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Whites PI3000 Modification Release

A

Anonymous

Guest
Hello!
I designed, built, and tested my modification to the Whites PI3000 (veteran? toy? you name it) pulse detector.
Modification is based on the substitution of the main timing chip (CD4017) that gives the timings to the TX pulse, target sample pulse , and ground sample pulse of my PI3000.
I substitute the CD4017 with a small board that carries a PIC16F628 microcontroller at 4Mhz clock speed and some other support logic.
The board is connected inplace of the CD4017 by means of a flat cable.
The board fits nicely inside the PI3000 housing.
I have added a rotary switch so that I can select up to 10 different programmed timing combinations to eventually adapt to different ground conditions or even simulate the original timings.
Thanks to Eric Foster's suggestions <img src="/metal/html/smile.gif" border=0 width=15 height=15 alt=":)"> I programmed a timing sequence that gives 2 more full inches of sensitivity on dimes (euro cents here).
The sequence is:
TX Pulse 60uS (originally 33uS)
RX Target delay 26uS for 15uS time (originally both 33uS)
RX Ground 50uS after RX Target and again for 15uS time (originally 165uS later ! for 33uS time).
All this to keep the original 3Khz sampling times even if I doubled the pulses and quadrupled the TX enery...
I programmed other timings as well but it's too long to list here.
I've not been able to move RX timing faster than 26uS because of the coil response in combination to the (nice) feature of automatic ground balance that the PI3000 provides.
I probably match an unmodified Whites Surf PI.
I post the schematic and I'll post a detailed "how to" only on heavy request!
The improvements reported are "on air tests" at the moment.....
HH <img src="/metal/html/lol.gif" border=0 width=15 height=15 alt=":lol">
 
Hello, it is impossible for me to make a real test here in my garage and I had no chance to try it in the open. Here I have iron everywhere.
I'll be at the beach during the weekend and I'll answer with a report next week.
The only thing I could test was that , under the same conditions ,so even inside the garage, by changing from the "original" mode to the new one I could gain about 2 inches of depth.
The improvement over the original mode is evident also with the other combinations I programmed, but only the beach will tell by how much.
There is also the possibility that the best combination for air tests won't be the best on wet sand.
I'll be back!
Greethings
HH <img src="/metal/html/grin.gif" border=0 width=15 height=15 alt=":grin">
 
Hi Luca,
Your mod; looks good. looks like you re-invented the wheel.2" increase not to good after all that work.I am sure that you will get it to go deeper.your mod is very impresive.
Reg,helped me with mine.took me about 5 mi to install a cap.with a 3" increase in dept on US nickel,Dont forget I later ruined PI-1000.dont have your skills.Now you need to find gold with it.
Been detecting over 40 yrs. might be able to help you there.Try what I call a military grid pattern on your beach. At low tide start about 70 ft above low water.walk in strate line down to water,keep coil flat about 1"from ground,keep it flat to end of your swing.at end of swing push coil forwards(still flat) half diameter of coil come back across with you swing.Do this both ways left and right.this is more than enough over lap for your PI.
The object is to locate all target.if you miss one you step on a mine and blow-up.
Now you need to walk in a strate line down to water and back up 70 ft. then back down to water until you cover the entire beach.I have watched a lot of people over the years, most of the can not walk a strate line.To over come this drag your foot in sand then look back,this will keep you strate.
Dont forget each time that you turn around on each approach, step over abut half length of your swing,and go back in strait line. good luck
Frank.
 
Hello Frank,
thanks for the suggestions.
Can I ask you which cap did you change on the PI1000?
Chris suggested to try to change the preamplifier opamp feedback resitor (1MOhm) to 1.2 or 1.5 MOhm (see PI1000 schematic below) to try to have a higher preamp gain.
That, in my opinion, should be changed with a potentiometer to adjust gain in accordance to ground conditions but this would mean to carry around wires that could pick up noise and get the situation worse
.
Anyway I did not try it yet, because I first want to evaluate my modification and play with it for a while.
I have to thank Eric Foster for his detailed articles about PIs that sparked my entusiasm and willing to share it with you all.
<img src="/metal/html/lol.gif" border=0 width=15 height=15 alt=":lol"> HH
 
wich is the objective to place the resistance of 5.6 in series with the coil, that it improves?
 
Hello,
I think that the 5.6 Ohm resistor in series with the coil has been put there by Whites to limit current flow in the Ztx750 transistor that has a maximum current rating of 2A. This is my opinion, maybe Eric has a better explanation....
<img src="/metal/html/confused.gif" border=0 width=15 height=22 alt=":?">
HH
 
Luca,
I am at public libary, might have told you about cap a month ago.this is from menery.Its the main cap at 4069 IC.might be 150pf dont go lower 120pf.You have all ready made a mager change.dont mess with this untill you finished your mod.
I think that Cris was talking about lowering the resistor at 4069 timer.its a 5 mi job.all you do is put one resistor on bottom(parall)of resistor.this will seedup timer and lower time delay.you can also put a pot ther for a non-ferrous ID.no need to remove resistors.
Frank.
 
Hi Luca,
Glad the technical info on this forum got your enthusiasm going for doing a bit of experimenting.
The 5534 in your schematic is running at a gain of 1000, which is more than I normally use for a front end. You need to look up the gain/bandwidth curves for the 5534 to see where you are sitting at the moment. Increasing the gain will reduce the bandwidth and may affect the earliest point that you can sample. The reason for this is that the amplifier takes longer to come out of saturation after the transmitter pulse. Using a 5534, I find that a front end gain of about 500 is about as high as I would want to go, to be able to sample at 15uS. If you were sampling later than that (which the White's did), then you can have more gain and a longer recovery.
Having leads and a pot to alter the front end gain is not a good idea. This will add capacitance to the feedback circuit which will result in even less bandwidth than you would expect from the curves.
I don't see the point of the 10K resistor from pin 2 to ground in the schematic.
The 5.6 ohm resistor limits the maximum coil current and improves the coil circuit time constant, so that the switch on current is not still rapidly growing at the point it is switched off.
Eric.
 
Hi Luca,
Good to hear that you succsessful upgrade your detector. You mention that you programmed different timing sequences to gain deph. That sounds interesting. Can you post your different sequences (TX, RX, Sample) you programmed into the forum?
Chris
 
Hello Chris,
I have a 10 positions rotary switch to select one out of ten different timing programs.
I choose the following to start with:
N. TX Dly Target Dly Ground (all times uS)
0 33 33 33 165 33
1 33 30 33 165 33
2 33 28 33 165 33
3 33 26 33 165 33
4 66 33 33 165 33
5 66 30 33 165 33
6 66 28 33 165 33
7 66 26 33 165 33
8 33 26 15 50 15
9 66 26 15 50 15
Prog 0 is same as the unmodified PI3000.
1 to 3 just a sampling speedup.
4 to 7 double TX time
8 and 9 are special and also the most sensitive.
If you look at the numbers in prog. n. 8 and 9 I shortened the whole Tx/RX cycle by sampling for only 15uS and allowing a 50 uS only delay between Rx target sample and Rx ground sample. Apparently this should give HALF sensitivity but since this whole cycle is long HALF of the original one (e.g. prog.n. 0) this one is executed twice in the same time period.
In other words the sampling capacitor is charged always by the same time length but the signal is taken more toward the 26uS area(! <img src="/metal/html/wink.gif" border=0 width=15 height=15 alt=";)"> ) where more signal is available .
I remind that I can't go faster than 26uS because I have a strange curve in the 20 to 26 uS that goes in the minus area (below ground!).
Probably this is an effect of the slow recovery speed of the 5534 Op amp as suggested by Eric. I'll try to lower it's gain to 500 times and see if this beaviour vanishes. In that case my programs will change again....
Sensitivity is really improved. I gain 2 inches on any target and I noticed a greater sensitivity to very small targets as well.
No doubts about it.
Cheers
Luca
 
Hello Eric,
your suggestions look like eaven to me! I'm good at digital by unconfortable with analog circuits.
I'll check the 5534 and half its gain to see what happens.
I admit my curve, measured at the output of 5534 is high and flat between 0 to 13us, decays to ground between 13 and 20uS, goes BELOW ground from 20 to 26-28 uS and is around ground from 28uS on.This prevents me to sample faster than 26uS.
Do you mean that I should try also to remove the 10K resistor to pin 2 and ground of 5534?
Thank you again <img src="/metal/html/smile.gif" border=0 width=15 height=15 alt=":)">
HH
 
Hello,
it is not exactly what you asked, anyway I tested a solid 18K gold ring with gems in dry sand and I spot it at true 10 inches depth.
My modification does not make miracles but works! <img src="/metal/html/biggrin.gif" border=0 width=15 height=15 alt=":D">
HH
 
Hi Luka,
thanks for the update. You should try to reduce the gain of your receiver like Eric suggested. This might help you to gain more deep.
Chris
 
Hi Luca,
I am a bit puzzled when you say that the waveform is high and flat from 0 - 13uS. This is the period when the transmitter is on, and is usually more than 100uS for an underwater detector. At the end of the TX pulse, the waveform will swing negative due to the reverse emf generated in the coil at this time. You can see this emf if you put the scope directly on the coil. It appears as a high voltage spike at the end of the TX pulse. When you look at the coil, everything is reversed, compared to the amplifier output, as the amplifier inverts the signal.
If you try a lower gain in the 5534, you could increase the gain in the dc amplifier to compensate. Can you post a scope picture of the 5534 output waveform?
Take out the 10K from pin 2 to ground. Pin 2 is referenced to ground by the other resistors connected to it.
Eric.
 
Hello,
I explain...
I measure at the output of 5534, pin6.
All times are measured from TX pulse END.
After a 33uS TX pulse from 0uS to 13uS I see a flat high signal (diodes levelling?) that starts to drop in a curve between 13.8 and 20 uS. From 20 to 26uS signal goes below ground and levels at ground after 27uS.
I can't sample between 20 and 26uS because the PI3000 goes unstable, mute or almost unsensitive in that area.
I have two explanations:
1) the 5534 is unstable till 26-28uS, and this seems to be the case looking at the 5534 gain/bandwidth charts
2) I have a faulty head.
One way to check that is probably to look at the signal before and after the 5534.
I'll do it next week, as soon as I can.
As for the picture of the waveform, my problem is that I need a good digital camera. I'll get organized as soon as possible.
About the 10K resistor, could it be there to avoid overvoltage to pin 2 of 5534? I have no other ideas other than this stupid one!
Thanks!!! <img src="/metal/html/smile.gif" border=0 width=15 height=15 alt=":)">
 
Sorry for quality, it's the worst digital camera ever built...
 
Luca,
Looks good.Does 3000 have audio or a viberater.
Frank.
 
Why not replace the 5534 with a National Semiconductor LM6365? If I'm not wrong noise figures are similar but LM6365 is as fast as 1uS at a gain of 1000 and they are pin compatible as far as I understand...
 
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