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

[moodz]

This is the FINAL version of the MAGPI v3.
There are NO changes to the PCB layout. ( ie use the originally published PCB )
Some components were added. ( Originally not fitted to PCB )
Some components were removed. ( Originally fitted to PCB)
Some component values changed. ( Originally fitted but must be changed )

The original choice of opamps was the best ... ( others were tried ... they died ).
ie LM6171 for U2 and NE5534 for U3.

CHECK THE SCHEMATIC.

Here it is.

[Gunghouk]

Thanks Moodz, board is 99.9% done and just waiting for your final deliberations on opamps etc.

[moodz]

Try this firmware .. note it is in DEVEL status but fixes some bugs from 1.4 and has the target modulated volume code.
However note that the manual volume control wont work ...

For full sensitivity there are some minor component changes .. but no changes to the 1.3 PCB.

[Gunghouk]

Post #248

2023-06-07 V1.4

[moodz]

Which version did you load .. ie post number?

[Gunghouk]

Hi Moodz, my breadboard Pic test cct transmits at 10kHz with a 50uS TX pulse width, did a timer change since v1.4?

[moodz]

Progress update .... found a bug in the software while field testing. Had to pack up and back to base. However fixed now.

The proposed final hardware configuration for MAGPI V3 is in place.

Hopefully more field testing this week.

In the mean time here is video of the MAGPI detecting a 0.1 gram nugget..... well under 1 us tau

[moodz]

Is it a 6th power ? = It is safe to assume that the signal strength decreases with the cube of distance (1/8 the signal at twice the depth), and that location errors build up with the square of depth.

Hi Pito .. I dont know for sure it is a 6th power. I can reference the work of Corbyn and this formula he derived for voltage in a recieve coil of a pulse induction metal detector.



In this case the determining variable is the h^^6 term. This is the target depth to a 6th power.

I wont repeat the calculation here but for a coke can you do expect about microvolt range signal at 1 meter from the RX coil and so I would expect a few volts of signal from the MAGPI with gain = 135db - approx 20 db of losses.
The MAGPI gets around 1 volt of deflection at 1 meter .. indicating I am in the ballpark of corbyns formula.

[moodz]

Thanks Moodz, lovin' these videos and envious of your workshop. Please keep 'em coming

From the freq response sim it would appear that there's only about.. 3.5dB of gain in the front end op-amp. Is this incredible sensitivity from the IN125 instrumentation amp?

The preamp gain is 1000 given by 100k ohm / 100 ohms.

That simulation above is from JLKing demonstrating the benefit of a 2nd order feedback filter element.
If you plot the gain of the preamp you do get 60 db gain.

So the overall gain is preamp gain x diff amp gain = 1000 x 6000 = 6 000 000 ( 6 million ) . This is a voltage gain so we have 135 db.

[pito]

Is it a 6th power ? = It is safe to assume that the signal strength decreases with the cube of distance (1/8 the signal at twice the depth), and that location errors build up with the square of depth.

[Gunghouk]

Thanks Moodz, lovin' these videos and envious of your workshop. Please keep 'em coming

From the freq response sim it would appear that there's only about.. 3.5dB of gain in the front end op-amp. Is this incredible sensitivity from the IN125 instrumentation amp?

[moodz]

There are various sources on the web that show that the detected target response from the transmit signal is proportional to a 6th power path loss.

If this is true then as the target approaches the coil the response should be very non linear. To test this we move a target to where the detector just barely detects the target then measure how much closer the target must be for maximum signal / overload.

So I took a coke can which just registers at around 50 cm off the side of the coil. With the MAGPI just moving the coke can in by 1 cm caused a full scale target deflection.
This demonstrates the target signal to noise ratio is highly non linear as the target approaches the coil.

Is it a 6th power ? Dunno .... But it must be quite significant to go from a sniff to a sneeze in just 1 cm.

Anyway heres the video
https://youtu.be/TsX0H9jwNN8?si=fAGdo4BgWHX5gKLd

[moodz]

Nice audio moodz!

Thanks ! It worked better than I thought it would.

[Altra]

Added target modulated volume control TMVC. The MAGPI uses a software VCO that responds to target voltage. The only problem with that is you have to listen to the tone when there is no target. When there is a target the tone frequency rises proportionaly to the target signal strenth the volume stays the same.

The TMVC now drops the no target volume to around 5% .... when a target is near the coil the volume will increase proportional to the target signal strength in addition to the VCO tone changing.

Much nicer ... like changing from a B&W TV to color.

Heres the video

https://youtu.be/X7eVNNC9yq4?si=BFzb0bdBHND8VilW

Nice audio moodz!

[moodz]

Nice. How does the hardware compare with the mods we've been discussing eg. the steeper low pass front end hardware, new op-amps or TX fundamentals?

Its the same hardware ... I am waiting for bits from element14 but they are coming from the UK. In the meantime I am amusing myself by working on the software.
This latest change was just software. It works fine on the original hardware.