Goldscan IV, 4 position switch question?

[bchampig]

Hi Reg,

Thanks, I'll check the timings and my component values to see if I messed something up. I also made sure the pulse delay isn't too short which would make the gold channel too 'hot'. I'm using TDI coils for testing, and I have tried a solid silver spoon for testing, as well as a copper clad PCB material. With these targets, the Gold channel still wins.

I'll let you know what I find after some more testing,

Thanks again,
Brian

[Reg]

Hi Brian,

To be honest, I haven't taken a microscope to the schematic so I can't say there are not other errors. However, this design uses two identical channels so they should have equal gain in the various stages up to the GB pot area. So, if you keep that in mind you can't miss.

Now, the easiest metal to get a negative response on is either silver or copper. So, you might try them first.

Next, what value do you have for a coil inductance? If this is way off, you can get strange results. I personally use the most common inductance of 300 uh. Also, if you have the delay less than 15 usec, it takes more iron signal to ground balance.

If you have the adjustments off on the various timing networks, the same problem can occur. You can set the pulse width at 100 usec and it will work fine. The sample width can be 10 usec to 15 usec. Play with the iron channel delay. If it is out too far, you can get the results you indicated. An actual delay of 10 usec to 15 usec is sufficient between the two channels. So, you might try 10 usec for the gold channel and 30 to 35 usec for the iron channel. BTW, I usually call the iron channel the GB channel.

So, there are lots of signals to check that could cause what you are experiencing.

Now, make sure to look at the output of U13b to see if it ever goes negative.

BTW, I changed the quad opamps to very low offset IC's so the need of zeroing the amps becomes basically no problem. The LT1014 or OPA4277 work well but can be pricey. I stumbled upon a good deal on the OPA's on Ebay. If the standard IC is used, the offset balance adjustments used to zero the amps become very critical and can cause all kinds of problems if not correct.

Reg

[bchampig]

Reg,

I've been playing around with my homebrew GS4, and I'm having difficulty getting the Low/High conductive modes working. It turns out, that on my board, the Gold channel always overpowers the Iron channel regardless of how the ground balance is adjusted.
In other words, I never get a negative target signal at the summing(GND) pot no matter what my target is...even a large piece of high conductive metal. The ground balancing appears to work as I can cancel out a large piece of metal as I move it towards the coil. I could probably fix this by adding some gain in the iron channel or increasing the value of the 22K resistor from the Gold channel to the GND pot, but I'm wondering if you have experienced this, or know of a schematic error in this area?

Thanks,
Brian

[bchampig]

Hi Reg,

Thanks for the drawings. I was thinking along the lines of your second drawing except the opposite sides of the diodes would be connected to the switches. So basically, instead of the gain for that path being zero(by shorting with the switch), that path is essentially open circuited with a switch instead.

Thanks again,
Brian

[Reg]

Hi Brian,

I have made a quick drawing of what I mean. Keep in mind the drawing doesn't show all the parts in the two stages, but simply shows how the switch could be connected.

Actually, there are two possibilities of how the switch could be used. The top portion of the pic shows how to open one or the other signal from the diodes going to the next stage.

On the bottom opamp, the idea is to drop the gain to nothing on the signal you don't want. When the switch is in the center position, all signals are available just like the original. Change the toggle to up or down and one or the other diode feedback ckts have the gain dropped to nothing. The rest of the circuitry would be the same.

Reg

[bchampig]

Hi Reg,

Let me see if I understand you with regards to replacing SW1B with a 'special' toggle switch. Are you saying that you would put a switch at the common node(pin 7 of opamp) of the two diodes, so that in the center toggle position, the diodes share the common node, and the other two positions each disconnect one of the diodes from the common node(pin7 of opamp)? So basically, in the center position, the circuit is configured like the standard 'all metal' mode. In Low or High conductor modes, the configuration is the same as 'all metal' except that one leg of its respective diode is 'open'?

Thanks,
Brian

[Reg]

Hi,

First, I would like to thank Alex for the schematic and the pcb layout. This is a very complex detector and he did an excellent job of recreating it. There may be some minor mistakes but they are very few. Given the complexity, I feel Alex did an excellent job.

This could be an excellent project to better learn how a ground balancing detector really works. With a few changes this detector could be simplified also. The fact is, there is no need for SEL 1 and 2 modes for most applications. Instead, other changes could be made to make it more like one of the newer factory made PI's that allows a person to select between high conductor and low conductor objects.

So, as a means of making the detector less complex, the multi position selector switch could be eliminated and replaced by a couple of simple toggle switches. In other words, sw 1-C could be eliminated. SW1-A replaced with a toggle switch switching between the gold channel and the GB control. SW1-b could be replaced by a toggle also. This would be a little different toggle that would be closed in the center position. In the other two positions, the signal going to the two inputs would be opened, with one open in one position and the other opened when switched the opposite way.

This simple mod would allow the operator to select either low conductor, high conductor or all mode. In the all mode, the signal from both diodes would go to the next stage. In either of the two modes, one if the two signals would be opened.

Now, to also clear up a couple of things on the schematic, you will see two diodes at the input of the preamp with both diodes installed with the same polarity. These are D8 and D9 on the schematic.

Now, everyone makes the assumption that one is backwards, right? Well, that may not be the case at all. Instead, the only polarity that has high voltage that needs to be clamped is the positive voltage, so using two parallel diodes may have been done deliberately. It will work just fine with the two diodes installed the way they are drawn.

Other simple changes would be to eliminate the piezo and the meter. Neither are needed. The hold button could be eliminated also. The reset button could be internal since the only time it would be used is in calibration.

So, there are ways to simplify the design and still have a very complex ground balancing detector.

Now, OA85 are really BAT85 diodes. Using Schottky diodes does give a lower forward voltage drop.

Reg

[Kevco]

D14 & D15

Hi Reg,

I had read those OA85 diodes are germanium. Are they used in this situation for their lower forward bias voltage, to isolate the low or high conductor signals?

All of this is quite eye opening, especially after getting my own HH working, and having a better understanding of the circuit operation.

Kevco

[Kevco]

Thanks porkluvr,

Now after understanding more of this circuit, I agree that one switch is mislabeled (backwards). I saved that schematic, along with the previous one I have (i.e. GS4.pdf).

However, I'm Kevco, and I didn't post that 5 page schematic elsewhere Someone else maybe?

Kevco

[porkluvr]

I know that my copy of Alex's schematic had the switch SW1C drawn incorrectly . (Ummh, that may have been MY bad editing.) My apology for any confusion this caused.

The attached version is correct so far as I know (except for either D8 or D9 needing reversed), but only Eric Foster can say for sure.

This page was extracted from a five-page pdf file found in a zip file that Kevco posted in another thread.

[porkluvr]

Something Reg said made me remember that there is a third switch gang that I did not consider. (Thanks, Reg.)

Going on the assumption that Sel2 is different from Sel1, am I correct in deducting that the legend for Sw1-C shown here is upside-down, and that SW1-C should be labeled 1~4 from bottom to top, instead of top to bottom?

That would explain a lot because the other two switch gangs as shown in Alexismex's original drawings do not imply any difference in Sel1 and Sel2 modes... .

Just ASKING at this point in time.

edit: I only just now noticed that the Isis drawing and Alexismex's original four part schematic do not agree regarding the mode switch... . A clue, a clue.

[Orbit]

I'm interested in whether there is a video clip that is somewhere gs4

[Orbit]

Likewise IRF740 much heated probe, which is given, range is also very small and most of all his selections is wrong and a lot of drifting.

[Orbit]

whether the gs4 someone did the first question ,another question kolko the unit is stable because I think there are a few deliberate mistakes that make it unstable.

[Kevco]

Reg,

I see you posted this a few hours ago. You must either be up early, or have insomnia.

Kevin