Analog Target Indicator for VLF

[Nandor]

I would like to try to make an analog target indicator for my vlf detectors. Did some tests with my TGSL pcb but the needle moves at all targets kinda intensitivity indicator not really VDI. My friend has got an Whites 6000 di Pro SL and it has a really good indicator, big iron the needle does not move but sound is good so it's very nice and usefull. I have found the Whites indicator schematic here on the forum but the board is huge and I think also consumes much current, my goal would be around 10mA.
Does somebody have some good ideas in what direction to go, Logic gates or simply amplitude or phase comparison?

[bernte_one]

lcd skala
https://de.elv.com/electronic-assemb...CABEgIBXPD_BwE

[waltr]

I have the VDI display on the TGSL I built. This works well and uses both channels and Trig math for the target ID.

Something similar could be done in analog circuits to a meter.

[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?

[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 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.

[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.

[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]

Soon I will post a schematic that Im working on.

[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]

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.

[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]

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

[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]

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.