse rates. Again I went outside to do the testing, but still could not detect any reduction in external noise.

Finally I replaced C15 with a 1uF capacitor to set the cut-off frequency close to 1.5Hz. The main difference I noticed with this change was the increased stability of the audio threshold. Unfortunately this also had the side-effect of crippling the recovery speed. By waving a coin near the coil and then moving it away at the same time, the signal would completely disappear after being moved only a couple of inches. Continuing to then wave the coin at the new position would allow the audio sound to recover. Testing outside gave a very smeared out sound when passing over buried targets, and not the usual crisp beep with C15 set to 100nF. Also, there was no apparent reduction in power line noise as far as I could tell.

I have now reverted the detector back to the original hardware and software settings.

Question: Is the frequency adjustment control we see on some pulse induction metal detectors really intended more for eliminating general external RFI, rather than specifically noise generated by power lines?

[lc516]

Found it -> https://sourceforge.net/projects/met...sed-on-rp2040/

It appears to be based on the Spirit PI.
Minimum sample delay is 20us, and TX pulse rate can be varied between 20pps and 160pps.
There is no Earth Field Elimination (EFE).
Interrupts are not used, as the timings are all generated by software delays.
Detection of a one Euro coin is claimed to be 20cm.

There may be a problem trying to implement the timings using low-level hardware timers and interrupts, as the Pico only has one 64-bit timer. Looking at the Pico SDK documentation, it seems that the timer peripheral is incremented every micro-second which could also be a problem, although there are 4 alarms tied to the lower 32-bits. Maybe this could be worked around using some assembler code, but (as I said previously) the Pico is much more complicated to use than the Arduino, and may not add any extra value in this application. I suspect the Arduino Nano will still generate more interest in the Geotech community due to its ease of use.

PICO is not fit PI project and more power hungry. Port performance is not as good as stm32F103 series.

[useus]

Found it -> https://sourceforge.net/projects/met...sed-on-rp2040/

It appears to be based on the Spirit PI.
Minimum sample delay is 20us, and TX pulse rate can be varied between 20pps and 160pps.
There is no Earth Field Elimination (EFE).
Interrupts are not used, as the timings are all generated by software delays.
Detection of a one Euro coin is claimed to be 20cm.

There may be a problem trying to implement the timings using low-level hardware timers and interrupts, as the Pico only has one 64-bit timer. Looking at the Pico SDK documentation, it seems that the timer peripheral is incremented every micro-second which could also be a problem, although there are 4 alarms tied to the lower 32-bits. Maybe this could be worked around using some assembler code, but (as I said previously) the Pico is much more complicated to use than the Arduino, and may not add any extra value in this application. I suspect the Arduino Nano will still generate more interest in the Geotech community due to its ease of use.

I understand the simplicity factor but, as I said before, working on it could become a very nice and advanced metal detector and very low cost being diy


however I would like to congratulate you on your project, soon I will start designing the pcb with DIP atmega328p.

[Qiaozhi]

google rp2040 metal detector

1st link

Found it -> https://sourceforge.net/projects/met...sed-on-rp2040/

It appears to be based on the Spirit PI.
Minimum sample delay is 20us, and TX pulse rate can be varied between 20pps and 160pps.
There is no Earth Field Elimination (EFE).
Interrupts are not used, as the timings are all generated by software delays.
Detection of a one Euro coin is claimed to be 20cm.

There may be a problem trying to implement the timings using low-level hardware timers and interrupts, as the Pico only has one 64-bit timer. Looking at the Pico SDK documentation, it seems that the timer peripheral is incremented every micro-second which could also be a problem, although there are 4 alarms tied to the lower 32-bits. Maybe this could be worked around using some assembler code, but (as I said previously) the Pico is much more complicated to use than the Arduino, and may not add any extra value in this application. I suspect the Arduino Nano will still generate more interest in the Geotech community due to its ease of use.

[surfdetector]

xaj_ schematics.pdf

Here is the schematic for the above pdf

[surfdetector]

Do you have a link?

xaj_v2_03042021 doc.pdf

I believe that this is the document referenced.

[useus]

Do you have a link?

google rp2040 metal detector

1st link

[Qiaozhi]

among other things, by doing a search on the net, someone is already working on a PI with a simple circuit.

Do you have a link?

[useus]

I've also been experimenting with the new Raspberry PI Pico, but I'm not sure whether it makes a good candidate for a metal detector project. It's physically bigger than the Arduino Nano (despite being called Pico) and is more difficult to set up. The Pico can be programmed using MicroPython (I've used Python before, and it's somewhat similar) but my impression at this point is that the Arduino is an overall more pleasant experience. Obviously the Pico is a more powerful device than the Nano, with a lot more memory, so I might change my mind later as I get more familiar with it.

In my opinion, if you work on the pico, it could become an excellent Voodoo-style hybrid. in the specifications I read that it has two programmable fast i / o that could lead to a considerable increase in performance.
among other things, by doing a search on the net, someone is already working on a PI with a simple circuit.

[lc516]

Have you programmed the Nano?
You must do that before you can measure -5V at TP3, as the Nano generates the external clock for the 7660.

yes,I think I know what's wrong after excluding other possible problems.
I used a unmatch "MC7660", it can't runing on1k external clock.
I'll buy a new icl7660.


thanks for your information

[MartyJ1963]

Thanks again Bruce,,,Will do. Regards, Marty.