Voodoo Main Discussion

[nixie]

Scope pictures of a couple targets at different distances from the coil. Amplifier out, linear and log amplitude. Amplifier gain=500. Scope time base varied depending on target, 2us to 50us/div.

Thanks a lot green, very kind

[Qiaozhi]

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No longer will our Australian friends have to order books via the USA, or at inflated prices from third-party book sellers.

[green]

Hello George,
could you kindly tell me the electrical characteristics (amplitude and frequency) of the signal generated by the eddy current entering the first stage of the preamp?

Scope pictures of a couple targets at different distances from the coil. Amplifier out, linear and log amplitude. Amplifier gain=500. Scope time base varied depending on target, 2us to 50us/div.

[nixie]

Thanks so much.

[Qiaozhi]

While the way it decays should tell us if it is a ferrous or not, ...

No ... you cannot tell the difference between non-ferrous and ferrous by looking at the decay curve.
This is explained in Chapter 2.

[nixie]

Thank you George, so the presence of the target either accelerates or slows down the curve we compose, right?
While the way it decays should tell us if it is a ferrous or not, obviously after a couple uSecs, and if we don't have a target the value shown is nothing more than the influence of the terrain that needs to be accounted. (SH_EF in your schematic)

[Qiaozhi]

I'd like to know the details of the signal present on pin 2 of U3 (NE5532) if possible...
everybody says we are working with microvolts but what kind of signal is it actually?

Have a look at the attached image.

The main sample pulse captures a section of the preamp output on each TX cycle and puts it through an integrator. This produces a dc level that increases in the presence of a metal target. You will only see a signal change at the input of the preamp if the metal target is close to the coil, as the changes may only be a few microvolts for small targets or those far from the coil.

Try monitoring the preamp output on an oscilloscope using channel 2 (while triggering channel 1 on the falling edge of the TX pulse) and bring a relatively large metal target towards the coil. Note that the rising edge (where sampling takes place) moves towards the right. This will cause the sample pulse to see a signal that becomes more negative.

Then try the same procedure with a small coin at a distance that causes a weak audio response. I guarantee that you will not be able to see any movement in the preamp output, but the detector is still able to extract this miniscule change. Since the signal change at the preamp output cannot be detected visually on the scope, you will definitely not see any change at the input.

[nixie]

Sorry ... I don't understand what you're asking for.
Please explain.

I'd like to know the details of the signal present on pin 2 of U3 (NE5532) if possible...
everybody says we are working with microvolts but what kind of signal is it actually?

[Qiaozhi]

Hello George,
could you kindly tell me the electrical characteristics (amplitude and frequency) of the signal generated by the eddy current entering the first stage of the preamp?

Sorry ... I don't understand what you're asking for.
Please explain.

[nixie]

Hello George,
could you kindly tell me the electrical characteristics (amplitude and frequency) of the signal generated by the eddy current entering the first stage of the preamp?

[MartinB]

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Order Total: ￡18.29

[MartinB]

Well then it's a 7660 for sure.
Tomorrow I'll check with a microscope and see if I can find traces of writing. The most important thing is for anybody that got my same problem to be able to solve it.

Well found nixie.
Whilst we all try and save money where we can, I think the answer is to buy from a reputable supplier.

[nixie]

I wired an LT1054 on a prototype board and connected it to an Arduino Uno to supply the sync pulse via a 2N3904 with a pull-up resistor (20k) to Vref.
It worked perfectly as per the datasheet.

Then I went back and read your first post concerning this problem. You stated that removing the transistor and pull-up resistor caused the device to run at its internal clock frequency of 10kHz. This is interesting, because the internal clock frequency of the LT1054 is actually 25kHz, whereas for the 7660 it's 10kHz. Very suspicious...

Well then it's a 7660 for sure.
Tomorrow I'll check with a microscope and see if I can find traces of writing. The most important thing is for anybody that got my same problem to be able to solve it.

[Qiaozhi]

Obviously I bought it from China!
But even if I had bought it in Europe the seller would have probably bought it from China...
and I belive you are right! It may be a remarked 7660, in case you think it could be useful I can gift one to you!!!

I wired an LT1054 on a prototype board and connected it to an Arduino Uno to supply the sync pulse via a 2N3904 with a pull-up resistor (20k) to Vref.
It worked perfectly as per the datasheet.

Then I went back and read your first post concerning this problem. You stated that removing the transistor and pull-up resistor caused the device to run at its internal clock frequency of 10kHz. This is interesting, because the internal clock frequency of the LT1054 is actually 25kHz, whereas for the 7660 it's 10kHz. Very suspicious...

[nixie]

As far as I'm aware, no-one else has encountered this problem. Also, I've used this configuration in several designs without any issues.

The 7660 does not require a pull-up resistor when driven by an external clock, which makes me a little suspicious of your LT1054's authenticity. Maybe it's a 7660 disguised as an LT1054.
I'm going to set up a test rig in a prototyping board, and test both an LT1054 and a 7660 to confirm that the former does not have an internal pull-up resistor.

Obviously I bought it from China!
But even if I had bought it in Europe the seller would have probably bought it from China...
and I belive you are right! It may be a remarked 7660, in case you think it could be useful I can gift one to you!!!