Moodz' Awesome Gold Pulse Induction Version 3 - MAGPI V3 Project

[moodz]

Hello. Please help. The metal detector is very sensitive, it reacts to a hand, a wooden ruler, a plastic ruler at a distance of 20-30 cm. Which screen for the coil would be better, aluminum foil or graphite. If graphite, what resistance should be at a distance of 1 centimeter. The only change I made was the use of LM7171. Thank you

If the coil is picking up your hand then most likely there is no faraday shielding on the coil.

You can use graphite paint ( make your own out of a few drops of varnish mixed with acetone and large spoonful of graphite powder in a glass jar.)
Lay down a very very thin copper wire connected to ground of the coil cable along the coil circumferance ( however do not connect the ends of the copper wire to form a loop !! )
Paint the graphite mixture onto the coil in a thin continous coat ... but leave a gap ( eg 5 mm ) so no loop is formed.

test and reapply graphite if necessary.

[AhmatAhmatov1959]

Please post some photos of the board top and bottom and we will try to assist.

Hello. Please help. The metal detector is very sensitive, it reacts to a hand, a wooden ruler, a plastic ruler at a distance of 20-30 cm. Which screen for the coil would be better, aluminum foil or graphite. If graphite, what resistance should be at a distance of 1 centimeter. The only change I made was the use of LM7171. Thank you

[AhmatAhmatov1959]

Please post some photos of the board top and bottom and we will try to assist.

Hello. Thank you for your reply. I replaced INA125 and it worked. I buy all the parts from ALI. The metal detector has a sensitivity of 20 cm on the hand. On ferrite 15 cm. On copper getinax 50-60 cm. The gold in the photo is compressed foil, it is not an ingot. 3-4 cm. The coil is homemade with a diameter of 20 cm. I made a rope from 44 insulated wires 24 meters long and 0.17 mm in diameter wound 26 turns, left 1 meter at the beginning and 1 meter at the end. The initial meter was wrapped with 10 layers of Teflon tape and put into a heat pipe, and on top I made a screen from the last meter, additionally added 5 wires to the screen for connection to the graphite screen. The Chinese tester showed 280 mcg, 0.4 Ohm and 850 kHz together with the cable, I thought that this was a good result. If of course the tester is not lying. I have several questions for you. 1 What to do with the voltage on the coil. 2 What transistor can replace the BSP230 for higher voltage. 3 How to reduce the influence of ferrite.
I soldered a 120 pf capacitor parallel to the coil but did not notice any changes. Thank you very much for the provided project. I enjoyed making it. Before that I made a Tecoro Lobo, but after 3 unsuccessful coils I decided to do only pulse metal detectors.

[moodz]

Hello. I assembled Magpi 3. The speaker produces a constant sound that disappears when the target is brought closer. If anyone has encountered something similar, please help. Sorry for my English

Please post some photos of the board top and bottom and we will try to assist.

[AhmatAhmatov1959]

Slow drift can be a symptom of two things.

The differential amp is overloaded / railed .... the voltage at the oupuf of the differential amplifier U1 should be fairly constant at around 1 to 1.5 volts. If it is closer to 0 or 2.8 volts then the amplifier is seeing an offset current due to ..

1. Leaky input capacitors ... the DC blocking caps must be high quality film caps C6 and C7. Even a very small leakage current will cause an offset DC votage that will rail the output. The integrator caps need to high quality too. Ceramic caps are way too leaky.

2. Overload input ... the samples from the integrators R13, R14, C4, C5 are overloading the input to the amplifier. If the difference between voltage inputs exceeds 150 microvolt approx then the diff amp will overload. Bad sampling windows will do this.

Hello. I assembled Magpi 3. The speaker produces a constant sound that disappears when the target is brought closer. If anyone has encountered something similar, please help. Sorry for my English

[Gunghouk]

Hi Moodz, what is the latest version for LCD display ? If there is, any chance of a copy please ? I'm currently using 1.5 with non working volume.
Thanks

[moodz]

Hi Moodz.
Thanks for answering my question.
I have done everything you have told me;
I have added a 10 Ohm resistor in series with the gate of the mosfet Q2.
I have put the PCB inside a metal box and connected the tab of the LM7805 regulator to the chassis of the box, as you show in the photos.
I have turned off the LED lights in my lab for a moment to see what happens.
The detector is powered by Lithium Batteries, not by laboratory power supplies.
I have also tried cutting the power supply track on the PCB that you mentioned in a previous post and I have powered the DG411 directly from the Battery as you were explaining in a previous post. All this without success and without noticing any difference. The only thing that works is changing the TON parameter value to 94 or 96 and the SIG parameter value stays oscillating around 400. It is the only way to make the detector work stably.
I have been playing with all the possible parameters, changing all the values and without success, except for the TON parameter, mentioned above.
I have noticed that the detector, working stably, does not reach the depth and sensitivity that your detector in the videos you posted reaches.
I have noticed that the detector rejects ferrite and hot rocks very well, but it does not have any kind of discrimination.
I would have liked it to have at least discrimination for High or Low conductivity like the TDI or the Minelab GPX4500, because here in Europe we also like to search for gold nuggets, but the fields and rivers are full of ferrous trash, etc. In Australia I suppose that in the desert there will not be so much trash and that all the positive signals are gold nuggets.
One last question and I'm done;
Is there any possibility of some kind of discrimination in this detector, since I have now got it working? I have searched all your posts and have not found any firmware available.

Best regards,
Eduardo

Hi Eduardo. Not all the signals in Australia are gold nuggets but we do have much less trash than the rest of the world ( except maybe Africa ).

Unfortunately to discriminate there would have to be some changes to the front end and firmware. It really needs an extra channel ... as some targets will disappear with discrim enabled. ( target appears as half ferrous / half non ferrous ).

[moodz]

Hi Eduardo, I have also been suffering from similar stability problems as you. Installing the 10 ohm gate resistor on Q2 cleared my spurious supply issues and more recently I have reinstated R6, the 10k feedback resistor on the preamp U2. This reduced a lot of the instability albeit for the loss of some signal gain. I am now working on finding working settings for the operating parameters starting from Ton 7.8, Smp 8.0, Xsw 1 and an Xss value of 1.46 us.

Things are looking up and I may try reducing the overall gain further to stop the slow drifting probably by playing with R7 on U3 to something less than the 100k shown in Moodz final circuit (the original was 10k).

I've found that presenting a target will stop/reset the drift temporarily before slowing taking off again.

Best regards

Slow drift can be a symptom of two things.

The differential amp is overloaded / railed .... the voltage at the oupuf of the differential amplifier U1 should be fairly constant at around 1 to 1.5 volts. If it is closer to 0 or 2.8 volts then the amplifier is seeing an offset current due to ..

1. Leaky input capacitors ... the DC blocking caps must be high quality film caps C6 and C7. Even a very small leakage current will cause an offset DC votage that will rail the output. The integrator caps need to high quality too. Ceramic caps are way too leaky.

2. Overload input ... the samples from the integrators R13, R14, C4, C5 are overloading the input to the amplifier. If the difference between voltage inputs exceeds 150 microvolt approx then the diff amp will overload. Bad sampling windows will do this.

[Gunghouk]

Hi Eduardo, I have also been suffering from similar stability problems as you. Installing the 10 ohm gate resistor on Q2 cleared my spurious supply issues and more recently I have reinstated R6, the 10k feedback resistor on the preamp U2. This reduced a lot of the instability albeit for the loss of some signal gain. I am now working on finding working settings for the operating parameters starting from Ton 7.8, Smp 8.0, Xsw 1 and an Xss value of 1.46 us.

Things are looking up and I may try reducing the overall gain further to stop the slow drifting probably by playing with R7 on U3 to something less than the 100k shown in Moodz final circuit (the original was 10k).

I've found that presenting a target will stop/reset the drift temporarily before slowing taking off again.

Best regards

[eduardo1979]

1. ground the the tab of the 7805 ( no long wires )
2. place 5 to 10 ohm series resistor in the gate of the damping mosfet Q2 ( if step 1 does not improve things )
3. Install the pcb in a shielded case ideally as small capacitive coupling will cause false targeting / drift as any capacitive changes affect the coil reactance.

Note also that the circuit is sensitive to temperature changes such as air blowing on the front end. ( the mosfet bias operating points will change with temp variation ).
In a closed box ... this is not an issue as the air movement is low and any delta Temp is not fast enough to make it through the filters.
It is easy to show this by just blowing on the circuit when it is operating.

Further note. If you are testing the detector in your lab / workshop and you have a switch mode supply such as used in LED lighting ... this may cause interference to the detector ( presume harmonics from the inductor in the switch mode supply. ) In my lab I cannot use the main lighting as it is LED and creates noticeable noise that appears as instability in the detector. Turning off the light restores low noise operation.
Presumably other types ( but not all ) of switch mode supplies may cause this issue also.

m.

Hi Moodz.
Thanks for answering my question.
I have done everything you have told me;
I have added a 10 Ohm resistor in series with the gate of the mosfet Q2.
I have put the PCB inside a metal box and connected the tab of the LM7805 regulator to the chassis of the box, as you show in the photos.
I have turned off the LED lights in my lab for a moment to see what happens.
The detector is powered by Lithium Batteries, not by laboratory power supplies.
I have also tried cutting the power supply track on the PCB that you mentioned in a previous post and I have powered the DG411 directly from the Battery as you were explaining in a previous post. All this without success and without noticing any difference. The only thing that works is changing the TON parameter value to 94 or 96 and the SIG parameter value stays oscillating around 400. It is the only way to make the detector work stably.
I have been playing with all the possible parameters, changing all the values and without success, except for the TON parameter, mentioned above.
I have noticed that the detector, working stably, does not reach the depth and sensitivity that your detector in the videos you posted reaches.
I have noticed that the detector rejects ferrite and hot rocks very well, but it does not have any kind of discrimination.
I would have liked it to have at least discrimination for High or Low conductivity like the TDI or the Minelab GPX4500, because here in Europe we also like to search for gold nuggets, but the fields and rivers are full of ferrous trash, etc. In Australia I suppose that in the desert there will not be so much trash and that all the positive signals are gold nuggets.
One last question and I'm done;
Is there any possibility of some kind of discrimination in this detector, since I have now got it working? I have searched all your posts and have not found any firmware available.

Best regards,
Eduardo