Goldscan IV, 4 position switch question?

[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

[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

[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).

[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