Analog Target Indicator for VLF

[Melbeta]

Since you mentions the center deflection of the meter needle, back in the 1980s Compass first came out with the Good/Bad meter, called the X70 model. Now the next year they came out with the X80, and it was similar to the X70, but had the TID meter. I was curious, and I drew out the MSC circuit, which was the meter sending circuit, of both the X70 and the X80. But at that time, I was too busy to do more with it. I liked the Coin Magnum, which was a VLF that used discrimination that was NONE MOTION, and I thought that machine with the TID meter would be awesome. Yes, I would have like to stick in the TID meter into the Coin Magnum machine. And that is where it ended with me. Never went beyond that drawing comparing the two MSC circuits. Is this kind of interesting to you? Somewhere I think I still have that sheet stored somewhere in my things.
Melbeta

[Nandor]

Well the first thing I noticed is that there was no effect on the detectors sensibility with the test circuit connected to the comparator inputs without any buffer or pre amps.
The bias current is very important, I waved the indicator and the needle also moves and thats not how it should be working. Taking the needle in middle position would be also nice maybe it would be not affected by detector waving.
VCO is in my head also, with some window comparators or just a simple vco oscillator at the needles input. I have many ideas, just have to pick the working one.
I really want to make this project, it's really useful. When detecting with my friends Whites 6000 we have observed that coins almost every time are in middle or a bit higher position on the needle scale but sound is the same. Things that drive the needle up to maximum are mostly alu cans and other junks close to surface.
Back to my drawing board.

regards

[waltr]

Remember that the TGSL circuit uses a Comparator, U106a & b, to indicate a good target. It is only when both input to U106 are above the comparator's reference (sen. pot) that the audio is triggered. Iron targets will have either the GB or the Disc channels outputs of U104, U105, go lower.

If you are picking off the outputs of U104 & U105 to produce an analog signal to drive a meter, then both iron and non-iron will cause meter movement. Maybe you need to have a DC Bias to put the meter needle at center with No target. Then non-iron moves one direction and iron the other direction indicating positive and negative phase of the GB verse Disc channels.

Not sure how you would get "king of ground cancel' with this.

Another thought I have once circuit works:
The analog signal could also be an input to a audio VCO. Then there will be a varying audio tone indicating target types. Probably would want the audio out be gated with the comaprators (U106) so audio is quite with no target.

[Nandor]

Yes, amplification is not really needed maybe for small signal levels. The prototype pcb I made amplifies the difference between the 2 channels and it moves the needle on coins and also on iron screwdriver but not really on aluminium, less difference. There is phase shift between disc and geb response to iron, thats what I would like to measure somehow so the circuit could discriminate iron for real and amplify only signal aplitude differences. Also if Geb channel is not triggered there should be no detection, this way I could make a king of ground effect cancelation on the VDI.

[waltr]

I have made a test circuit, differential amp with 10x gain and a comparator stage. Well it drives the needle but it works the opposite way because it's showing mostly ferrous targets. Non ferrous targest get lower indication than iron objects, maybe because the amplitude difference betwen the 2 channels.
This circuit must be done more complicated like the the Whites one.
Anybody some ideas?

Sounds like maybe you just need to invert the signal with another op-amp.

[Nandor]

I have tried it , really happy about my o-scope! Forgot to tell, Blue is Disc and Orange is Geb channel. Pure ferrite makes some insane amplitudes in the Disc channel only, so I think even the VDI should be ground balanced somehow, will see. I was thinking about a phase difference detector to cancel out ferrite and iron objects with logic gates, I don't want to copy Whites stuff.

regards

[Qiaozhi]

Did some measurements today, and had some interesting results. Using the oscilloscope it's easy to see the differences between the reaction of Geb and Disc channels to different targets that pass the coil.
These are: Iron screwdriver, Alu pulltab, 1Euro, and an old Silver coin.

https://i.postimg.cc/KYHmdBX1/Iron.jpg

https://i.postimg.cc/8cVgSbZz/Aluminium.jpg

https://i.postimg.cc/GpBWTfNp/1Euro.jpg

https://i.postimg.cc/pVQgjz0K/Silvercoin.jpg

There is amplitude difference between non ferrous targets, and there is amplitude phase difference between the 2 channels if ferrous objects are detected. I think this needs to be measured somehow apart from signal difference to discriminate iron with analog VDI.

channel.
If you put your scope into XY mode, you can then plot the GEB channel versus the DISC, and see the phase and amplitude changes in a polar plot.

[Nandor]

Did some measurements today, and had some interesting results. Using the oscilloscope it's easy to see the differences between the reaction of Geb and Disc channels to different targets that pass the coil.
These are: Iron screwdriver, Alu pulltab, 1Euro, and an old Silver coin.

https://i.postimg.cc/KYHmdBX1/Iron.jpg

https://i.postimg.cc/8cVgSbZz/Aluminium.jpg

https://i.postimg.cc/GpBWTfNp/1Euro.jpg

https://i.postimg.cc/pVQgjz0K/Silvercoin.jpg

There is amplitude difference between non ferrous targets, and there is amplitude phase difference between the 2 channels if ferrous objects are detected. I think this needs to be measured somehow apart from signal difference to discriminate iron with analog VDI.

[Nandor]

I have made a test circuit, differential amp with 10x gain and a comparator stage. Well it drives the needle but it works the opposite way because it's showing mostly ferrous targets. Non ferrous targest get lower indication than iron objects, maybe because the amplitude difference betwen the 2 channels.
This circuit must be done more complicated like the the Whites one.
Anybody some ideas?

[Nandor]

Soon I will post a schematic that Im working on.

[waltr]

Looking at the Whites schematic, I have figured it out that comparing the 2 channels with a differential amplifier would be the best way. To amplifie the signal difference between the Disc and Geb channel as almost each time a high conductive target is in the detection field Disc channel reacts better than Geb so there should be an amplitude difference.

That sounds like a good way to go.
Very interested in how well this works.

[Nandor]

Looking at the Whites schematic, I have figured it out that comparing the 2 channels with a differential amplifier would be the best way. To amplifie the signal difference between the Disc and Geb channel as almost each time a high conductive target is in the detection field Disc channel reacts better than Geb so there should be an amplitude difference.

[Nandor]

I have understood the working principle of the digital VDI as I have learned trigonometry, but in analog electronincs it's a bit more complicated.
My goal is too use as few components as possible and with low power consumption.
Fisher CZ series also have nice analog VDI Meters.

[waltr]

I have built the arduino VDI but it needs separate battery and consumes almost as much as the detector, also the LCD screen at daylight is a bit weak.
Analog is more interesting to me, I was thinking of making some kind of comparator VDI circuit that compares the amplitude difference between the Disc and the Geb channel.
Geb channel has weak response to good conductivity targets because of the amplitude change but Disc channel is more sensitive to phase shift.
For example the TGSL disc is based on a logic gate, the comparator is basically an AND logic gate, it opens if boot channels are high. But it cant see the difference between their amplitudes, so maybe if there would be a circuit to measure and process this signal difference it could drive the Meter to show some kind of target identification, right?

That is what I was aluding to:
Build analog circuit that are not 'comparator' like what is already in the TGSL.

You have inputs 'X' & 'Y'. In the VDI these are then used with math as arctan(Y/X) = target ID angle.
simply Y/X would give something that is related the target 'angle'.
Analog divider circuits are around but barely used anymore due to cheap, fast processors. Search internet for idea.

[Nandor]

I have built the arduino VDI but it needs separate battery and consumes almost as much as the detector, also the LCD screen at daylight is a bit weak.
Analog is more interesting to me, I was thinking of making some kind of comparator VDI circuit that compares the amplitude difference between the Disc and the Geb channel.
Geb channel has weak response to good conductivity targets because of the amplitude change but Disc channel is more sensitive to phase shift.
For example the TGSL disc is based on a logic gate, the comparator is basically an AND logic gate, it opens if boot channels are high. But it cant see the difference between their amplitudes, so maybe if there would be a circuit to measure and process this signal difference it could drive the Meter to show some kind of target identification, right?