TGSL Experiments

[dfbowers]

Really appreciate your efforts for the rest of us!

Maybe we'll actually get to the bottom of what's going on here.

-SB

Simon,

The latest noise measurements of Pin 6 on my LM308s.. Far away from the house, the only local source of noise would be my scope, and one incandecent light bulb.

U104, about .02v p-p
U105, about .01v p-p

Measurements are somewhat subjective.. but my best estimate is above.
The noise seemed to be random, nothing to sync on.
Offset was ever so slightly positive, maybe .005v or less.
Results were reproducable.. In fact, I moded TGSL#2 (the version with TL071s instead of LM308s). Both circuits function identically. Bench test of 40cm+ with no chatter... even two different coils.

Now for the real results: No increase in detection depth on buried objects.. just an impressive bench test. It DOES however improve the audio somewhat on detectable targets, so increasing sensitivity does offer some benefits in my opinion...

Overall, a setting where a 1e coin can be detected in air at 30 to 35cm seems optimum. Nothing I have can do much better in my dirt. Maybe better in sand? Anyway.. Don't have to "scrub" the surface to pick up shallow objects!!

Here is an easy mod I did with TGSL#2. Just clip one wire to the 10k pot and move it to the closest jumper with -5v. (Look close at the 2nd pic.)

Don

[simonbaker]

Simon,

The latest noise measurements of Pin 6 on my LM308s.. Far away from the house, the only local source of noise would be my scope, and one incandecent light bulb.

U104, about .02v p-p
U105, about .01v p-p

Measurements are somewhat subjective.. but my best estimate is above.
The noise seemed to be random, nothing to sync on.
Offset was ever so slightly positive, maybe .005v or less.
Results were reproducable.. In fact, I moded TGSL#2 (the version with TL071s instead of LM308s). Both circuits function identically. Bench test of 40cm+ with no chatter... even two different coils.

Now for the real results: No increase in detection depth on buried objects.. just an impressive bench test. It DOES however improve the audio somewhat on detectable targets, so increasing sensitivity does offer some benefits in my opinion...

Overall, a setting where a 1e coin can be detected in air at 30 to 35cm seems optimum. Nothing I have can do much better in my dirt. Maybe better in sand? Anyway.. Don't have to "scrub" the surface to pick up shallow objects!!

Here is an easy mod I did with TGSL#2. Just clip one wire to the 10k pot and move it to the closest jumper with -5v. (Look close at the 2nd pic.)

Don

This is great. Thanks for getting done in a few days what I can't do in a year!

The mod is beautiful and no real reason not to do it, IMO. I think for marketing reasons Tesoro didn't want people to turn on their machines to a lot of beeping.

[As a temporary measure, people can skip cutting the wire and just run a jumper to the -5V buss (just wastes another 5 milliamps through R37). Whatever is easier.] (let me think about this, I may not be correct as to dfbowers mod)

I will continue my thoughts with a post to the TGSL Tuning thread.

Many thanks.

-SB

P.S. dfbowers -- the Geotech site is having some kind of trouble with your photos, oversized margins. Any ideas?

[simonbaker]

Hi dfbowers -

Looking at the photo of your mod on the PCB, it is hard to tell the connections. It looks like the pot wiper goes directly to the LM393 inputs, although your schematic indicates should pass through the 220k resistor first.

Also not clear what happens with the jumper wire that was there -- is it intact or altered?

Can you describe a little more?

-SB

[dfbowers]

Hi dfbowers -

Looking at the photo of your mod on the PCB, it is hard to tell the connections. It looks like the pot wiper goes directly to the LM393 inputs, although your schematic indicates should pass through the 220k resistor first.

Also not clear what happens with the jumper wire that was there -- is it intact or altered?

Can you describe a little more?

-SB

Simon,

I would just go by the shematic that I posted. I just moved the wire that was connected to one end of R36 FROM R27 TO -5v.
The bare jumper wire was a convenient place just to test.

You are right about the change is current consumption. A 20K pot would now be a better choice for R36..
Don

[simonbaker]

Simon,

I would just go by the shematic that I posted. I just moved the wire that was connected to one end of R36 FROM R27 TO -5v.
The bare jumper wire was a convenient place just to test.

You are right about the change is current consumption. A 20K pot would now be a better choice for R36..
Don

I get it finally... my eyes thought the jumper wire was cut and pot wire went through a hole - just shadows now I see... all is consistent with your schematic!

So my comment about extra current really doesn't apply to the way you do it, not worried about the little extra current through the pot. I was thinking some people may want to just do an alligator clip jumper from pot lead to the -5 without moving the wire. Then you get constant drain across the 1k resistor. Your mod is fine and dandy as is. Now I've got to get my carcass out there with my rig and test noise and play with mod.



-SB

[dfbowers]

I get it finally... my eyes thought the jumper wire was cut and pot wire went through a hole - just shadows now I see... all is consistent with your schematic!

So my comment about extra current really doesn't apply to the way you do it, not worried about the little extra current through the pot. I was thinking some people may want to just do an alligator clip jumper from pot lead to the -5 without moving the wire. Then you get constant drain across the 1k resistor. Your mod is fine and dandy as is. Now I've got to get my carcass out there with my rig and test noise and play with mod.



-SB

I'm thinking that this mod is more of a Bandaid approach.. But it proves a point. The real fix is to install an offset circuit for each LM308, or TL071. It would only take 2 resistors and 2 pots. Your "stereo" idea is probably the more correct thing to do. Do we really know what the threshold level of an LM393 really is? The data sheets only say that it compares signals "near" 0.

Also, what happens when we fine tune the connections between the LM308s and the LM393s? Suppose we can get both channels to switch more closely in unison? In playing with mine, adjusting the reference point for the comparators effects things in a very non-linear way around the threshold.

The original design most likely has a ton of marketing influence built in.. Minimal parts, minimal returns for repairs under warranty.. Machines that work under a wide range of environments. Sell the thing, make a profit and never see it again! But that's not what we are trying to do.

[simonbaker]

bust

I'm thinking that this mod is more of a Bandaid approach.. But it proves a point. The real fix is to install an offset circuit for each LM308, or TL071. It would only take 2 resistors and 2 pots. Your "stereo" idea is probably the more correct thing to do. Do we really know what the threshold level of an LM393 really is? The data sheets only say that it compares signals "near" 0.

Also, what happens when we fine tune the connections between the LM308s and the LM393s? Suppose we can get both channels to switch more closely in unison? In playing with mine, adjusting the reference point for the comparators effects things in a very non-linear way around the threshold.

The original design most likely has a ton of marketing influence built in.. Minimal parts, minimal returns for repairs under warranty.. Machines that work under a wide range of environments. Sell the thing, make a profit and never see it again! But that's not what we are trying to do.

Well said!

Hopefully we'll accomplish fixing our TGSLs that are way out of whack, and maybe improving slightly the good ones. Then on to radical mods for interesting variations.



-SB

[simonbaker]

(Please don't read this unless you want to hear me complain. No new information.)

I thought I'd try a "quick" field test. It was pretty much a washout.

Urban environments and tinkering don't go together -- or at least with me.

Only place nearby with a table is local park, so decided to head there.

Quick checklist - coils, PCB, battery pack, ammeter, voltmeter, alligator clips, targets. Oh yeah, schematics, pen. No oscilloscope this time -- worried the 35 year old scope might not handle any vibration, can't afford to lose that baby...

Packed the stuff up in a knapsack (poor choice, not much support, but had to walk a distance).

Drive, park, lock car, hike to picnic area. As I unpack the stuff, I realize I pretty much look like a terrorist building a bomb. (After saying that, this message probably just got a hit on the NSA scanning computers...)

Ok, forgot to bring pad for taking notes. Add to next checklist...

Wire broke off battery pack in transit. Fortunately alligator clip barely grabs broken end.

Lay everything out, power on, and attempt to null the loose coils with AC voltmeter. Naturally too distracted to put probes on the right points the first time. Finally get my head straight and get volts pretty low on output of LF353, hear some chatter on speaker. I could have checked DC volts on capacitors C15, C12 for phase estimate, but what the heck, can do that later.

Test with target -- no big difference from in my "workshop". Also, chatter seems fairly similar, maybe more.

I realize that wind is a problem, blowing over the loose coils. The horseflies biting me don't help either. Nice big ones and lots of them. Keep you pretty busy.

I try to block the wind a little with my knapsack, but I can't trust the setup. Is it wind, is it electrical EMI (there is a light pole and building about 25 to 50 feet away, and you never know what is underground).

Without a scope, I'm blind. But I realize anyway there will always be possibility of wind outside. I have a feeling it will be extremely impractical to ever test loose coils outside -- probably only can use a nulled and glued coil.

For the heck of it I try the dfbowers mod with an alligator clip jumper. Pick wrong points, finally get it right. I can get a lot more chatter by turning sensitivity pot, but impossible to test depth with all that chatter.

People are walking by and I'm feeling pretty out of place with my nice little picnic of wires.

So, shut out, I pack it all back up and head back to car, hoping the headlights are off. Miraculously, they are.

So I'll need to regroup and plan another attack and decide on possibly gluing the coils dfbowers sent me -- then nulling experiments will no longer be possible of course.

I think these tests are important next step to understanding our TGSL performance and differences. Hopefully, others will do better with tests.



-SB

[lunamay]

i will try to make some mesurements on weekend(student:P) i have to make some changes in board..and finally test md in low nois place

best regards

ps i hate horseflys:P bad expirienc:P

[simonbaker]

pebe's complaint

pebe made a good point that my graphs at the beginning of this thread showing Bode plots for each stage of the TGSL filter (LM358, LM308 ) and the overall gain don't make sense.

The reason, I believe, is that my original plots plot the signal difference across the stages and compare them. For Bode plots, I believe you must plot the signal ratios across the stages for them to be useful for comparison and combining.

So attached is a new image of the same stages that I plotted at the beginning of the thread, only using signal ratios instead of signal differences.

Thanks pebe for bringing it up.

(I should note that these two stages in my LTSpice simulation do not represent the entire filter of the TGSL. Actually, the capacitors C15 and C12 and resistors R26, R25, and R27 also provide a low-pass filter stage that we need to calculate to find the total filtering going on. I have not figured out the best way to do that yet, but it should be done.)

-SB

P.S. I have probably made a similar mistake on other graphs, so watch out. However, usually both types of graphs reveal the same conclusions.

[nelson]

Excellent job dfbowers!

Just one question, can this mod apply to TGS also, not only TGSL and how?
In advance, many thanks
Regards
Nelson

Simon,

The latest noise measurements of Pin 6 on my LM308s.. Far away from the house, the only local source of noise would be my scope, and one incandecent light bulb.

U104, about .02v p-p
U105, about .01v p-p

Measurements are somewhat subjective.. but my best estimate is above.
The noise seemed to be random, nothing to sync on.
Offset was ever so slightly positive, maybe .005v or less.
Results were reproducable.. In fact, I moded TGSL#2 (the version with TL071s instead of LM308s). Both circuits function identically. Bench test of 40cm+ with no chatter... even two different coils.

Now for the real results: No increase in detection depth on buried objects.. just an impressive bench test. It DOES however improve the audio somewhat on detectable targets, so increasing sensitivity does offer some benefits in my opinion...

Overall, a setting where a 1e coin can be detected in air at 30 to 35cm seems optimum. Nothing I have can do much better in my dirt. Maybe better in sand? Anyway.. Don't have to "scrub" the surface to pick up shallow objects!!

Here is an easy mod I did with TGSL#2. Just clip one wire to the 10k pot and move it to the closest jumper with -5v. (Look close at the 2nd pic.)

Don

[dfbowers]

Excellent job dfbowers!

Just one question, can this mod apply to TGS also, not only TGSL and how?
In advance, many thanks
Regards
Nelson

Hi, Nelson.

I have not tried the mod on TGS but maybe someone else can comment. I even thought about trying it on a Whites classic I as the circuit seems similar enough.

Don

[simonbaker]

10 days

I'll be offline for about 10 days, so threads won't be cluttered with my musings for a while.

Hope to read about all the deep TGSLs when I get back.

Have fun!

-SB

[nelson]

Ok dfbowers, i will compare TGSL with TGS schematics to see if there are any diferences.
Regards
Nelson

Hi, Nelson.

I have not tried the mod on TGS but maybe someone else can comment. I even thought about trying it on a Whites classic I as the circuit seems similar enough.

Don

[Dennis the Mennis]

DFBowers sens.control mod on TGSL

Ok dfbowers, i will compare TGSL with TGS schematics to see if there are any diferences.
Regards
Nelson

Hi Nelson ,

I think the sensitivity control mod DFBowers made on his TGSL is also usable for the TGS , the schema is in this part the same.

Only DFBowers used a 100K pot and in the TGS there is a 10 K pot connected via a 1K resistor to the ground , see attachments .

I will try to test this mod this week on my TGS as soon as I 'll get my MD more stabil , otherwise it won't be much of use in this case .

regards

Dennis the Mennis