XL500 sample timing

[homefire]

Someone Provided me with a XL200 and XL500 Sea Hunters.

Deal was if I get them working I could keep one.

Done Deal.

They will detect a ladies wedding ring in the air about 6-8 inches.



Looking to mod the sample times on mine.

What do you think? Will this pull in the Sample time within a usable amount?

Just trying to make it hotter on small Rings and Gold.

Stock Coil will be replaced later.

[porkluvr]

Hmmm.

I have some ideas, but when you cover up part of the schematic you are not playing fair.
Why don't you enlighten us with the COMPLETE diagram??

[homefire]

Here Ya Go! I found this schematic here on the forum some place.

[Unregistered]

It's me, porkluvr. The system won't keep me logged in.

Good deal - I always appreciate a highly readable PI circuit diagram. I had searched on XL200 and XL500 and came up empty except for your first post.

Yes, that 12k is the resistor to work with. Motorola recommends the timing resistor be no less than 5K. Use anything smaller and the t=RC formula would be rendered invalid. A 5k resistor will give you from about 17usec to 56usec, as you vary the 25k pot from zero to max).

The timing capacitor (.0033 in your case) could instead be reduced if you really want to go to 10usec. The MC145158 datasheet has a chart showing 10usec as the absolute lower limit of pulse interval for this part. It could probably be forced lower, but again, the t=RC formula will be invalid. (And things get tricky. Read on.) I calculate your stock configuration's delay to be about 40usec to 122usec. With a 1nf (.001) capacitor and your stock "R" components you could range from about 12usec to 37usec. But, there's a catch*:

If you intend to use the detector on land or maybe in fresh water, there can be great benefit in shortening the timing - *so long as you don't take your sample at a time when the preamp is recovering from the "bang". First stage gain and type of amplifier are two important factors here. The coil's time constant also has an affect because of it's decay time. If you read through some other PI threads in these forums you will get some concrete ideas. You may get some benefit (quicker recovery from overload) by replacing the 709 with NE5534, but I cannot guarantee it. I have one of those NE5534 (and also a HA-2625) that you could try if you want (just send me money for postage and a mailing envelope). There are other opamps that folks swear by. The stock compensation and offset null components on pins 1,5,8 would need to be modified to match any new amplifier you wanted to try. It's not easy, but 709 is a very ancient and fairly noisy device. It might be worth the trouble, but please don't do this unless you have an oscilloscope to check your results, before and after! You would need to re-null the amplifier, and check for proper operation.

If you try to take your primary target sample at less than (this is a wild guess) about 15usec, I highly recommend that you use an oscilloscope to make sure that 709 output (pin-6) has settled back down after it's bang-shock. If you change search coils, you will absolutely need an oscilloscope in order to determine the best damping resistor value, unless you can follow somebody elses work. There are various R-L combination that have been used in various other PI units, but it relies on your having duplicated the other coil very closely. You shouldn't just make a guess, then toss in a damping resistor and go hunting. You probably won't destroy anything, but it won't be an optimal setup.

When you are in a salt water environment, there is much less advantage from taking quick samples. Salt water is highly conductive. Backscatter will smear and obscure your target response. The effect is similar to the ground response of a land based detector, but much more pronounced. In your favor, if you scuba-duba, - the backscatter effect should be relatively constant.

Before tightening the sample down, some folks will reduce the first stage gain, (and perhaps add another amplfier to make up the difference) in order to reduce the time it takes for overload recovery.

Oh yes, I almost forgot: it might be advantageous to use a concentric, an omega, or a DD coil. But this is hardcore stuff. IF correctly made, these coils give essentially zero output in the absence of a target, even after the high voltage bang. But, they must be accurately nulled to get maximum effect. It takes a plan, careful mechanical alignment, and a test procedure. Of the three, DD is probably most often used, maybe because it doesn't require quite so much expertise to simply physically align two coils, compared to adding a bucking coil and THEN doing an alignment. I think most PI detectors use a single coil for both transmit and receive. No alignment necessary.

I'm not pretending to be an expert, but what I've told you is correct to the best of my knowledge.

Maybe I'll come up with some more ideas, but that's about all I have for now. Good luck, and thanks for the schematic!! If you have any questions, ask away, but I don't want to write a book.

[homefire]

Thanks for the help!

The timing capacitor (.0033 in your case) could instead be reduced if you really want to go to 10usec. The MC145158 datasheet has a chart showing 10usec as the absolute lower limit of pulse interval for this part. It could probably be forced lower, but again, the t=RC formula will be invalid. (And things get tricky. Read on.) I calculate your stock configuration's delay to be about 40usec to 122usec. With a 1nf (.001) capacitor and your stock "R" components you could range from about 12usec to 37usec. But, there's a catch*:

Timing is based on the MC14538B not the MC145158 PLL !

Is that a typo? Are the times you state valid?

[Unregistered]

Whoops, MC14538 is not MC145158

Porkluvr here. (Still being dumped out of the system)

Yes, typo. I have MC145158 on the brain because I have been trying to invent a VLF ground track circuit using its cousin the MC145152 PLL. Sorry about that.

MC145128 is a serial loaded PLL controller and has NO place in this discussion.
MC14538 is a dual retriggerable oneshot. (I do know the difference, believe it or not.)

Just for your info:
MC14538 and the following parts are -almost- the same:
CD4538
CD14538
HFC4538
There are others that are almost the same, but lets move on...

(I tend to ignore the "B" suffix, it mostly "understood", I guess). But here's an important point:
These last three part numbers have the same pinout and similar function. But there are subtle differences -. Some have hystereses on one or the other input, or both, and some have limits on the minimum value of the timing components. In my experience, you can get into trouble if you generalize "4538" devices, because of those differences.

For instance: I thought I was getting a bargain when I bought about 20 of "4538" on the surplus market. What I actually bought was a bunch of HCF4538, made by ST. I eventually learned how to use a computer, and one day I downloaded the HCF4538 data sheet. I learned that there is hysteresis on the both inputs, which is good, but, there are minimum limits on the value of both the R and C timing components (5k and 4n), and a minimum pulse width of 20usec, which is BAD BAD BAD!
That's the breaks! No typoos in this messige (I hop)e.

[homefire]

Cool Beans!

Thanks for the help.

So it looks like it's running Stock config delay about 40usec to 122usec.

This is looking better then I had expected.

Replacing the 3.3n with 1n looks to be the option.

Getting down to the 17us Changing the 12k resistor to 5K is pretty damn close to me.

With a 1nf (.001) capacitor and your stock "R" components you could range from about 12usec to 37usec. This would be better on gold!

With a new coil, I may be able to use it as a nugget shooter. We Shall See.

Worst come to Worst, it will respond to smaller gold rings and trinkets.

I don't have a scope but do have access to one.

As far as replacing the 709, I would rather not go that far if I can achieve what I need with out it.

709's are prone to toast quite easy too if I remember right.


I plan on using it for Relic, GP metal detecting, "DIG IT ALL" and prospecting if I can get it to see small nuggets.

As for the person that provided them, He is a beach Ring chaser. This should give him a bit more on the smaller ones.

The XL 500 coil cable is not even shielded.

The XL 200 coil cable is braid shielded.

The Circuit Board appear to be the same.

With only a 6" coil, they are surprisingly good.

They will detect my wife's wedding ring at 6-8 " in air quite strong.

I have no plans on getting mine wet, so I can mess with it a lot more.

My Back Ground.

I did build a old Sandbanks PI. Ha! I still have a complete kit untouched stashed away.

Sample rate on it was something like 50usc and it could just see a quarter at 6 inches.

Ex USAF Telecoms Weeny I have a good Electronics back ground so should be able to sauce out most of it.

Thank You For yer Help on this and Please Stick with me on this!

The way they set up the timing on it seems to make the delay and actual sample time flexible to a point.

The Eliminator just backs off the sample so it will not see certain objects. Not so good on the Good Stuff like Gold.

I am perplexed by on aspect of the design.

Can you explain why the are feeding the op amp with the same sample and no Reference Voltage?

Both Fets seem to be providing the same sample with only the Propagation delay between them. What is the advantages on this?

The Op Amp looks to be set up in the Differential mode. Are they using the slight change between the two to Provide the difference?


It's 3:30 am , I just got off work and getting long winded on you this time.


Thanks again.

[porkluvr]

I'm back!!

Me again- I'm still getting bounced right after login. (Is somebody trying to tell me something?) I'm thinking about changing my Login identity, maybe the problem will go away
Oh, think the reason why my search results came up empty is probably because I left out the "-" in XL-200. Google probably would have found it, but I used the THunting.com "Search" button.
************************************************** *************************

I'll run set up and run an LTspice simulation and try to give you some more pointers. The timing and target sampling has many similarities to the Goldscan IV which I have already built in a virtual lab. But first, I want to work on my MC14538 symbol - add pin numbers, and check the timing on the model; I think it was adapted from the 74221 which uses a different formula. I'm hoping it takes longer to write about it than to actually do.

I looked over the schematic, and after encountering the MPSU60 (or MPSA56 in a one of the other versions) and MJE3055 in devices, I noticed that I had no models to represent these. I was up until about 5:30 this morning hunting down spec sheets and spice models for these transistors and also for miscellaneous others transistors cluttering my parts bins.

I almost got them all, but models for those two MPSxxx devices are missing. I'll have to determine a viable substitute, one for which a spice model can be found.

I'll try to explain the circuit topology in a day or two, but I still have some foundation work to take care of.
Cheers!

Yow, I just got re-logged in as I clicked the reply button. I'm, confused.

[homefire]

Ultimately, The Coil driver will be replaced with something like a IRF740. You can do away with the driver transistor and run right off the 555.

[porkluvr]

Can you explain why the are feeding the op amp with the same sample and no Reference Voltage?
Both Fets seem to be providing the same sample with only the Propagation delay between them. What is the advantages on this?
The Op Amp looks to be set up in the Differential mode. Are they using the slight change between the two to Provide the
difference?

The circuit is more easily analyzed if you break out the individual portions of the dual one-shots (os).
Consider that the 555 pin-3 output is active low. The "A" and "B" inputs of the os can trigger the device on
either a leading, or trailing edge, respectively - depending on which input is used.

The first os's input is to leading edge trigger "A", pin-4. The sample timer is initiated as the 555 turns OFF.
(Q\ connected to pin-5 sets a non-retriggerable mode of operation, but that is tertiary issue. I'm just pointing that out
because the device data sheet may not explain that connection.)
The first one-shot sets the initial delay between the rising edge (turn-off) of the sync pulse and the start of the primary sample pulse.

The second os sets the delay between the primary and secondary sample pulses. The two diodes (connected to pins 2 and 14)
cause the third and fourth os to share a mutual RC network so that their outputs have the same duration; as determined
by 22k resistor and 1nF capacitor. This diode trick only works when there is adequate time for recharge between outputs.
I learned this from reading Carl Moreland's Hammerhead pdf.

Think of the JFETs as normally off switches that are turned on by the sample pulses from pin-6 (primary sample) and pin-10 (secondary
sample). In the third os, notice pins 6 and 4 connected; this is how you do a non-retriggerable mode when the "B" input is being used.

Yes, the CA3140 is used in a differential mode, but the + and - inputs are fed at different times. The non-inverting input gets the
first,"target" sample. The inverting input subtracts the second, "noise" signal which has been delayed by time t=RC,
the 150k and 1nF on the second os. Because we subtract the 'noise' signal from the sum 'target plus noise', theoretically we
send only target to the following stages!

I looked at the Coil Parameters info (found here:
http://geotech.thunting.com/cgi-bin/...&file=info.dat) and it says the Sea Hunter coils are
about 150uH, 1.2 ohms. Is this correct? Can you measure your coil and post the data?

Something bothering me: the MJE3055 transistor is rated at about 60V, and my simulation clearly shows the device is subjected to over 100V
on the flyback! That voltage level would be highly affected by the search coil's inductance.

To increase sensitivity I would -otherwise- suggest the possibility of changing the timing components on the 555, but you have to
consider battery drain, and, possibly exceeding the safe-operating zone of the MJE3055 if the device gets too hot.
(I simply do not understand why this particular transistor is employed here.

If you change coils, you could possibly subject the MJE3055 to catastrophic failure!) Thermal runaway is a danger. I think that with some
simple resistance changes, a MOSFET (perhaps either IRF640, IRF740, or IRF840) could be substituted in place of the MJE3055.
I think you would get much faster turn-off, and, over-voltage would not be an issue as you try different coils!

I will modify the LTspice circuit and analyze this possibility. I think that IRFxxx could fit on your board if
you simply turned it 180 degrees. Mosfets aren't perfect, but they are not subject to thermal runaway.

Also, it is easy to increase pulse rate and decrease pulse width. The right combination might give better sensitivity and still not cause
significantly more battery drain, if approached intelligently. You want to have an oscilloscope handy before you attempt these mods,
but it's not really major surgery, imo!

It's late, so I'd better go before I say something else really crazy.

Hey, I just got notified of a private message but the THunting system won't let me access it, it logs me out.
Yeah - I keep some crazy hours. No, I'm not on skype, I'm an old fogey. (I read my oe email).
Hey, I just read where you mentioned the IRF740, good idea. You still need the predriver transistor to invert the drive
unless you change the 555 duty cycle drastically!

[porkluvr]

Ultimately, The Coil driver will be replaced with something like a IRF740. You can do away with the driver transistor and run right off the 555.

Your original schematic shows a 10Ω resistor in the MJE3055 base circuit. It should probably be 100Ω.
Battery supply current decreases from 150ma to only 100ma when 100Ω is used and it makes for a more reasonable voltage
across the MJE3055. I think schematic may be in error. But this is moot because:

You want to use an IRF740 but remove the MPSA56? Remember that you have a negative drive pulse coming from the 555.
If you were to remove the predriver without also reversing the 555 duty-cycle, then you would be energizing the
coil 97% of the time instead of the normal 3%. That is not a good thing.
Leave the MPSA56 in the circuit, you need it for signal inversion.

With an IRF740 you should change the value of the damping resistor. My virtual test circuit shows that something around 360Ω is
best - assuming that your coil is like the one that I am modelling. 360Ω might be OK, but no more. I think it should be rated for => 1watt.

I played around with timing and transistor bias. Here is something that might work.
The flat top on the coil flyback pulse is about 240ns duration, with a 8.4V supply and 150uH coil.

I looked at your original circuit, and that old MJE3055 might be good for salt water, but it turns off extremely slow compared to IRF740.
You don't want that for hunting small gold.

note: don't look at I(Rtrg1) in my simulation and assume that the 555 outputs a positive edge at that time.
If I were to flip that resistor around, then the signal would be shown going in the other direction.
One other thing: be sure and reverse the IRF740 orientation before you solder it in place of MJE3055!

[homefire]

Cool! Thankx ! It's going to take me a while and a beer to digest this.

[homefire]

Yes, the CA3140 is used in a differential mode, but the + and - inputs are fed at different times. The non-inverting input gets the
first,"target" sample. The inverting input subtracts the second, "noise" signal which has been delayed by time t=RC,
the 150k and 1nF on the second os. Because we subtract the 'noise' signal from the sum 'target plus noise', theoretically we
send only target to the following stages!

Ok! Let's see if I have this right.

Sample one is taken and subtracts noise .

Sample two 15 us later is taken containing noise plus target signal.

Q: Why is the target signal not present in the first sample?


I will try to verify L of coil with this method.

http://www.coilgun.info/theoryinduct...inductance.htm

[homefire]

Here! Just to muddy the waters, I have this one too from the forum:

[porkluvr]

Ok! Let's see if I have this right.

Sample one is taken and subtracts noise .
Sample two 15 us later is taken containing noise plus target signal.
Q: Why is the target signal not present in the first sample?
I will try to verify L of coil with this method.

No, that's not what I said. You got it backwards. So, while I'm typing my little fingers off, you are kicked back doing the 12 ounce curls? That ain't right.

Here's what I was trying to say:
Sample1 is (target+noise)......... [ pin-3, + input, aka noninverting input]
Sample2 is (noise)................... [ pin-2, - input, aka inverting input]
Result: (target+noise) - (noise) = target

That's my eighth grade educashun paying off! Time, well, spent.

Before I forget, some PIs have a small value resistor in series between the MOSFET drain and the coil. Something like 2.2Ω maybe more. I just today saw a design using 5.6Ω. I don't think it's absolutely necessary, but it's intended to help the FET run cooler. FETs get NOISY when hot (so do resistors, come to think of it) - which could reduce sensitivity to small targets. Just keep that in mind. Also, a small heatsink on the FET is good - if you can fit it in. IF not, then don't. I would use both a 2.2Ω
resistor AND a small heatsink, but that's just my being a stubborn perfectionist.

Although the average dissipation of the resistor may be less than 1/4W, it needs to handle 7W peaks. So, don't use a pee-wee. 2W should be good. You might be able to lift the drain lead on the IRF740 and solder a resistor to it to avoid having to cut the trace - should you deem it appropriate to use a resistor in the first place.

note: Although the MC14538 has internal connections between the capacitor discharge terminals (1&15) and pin-8, the datasheet suggests that an external path be provided. I suppose it's no big deal - because we are dealing with microsecond pulses. But that's why I show pins 1 and 15 connected to the negative supply and your schematic doesn't.