Goldscan IV, 4 position switch question?

[Kevco]

I am not building this, but am looking at the timing signals for this detector.

I would like to know what the two "Sel 1" and "Sel 2" switch positions do.

The schematic by Alex, shows a four position switch, which appears to change the ferrous discrimination (or so it looks like). The schematic shows the four positions to be:

Off
All Metal
Sel 1 ??????
Sel 2 ??????

Anyone know what those last two "Sel X" do?

TNX,

Kevco

[porkluvr]

<snip>
I would like to know what the two "Sel 1" and "Sel 2" switch positions do.
<snip>

TNX,
Kevco

Sel1 & Sel2 (which look identical from my copy of the GS4 schematic) are discrimination modes. I included only enough of the diagram to go with my explanation. Be aware that SW1 and SW1-A face different directions in this picture.

(edit: Ch2 is "Gold" channel and Ch1 is "Iron".)

Notice that Ch1 (top half of the original page) is inverted before going to the summing pot. Ch2 and Ch1 are fed anti-phase to their respective ends of the summing pot.

Note that U13C can amplify either pin-9 signal by -1, or else amplifies pin-10 signal by +1, depending on the polarity of U13B output, and note that Sel(x) uses the signal from the potentiometer tap.

If the output from Ch1 overpowers the Ch2 output as seen at the tap, it will be seen as a positive signal at the U13B output, and D15 will block its passing to U13C.

All Metal Mode: if Ch1 overpowers Ch2 a positive signal will pass through D14 and switch Sw1. If Ch2 is stronger, a negative signal will pass through D15, and get inverted before passing to the output. Either Ch1 or Ch2 can "ring the bell".

Disc "Off" bypasses the summing pot and uses only the Ch2 signal which passes through D15.

Read this: http://www.finders.com.au/Finders_Ar.../Goldscan4.htm

I don't see the foil rejection control mentioned in the reference. That might be a sub-circuit that differentiates Sel1 or Sel2 (just a guess).

[Reg]

Hi Kev,

First, let me say that the two positions Sel 1 and 2 were added to make it possible to use the GS 4 to hunt certain English coins. When initially designed, there was one coin that didn't exist but when it became available this coin would be rejected. So, the SEL 2 mode was added such that the circuitry shifted the second or GB sample enough that this new coin could be detected while still ignoring nails. This allowed the new coin to be accepted rather than ignored.

One other important thing to note about these two settings is they add something presently found on the TDI and that is the ability to select low conductor objects only and ignore high conductor objects above a certain size. This is how the nail is rejected.

Stated a little differently, these two modes would allow the operator to detect certain coins while ignoring nails (if the ground balance was set correctly).

Basically, the manual states "These settings on the DISCRIM switch have a special function in iron rejection. This means iron does not read at all......

Set the DISCRIM switch to SEL 1 and the G.Can/Disc control to the 11 o'clock position and scan over a nail... There should be no signal... However, you should be able to detect the L2 (British coin) coin and a large percentages of rings and coins...

It also says that SEL 2 position was added to broaden the acceptance range to bring the L1 coin in and not be rejected.

So, in a nutshell these two modes were added to allow the GS 4 to work as a coin detector while rejecting most nails. Also, to be honest, these modes are basically similar to the low conductor mode on the TDI.

Personally, if I were to build this unit, I would not add the SEL 2 mode. This simplifies things a lot. Instead, I would a simple switch at the ground control that would allow one to select taking the signal from the ground pot or from the gold channel. This would give a person either GB or no GB.

I would also change another stage that would allow me to select low conductors or high conductors. This would be done at U 13 b and c (refer to the schematic portion noted in the previous post). Presently, SEL 1 or 2 allows one to select a low conductor mode. Looking at U 13b, you will note the signal going to the next stage comes from two places and goes to both the inputs of U 13C. In the SEL 1 or 2 mode, only one input is provided a signal from the previous stage. By modifying the design a little and allowing one to select the signal coming from either diode, then one could select either high conductor or low conductor. Using the position where both signals are used provides the all metal mode.

As for what is written on the Finder's forum the discussion of SEL 1 is not totally correct and is probably one of the key reasons the GS 4 never really caught on. If selected to SEL 1 or 2, large gold or high conductors would be ignored. Unfortunately, that isn't noted so it could lead a person to believe there was something wrong with the detector when in fact, everything was working as designed.

In retrospect, had I realized what was being done in this circuitry when I first studied it, I would have added the single tone feature to my GS 5 a lot sooner than I did. I would have also added it to my GS 4.

Reg

[Kevco]

Thanks for the explanations.

I was aware others were building homebrew detectors with an additional timing & integration stage, which was to provide some ground or disc abilities. So I wanted to look at what the GS 4 was doing. But until now I had no clear description of what those two Sel X positions did. I couldn't find any GS 4 user manuals, or clear photos of the control panel either.

Thanks!

Kevco

[Kevco]

Reg,

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

Kevin

[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.

[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]

I&#39;m interested in whether there is a video clip that is somewhere gs4

[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.

[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.

[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

[Kevco]

D14 &amp; 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

[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

[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 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