MPP revE need help

[Albdelafar]

I my name is Alban I am 28 years old I am from France and I like diy projects.
I wanted to start an electoniqual project and i chose to try to build a metal detector so i find this forum and i choose to build the MPP rev-E for the good builds instructions and the support on this forum.
I buy all the parts and an old oscilloscope for 50? and let's start the project.

The three first step was ok !!

For the fourth i choose to replace R4 by a 1k ohm potentiometer to provide adaptable damping resistor if i wanted to adapt new coils on my future MMP E.
I build a coil with 17 strand of 1mm outer diameter copper wire. i used a loop diameter of 25cm and I find a 250mH inductance on this site https://www.allaboutcircuits.com/too...ce-calculator/

-1st theat : did this coil can fit with this detector because of is low inductance ? or i need to build a new one with thinner wire ?

-2nd threat : i tried to adapt the damping resistor but i don't understand the signal that i read on tx pin... i see the diferences on the signal when i moving the damping resistance but i' m not sure to see the critical damping value. I puted the resistor to 300ohm that seem to be the point of change on signal reaction and i chosed to continue.

Now i'am on the fifth step and when i put a cola can on the coil i just see a very little movement on tp3. Is my osciloscope signal is normal ? Do I need to change something to get a better signal reaction when i put a cola can on the coil ?

I hope you will help me to improve my knowledge on how i could build this metal detector.
Thank you for helping !!
Best
Alban
https://drive.google.com/open?id=1Rx...7cCboeqeh3AUFC
https://drive.google.com/open?id=13B...WNcnRz0CUQpLlo

[waltr]

Step 5 says:
"Place a cola can close to the coil, and the voltage should drop
by a few 100s of millivolts. A coin will only drop the voltage by a few 10s of millivolts."

What is the vertical scale on the O'scope?
I am guessing 1V/div. Then the Voltage drop with a coke can is about 100mV.
This is a less than Step 5 indicates and may be due to the lower coil inductance.
Otherwise it looks like Step 5 is working.

[Qiaozhi]

There is a coil calculator on the forum here -> Coil Calculator

A metal detector is not the easiest project to start with if you're trying to learn electronics.
In order to gain a better understanding of how metal detectors work, you will find this useful -> Inside the METAL DETECTOR - Second Edition - Published 2015

[Albdelafar]

Thank you for your reply.
I made two short videos to explain what i see.
https://drive.google.com/open?id=1QJ...45Mfw1rl7KIYjI
https://drive.google.com/open?id=1SR...M9aGaSF2POxLQw
Sorry for my poor english level i do my best
I will do a video for the damping resistor signal too.
Before to do step 6, do you thing i need to modifie the tx pulse width and rate ? i use the mono coil recomandations with pulse width of 100?s and pulse periode of 1ms.
https://drive.google.com/open?id=1kF...i6ApbHpqhoYGeW

EDIT with good links and damping resistor video ....

[Albdelafar]

Thank for you reply Qiaozhi
To easy project are not enought interesting for me. I used this coil calculator before but it doesn't give to me the same results that i find on others online inductance calculators. I don't know why perhaps you can give me the reason why ?
This calculator give me a 450mH coil inductance and it's why i choosed to build my coil like this at begining of my project.

For the paper to help understand how metal detector work i tryed to read it but it's sometimes hard because of my english level

[dbanner]

mH - millihenry

uH - microhenry

[Qiaozhi]

Thank for you reply Qiaozhi
To easy project are not enought interesting for me. I used this coil calculator before but it doesn't give to me the same results that i find on others online inductance calculators. I don't know why perhaps you can give me the reason why ?
This calculator give me a 450mH coil inductance and it's why i choosed to build my coil like this at begining of my project.

For the paper to help understand how metal detector work i tryed to read it but it's sometimes hard because of my english level

You may have input the diameter of the coil into the calculator instead of the radius?
The Geotech calculator gives 199uH for 17 turns of 1mm wire on a 125mm radius.

The online calculator that you used is not suitable for air-cored coils. It is for calculating solenoids.

[Albdelafar]

ok thank you for your explaination !!
So do you think i need to build a new coil if 199?H is not enough for this detector ?
I have a 1kw motor winding with very fine wire i can try to do something with it.
Because of covid-19 i can't buy anything and i need to find something to do or i will be crazy

[Qiaozhi]

ok thank you for your explaination !!
So do you think i need to build a new coil if 199?H is not enough for this detector ?
I have a 1kw motor winding with very fine wire i can try to do something with it.
Because of covid-19 i can't buy anything and i need to find something to do or i will be crazy

Try 21 turns of the 1mm wire for an inner radius of 125mm. This should give an inductance of 297uH.
22 turns will give 324uH. Either should be ok.

The damping resistor will probably need to be 680R. You should not simply use a potentiometer for adjusting the damping resistor value, as it will most likely burn out due to the high flyback voltage. You should use a damping resistor tool (see attachment) to determine the correct value.

[Albdelafar]

Thank you Qiaozhi I will build a new coil with an internal radius of 125 mm and 25 turns to be as close as possible to 400?H.
For my potentiometer, I buy this :
https://fr.farnell.com/bourns/3590s-...-1k/dp/1700941

It is given for 2w, 1500 VAC minimum and an insulation resistance of 500 VDC I hope it could survive because I do not have the necessary resistances for your damping resistance tool. If it burns, I will buy the resistors to build your damping tool but I do not know how long I will wait before being delivered ...

[waltr]

The resistor values in the Damping Tool can be changed slightly.
For example: the 2k could be 2.2k or 1.8k and the tool will still do what you need. That is find the optimal damping R value and this is done after adjusting the pot then remove the 'Tool" and measure what the resistance across the Tool is.

What you need from the Tool is a resistance of about 200 to 2k Ohm. Since you are learning electronics calculating the Tool's resistance range is a very good excise.

[Albdelafar]

Thank you waltr for your reply. I know resistor calculation rules but i have just 1000ohm CMS resistors and no 10k potentiometer.
I need to invest i think...
i will try to continue whitout optimal damping resistor and come back later to adjust it.

[Qiaozhi]

Thank you waltr for your reply. I know resistor calculation rules but i have just 1000ohm CMS resistors and no 10k potentiometer.
I need to invest i think...
i will try to continue whitout optimal damping resistor and come back later to adjust it.

Just fit a 680R damping resistor for the moment. You can check how well it's working by looking at the preamp output on the scope, and then either fit a higher or lower standard value if necessary.

[Albdelafar]

I need a 2w resistor for damping the high voltage it's right ?
Can you confirm that my montage is good for damping calculation and o'scope measurement please ?
Thank again for your help this forum is really helpfull !!

[Qiaozhi]

I need a 2w resistor for damping the high voltage it's right ?
Can you confirm that my montage is good for damping calculation and o'scope measurement please ?
Thank again for your help this forum is really helpfull !!
[ATTACH]49762[/ATTACH]

Your diagram is very confusing. It looks like you've taken the two wires from the coil and shorted them together.
However, I think you're confused by the fact that the MPP can use either a mono coil or a two separate TX and RX coils.
Your coil is a mono coil, which means that the same coil is used for both transmit and receive. There is not one wire designated TX and the other RX, as shown in your diagram.

Forget about the electrostatic shield for the moment. Connect one wire from the coil to PL1 pin2, and the other wire to PL1 pin1 (or pin3). Then short PL1 pin2 to PL2 pin2.
The damping resistor goes between PL1 pin2 and PL1 pin1 (or pin3).