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New member
Those wanting to know more about how a P works probably should read this post.

One thing I strongly recommend is you read my article "Understanding the PI metal detector" to learn more about the PI and how it works. The reason I say this is there is a lot of miss-understanding about how a PI works and what is needed to obtain the greatest depth. Here is a link to that article.

I wrote the article in 2003 and basically everything wrote then is still correct today. Also, the article went viral on the net.

What I should have emphasized more in the article is information on the factors that determine the potential depth capability. I also should have provided more information about what I call low power and High power PI detectors. However, the article was already two pages long and was written because of a request from Bill Southern, owner of the Nuggetshooter forum.

With all that said, I will try to fill in some of the missing blanks now. So, to begin with, the PI relies on a strong current pulse into the coil to obtain maximum depth. The more current into the coil the greater the potential of obtaining more depth. Next a very low noise front end of the detector is required so any return signal can be noticed. This takes a lot of engineering time and energy to maximize this feature.

So, as a general rule, high power PI's go deeper and those that have the greatest development in reducing front end noise also have a tendency to get the greatest depth.

Typical high power PI's include the ML PI series, Eric Foster's Goldscan series, and the Whites TDI. There are others, but these are the most often referenced. These detectors use heavy batteries so they can provide the greater current needed to maximize depth.

Lower powered PI's such as the Beachscan, Goldquest, Headhunter (HH), Whites dual field (DF), C Scope CS 6 and some of their other PI series, as well as other brands using typical AA batteries are usually a low power or lower powered than the above mentioned high powered PI's. Normally, these detectors have a current limiting resistor in series with the coil to limit the current, which in turn extends battery life.

Many of the low powered designs used today or even those of the past are detectors originally designed by Eric Foster. The evolution of some follow closely to Beachscan. The Headhunter is one of them that I believe was built between the Beachscan and the Goldquest. All of these are low powered PI's and most, but not all use, 8 or so AA batteries.

Now, the HH uses two 9v batteries. This is more voltage than that produced by 8 AA's but the two 9V cannot and do not produce nearly as much current into the coil as the AA battery pack and it is the current into the coil and not the voltage that is what is needed to produce the stronger pulse. The two 9v batteries was chose to provide the necessary voltage for much of the control circuitry and not to provide more coil current and do so at a lighter weight. In fact, I suspect a larger value resistor is installed in the coil circuitry to reduce the coil current than found on the Beachscan series. Also, keep in mind that is is a larger pulse current is what is needed to increase depth.

Keep in mind the Minelab PI series uses a heavy duty 6 volt battery that also uses more pulse current than other PI's that I know of even though the voltage is less than most others including the GS 5 and TDI. Also keep in mind that the ML PI also goes the deepest and does so because of both the increased current into the coil and the advanced engineering in reducing noise generated in the front end of the electronics. This advanced depth capability is also reflected in the price of the detector.

So, in a nutshell, the operating voltage by itself has little to do with just how deep a PI will go.

Now, discussing the low powered PI's such as the HH, the Beachscan, the Goldquest, and others, the low power label has little to do with the depth potential. Yes, they do not put the largest pulse into the coil, but the increased repetition rate helps in the depth potential. No, the low powered PI's won't go as deep as the high powered PI's but the depth difference isn't as great as one might think.

One other factor that applies is the fact that the technique presently used to ground balance a detector such that one can ignore or cancel the ground mineral signals is done by adding more samples taken later in time and then subtracting the later signals that have been amplified enough such that the signal from the ground in the later sample equal the main sample and the later is subtracted from the main sample. The result is the ground signal appears to go away. What also goes away is part of the signal from many targets. Thus, ground balancing detectors will not have the depth on many of the same objects as a straight PI of similar design not using or having the ground balance feature. This is why on the TDI, turning off the ground balance (GB) in areas where it can be done results in more depth on most objects. This is also why many TDI owners complain that they can see an obvious depth loss on some objects when testing with the GB on versus GB off.

I mentioned the low powered PI's do quite well against other PI's and part of the reason is the fact they do not have the ground balance system. So, they do not suffer the depth loss caused by using a GB design.

The bottom line is there are a lot of factors involved as to what affects the depth potential of a PI detector. Also keep in mind, there are anomalies not related to the detector that can influence the potential depth one might be able to detect an object. Normally, this occurs on a one time basis and is not the norm. Normal repeatable depth capability is generally much less. However, we as humans demand our bragging rights and take advantage of them. So, it is not unusual to read of abnormal depth capabilities being posted. This is far more common on some of the gold prospecting forums where one can easily picture a fisherman's ruler being used.

Now, with all the above mentioned info I will also mention that something as simple as EMI (external electrical noise) plays a large part on what one might be able to detect an object, so testing should be done often and at different locations to really get an honest evaluation. While building coils I ran into some strange things that stumped me at first and one of those things was the noise factor. I found while tweaking coils I could get depths ranging from about 11" to a maximum of 17" air test on a nickel. Now, that is a huge variation that caused me some problems because it wasn't consistent. So, coil testing became a challenge. The final result was there was very little depth difference between two similar size and shape coils once all the other factors were equal. Oh yeah, the realistic nickel depth of detection on my low powered PI's was about 11" to 12" and that was on a good day.

I hope this clears up some of the errors in thinking many people may have as well as why one can read such dramatic differences in postings about depth of detection.



New member
My first post pointed out some of the features as to what can cause one PI detector to detect deeper than another. Issues such as current into the coil and unique design that generates very low noise are two key issues. However, there are more things and issues that are sort of intertwined that also need to be factored in.

Talk to a Minelab PI owner and sooner or later the discussion will get around to noise. The discussion may include everything from solar flares to lightening, and of course power line noise. On some of the OZ forums, they even discuss how power line noise extends out because of air currents.

Now, in all fairness I really need to explain that Minelab has pushed the limits on sensitivity and taking the sensitivity to such limits causes their detector to be much more sensitive to some of the noise problems. This increased sensitivity is part of the key as to why the ML normally has greater depth than other PI's.

Other PI's are also affected by some of this external noise but usually the effects are not as dramatic or obvious. Try to use a ML PI in town and usually any depth comparisons will be dismal at best. Change the location to a more remote area and the ML really shines and displays exceptional depth, usually far superior to most competition.

The point is, all PI's are affected by noise and this effect can be quite dramatic. In my previous post I mentioned that with my low powered PI, I could get a wide depth range on a nickel and the reason for that depth range was simply noise. One thing that many PI owners don't notice or realize is if they raise the coil above the ground, higher and higher the noise level most likely will increase. Depending upon how close one is to the noise sources, the amount of increase in noise as one raises the coil will usually determine just how much influence the noise has and this can vary significantly just because of the noise source itself. In other words, the issue is far more complex than many people realize.

One other effect many PI owners don't notice or realize is the coil edge effect or stated differently, the coil edge sensitivity. Take a very small gold nugget or any other non ferrous object and pass it under the center of a large coil and chances are you might not get a noticeable signal. However, take that same object and pass it right under the coil winding on the edge of the coil housing and bingo, it is very possible to get a decent signal. The reason for this occurs because of the coil field effect. In simple terms the field strength is quite intense right under the winding itself and this intensity extends out a few inches depending upon the coil size and the object itself.

Now, take this edge effect and a unsuspecting operator and it is quite possible for someone to believe the object they are detecting is under the center of the coil when in fact it is under the edge of the coil. Because the signals sound the same, it is extremely easy to have this happen. Alter the angle of an object such as a flat coin in the ground and the maximum signal doesn't necessarily happen under the center of the coil. Since in most cases the object is centered at the largest response, the natural thought process says that is where the object is when one begins to dig. Usually, the hole gets quite deep before the object drops down into the hole and disappears. When this happens we don't give up but dig deeper, which then recovers the object. The next logical condition is by chance the object is scooped out just prior to it falls but the hole is still much wider and deeper. In any case, the first thought is the buried object had to be at the deepest depth. Unfortunately, if one tries to repeat the process, this same object won't be detected even close to the depth one thinks it was found. Unfortunately, this causes people to believe in the halo effect is what caused the object to be detected deeper and now can't be detected since the "halo" was disturbed.

Keep in mind that the halo oxides of any metal are such that they do not produce a signal that is detectible. The exception really isn't an exception is rusty ferrous metal. Normally as the object deteriorates, the signal strength gets weaker. However if the rusty object deteriorates to an iron oxide and becomes part of the ground minerals and than that oxide is transformed by intense heat to maghemite, then the ground signal will change. Something as basic as a fire or a lightening strike can cause the ground to suddenly change which can cause a signal quite often sounding like a deep target.

Then there are all the strange anomalies that occur that seem to enhance a signal. Normally, these seldom happen but they can and do. Something as strange as iron oxide such as black sand can concentrate or distort the signal such that in certain cases objects can be detected deeper than they would be under more normal conditions.

One other unique issue is many people firmly believe that a PI will or can detect burnt wood, so when they come across an old campfire they will get signals from the burnt wood. Unfortunately, this is not the case. What has happened is the intense heat from the campfire causes a transformation of the ground mineralization to maghemite which is quite detectable. Old red bricks were made using different levels of iron oxide to get the brick the red color. Test an unfired red brick and nothing, nil, nada in terms of any distinct change. However, subject that same brick to the heat used in firing that brick and suddenly that same red brick becomes a strong ground signal change. In fact, one can use an old brick to simulate some of the old positive indicating hotrocks when testing a VLF. On a PI, the signal comes through as a sudden change in the ground signal but may sound like a very large deep target response.

Now, do any of these things I mentioned occur even to those really experienced operators? Of course they do. In fact, I have seen them directly, and in the case of one small nugget had very well known AZ nugget hunter stumble when he tried to show how his detector was so effective he detected that small nugget at a depth of several inches. Unfortunately, when he tried to show me how well his detector did on detecting the small gold by passing the same coil over the small nugget he couldn't get the slightest of signals. Because of my knowledge of this nugget hunter, I knew he found the nugget with his detector. So, what probably happened was something enhanced the nugget signal allowing the guy to get a decent signal, which finally led to the recovery of the small gold piece. As I think back, I realize now we didn't check the coil edge to see if that would generate a response.

So, in my 40 plus years of metal detecting of which 25 years was dedicated to serious nugget hunting and the last 12 years with a PI, I have pretty well seen it all. Compound this with the fact, I needed a light weight ground balancing PI when there wasn't one available, led me to learn what I could about PI's. As a result, I ended up modifying different PI's until I was able to buildup a low powered PI for my dad to use because his bad back wouldn't allow him to hunt with any of the heavier PI's any more.

The learning process, along with the experimenting needed to design and build the changes forced me to learn an awful lot about the PI and how it works. My BS degree in Electronic Engineering Technology gave me the technical background from which to build.

What I can greatly attribute my PI knowledge and experience to is to Eric Foster and his willingness to share his knowledge and wisdom over the years I have known him. Eric was and still is a wealth of knowledge when it comes to PI's and what makes them work the way they do. BTW, I still search a lot of his earlier posts on this forum when looking for answers, even today.

As a final note, some of you won't believe much of what I have written and that is fine. Feel free to experiment and prove me wrong. That is easy to do on a one time basis, because of all the potential factors that have an influence. However, feel free to do it on a consistent basis. The key to really learning the PI is something that takes years of experience and knowledge to fully understand just what is going on. I strongly recommend people take the approach to prove or disprove an issue and do so on a regular basis. What you will find is what seems logical when you first start out, may change dramatically as time goes by.

I mention this because for many years people believed that gold would be detected using somewhat of a constant thought on gold itself. By this I mean a 1/4 oz nugget would act much like any other similar size nugget. Unfortunately, nothing could be further from the truth, but that is another issue that has been discussed and can be discussed again.

So, getting back to which is the best PI with the greatest depth, the answer can be quite complex since what works in one location may not work that well in another. Noise, ground conditions, hot rocks, and other factors all can influence how well a detector works.

Now factor in a person's experience with only one detector and it is easy to understand why questions get so many different answers as to which is the best today.


Dave J.

New member
More great stuff, Reg.

--Dave J.

PS: funny you should mention fired bricks. Back in the 80's when I was playing with PI, I had a prototype which I called the "CodFisher" (and therein lies another tale which I probably posted about in this forum a few years back). Not knowing about magnetic viscosity I was puzzled that most of my prototypes had exhibited a slight amount of ground pickup in many places, but were dead silent on a sandy freshwater beach a few miles from town where we did quite a bit of our testing. One day I took the CodFisher home and it sounded off over my brick walkway like the walkway was metal. I'd laid those bricks myself and knew that although there might have been tramp metal here and there in the soil underneath, the walkway was not underlain by solid metal. I yanked a brick out and it sounded off like metal. I busted it up with a hammer: no metal in there. That was my rude awakening to magnetic viscosity.

My next prototype had a switch to flip to maghemite balanced operation. It split just above pulltabs, and this was before zinc pennies became numerous. The split knocked out most iron and made any other iron inconsistent. I was able to search the mineralized beach in what was like a TR disc mode without digging any trash I didn't want to dig, a real sweetheart of a machine, fully static operation, stable, and ran on a single zinc-carbon 9 volt battery. But it didn't have enough hots to be a marketable product.


Well-known member
Love reading your posts and they have helped me to learn to use the PI's I have built.
Found everything you wrote to be true.
I always refer people to your Nuggethunter PI page................

As for depth of my PI's, they do air test excellent with the coil lying flat on a pc. of partical board shelf, I do find in actual hunting
that that depth might not be attained. EMI and other ground noises fluctuate the threshold above the faint target sounds achieved
during the air tests. Here we have black sand mixed in the beach sands and sandy dirt and on the main beach area, there is some strong EMI.
With some adjustment I can still hunt the area...... I have been using the 7" search coil there mostly since the sand depth is normally less than a foot.

I am enjoying the PI and just had some new PCB's made from schematics Smitty sent me that should be more sensitive to smaller gold and be deeper as well.
I just sent him 3 of the boards to build up. Just to let you know, you have had influence on the re-designs of Carl Morelands Hammerhead that Smitty worked on
that we are building.

Keep on posting


New member
Hi Dave,

Thanks for the kind words. Coming from you your compliment really means something to me.

BTW, for those of you who don't know Dave J. Well, chances are you will recognize some of his metal detector designs which include some of the more popular detectors such as the Tesoro LST, the White's MXT and GMT, the Troy X5 , many of the earlier Fisher models including the Gold Bug, Gold Bug 2, the 1200X series, etc, etc.

Now, Dave is now the chief Electrical engineer for First Texas and is responsible for many of the new Fisher, Bounty Hunter, and Technetics detectors. So, the odds are that if you have a few detectors, chances are you have owned one designed by him.

So, in very simple terms, Dave J. is one of the key engineers responsible for the shaping of the detector technology used today.

With that said, your discussion of the CodFisher has me really interested, especially because you built a PI that operated on a 9V battery. Better yet, being able to balance out magnetite also sounds interesting. So, hopefully, you will provide more technical details of this detector and whether it might be the basis for something new in the future.

So, now I really hope you will provide more detailed information about this PI design of yours. Knowing more about different PI designs and what happened with them and why is a big part of what this forum is all about. I am sure many if not most people didn't know the Codfisher even existed. I know I didn't. I also think there are a lot of other people who would like to learn more especially as to the general technology involved plus maybe why it was never taken to market in greater detail. The exact details aren't absolutely necessary but a more detailed overview would be nice.


Dave J.

New member
Back in the early 1980's, Jim Lewellyn (then the president of Fisher) got the idea that he wanted a fully static ground balanced discriminator. At that time I was prototyping multifrequency stuff and told him that motion was the way to go, we'd already proven that with the 1260-X. But he kept on begging for fully static operation, and I'd heard a thing or two about PI (although not seen any schematics or had a PI unit in my hands) and figured that with PI you could do it fully static-- again, not that I thought fully static discrimination was a good idea. His answer to my proposal to develop a PI fully static discriminator was simple-- "PI's are no good." He didn't know any more about PI than I did, but that much he was certain about.

So I figured out a way to build a PI and started bootlegging them when the boss wasn't looking. After a few rounds of prototyping I finally had something that worked-- not well, but at least well enough to illustrate proof of principle. Jim didn't want to talk about it. So, I realized I was going to have to trick him into capitulating.

I built a PI that looked straight out of an old-fashioned hillbilly cartoon. I wound the searchcoil around the rim of a wicker basket, tied it to a branch pruned from a sycamore tree, put the electronics in a little wooden dried codfish box minus the codfish on the other end of the stick, wired the searchcoil to the box with lamp cord, and instead of a real knob on the threshold control I glued a Pepsi bottlecap. Our graphic artist made a label for it that said "CodFisher". And, it really worked, fairly well by the PI standards of that time. About 7-8 inches on US coins, more like 6 inches in disc mode which was just a long pulse delay that lost everything below zinc pennies although iron still came through.

I took it down to the local fish store and worked out a deal with the proprietor Olga. We put a tag on it that said $10, and she put it on top of refrigerator out of sight of the public. If someone walked asking to take a look at a funny metal detector, she'd take it down and they could take a look. And if they wanted it for ten bucks, sell it to 'em for ten bucks and keep the ten. The kicker is that the only person besides me and her who would know it was there, would be Jim Lewellen, because I'd tell him. I was a regular customer and she was willing to have some fun, so it was a deal.

I told Jim that I'd seen a weird beep at the fish store of all places, he oughta drop in after work and check it out. He ignored me, and it, for two weeks. Finally he said, "Dave, I know you're up to something, just tell me." I told him, "Yep, I'm up to something, but the only way you'll find out what, is to go to the fish store." He went, saw it, cracked up, paid his ten bucks, tried it out, and that was the end of "PI is no good".

Several prototypes later we had ourselves a design that was almost ready for production. Fully static with discrimination, meter target ID, and manual ground balancing. He thought it was great and was shocked when the distributors told him they didn't want it. From that disaster emerged the Impulse for underwater work, and a return to multifrequency for land machines which led to the CZ which is still in production virtually unchanged.

The Codfisher-Impulse type PI's were patented together with multifrequency back in the late 1980's (award may have been early 1990's, I don't remember). Simple circuit, ran on a single 9 volt battery, fully static, stable, easy to add discrimination and ground balancing capability, no high voltages, no earth field pickup problems. I suppose that someone's published the Impulse schematic over on Carl's Geotech forum. The thing is so easy to build and get working right that I'm surprised that the hobbyist PI folks never took any interest in it. It's a lots better hobby project than the stuff I see them screwing around with over there.

From a commercial perspective, the end of the Impulse was the end of the Johnson PI platform. Within its limitations it works surprisingly well, but I know enough about its limitations not to be tempted to resurrect it.

--Dave J.


New member
lol!!!!!! i love to read some of the behind the scenes detector evolution stuff that you write about! i always wanted to know more about the "surf and turf" pi machines that fisher had and the pi/vlf combo that never made it to market.


New member
Dave J
great story thanks
Is the Codfisher-Impulse type PI schematic available please ?
I would like to have a go at it
by searching around the net finds a lot of "fishy" stuff but not the schematic or patent info

Dave J.

New member
The Codfisher schematic is a pencil sketch that never made it into the public domain. And won't make it there, it ain't worth the trouble.

The block diagram of the Impulse and various other stuff is in the "returning energy to the power supply" Fisher patent.

The Impulse schematic is linked from one or more posts on the G**t**ch forums. It may look a bit much for a hobbyist experimenter, but a lot of what's there is not necessary for an experimental unit and can be stripped out. What remains is easy to get working right, an advantage it has over conventional PI which is fraught with challenges that often defeat newbies.

--Dave J.


New member
Hi Reg,

great contribution from you again. Thanks.

It is strange that light batteries in and old Fisher Impulse design can last almost 100 hours in comparisons to some high tech's modern design which asking for heavy "car" batteries lasting as little as couple of hours.


Fisher Impulse uses half-bridge tx stage. same sure as in Fisher CZ-5.


New member
Geez ...I don't see what the fuss is about all I see is a lot of rambling on...the person that invented the pulse induction detector deserves all the credit nobody else is getting a pat on the back from me ...start posting some real technology advancements and I for one will be the first to jump up and applaud .This is just another look how smart I am post haha LOL...please don't take this as an insult and I do apologise I'm not at all trying to offend anyone ,i just say it the way it is ...we've seen these posts over and over .....please it's time to talk new technologies you guy are really not doing anybody any favours with these comical posts...just my 2 cents worth...thanks guys


I am wondering about the statement that low power pi’s have a resistor in series with the coil to limit current. Most I’ve seen do not. The few I have seen with a small resistor are perhaps higher voltage sourced and depend on a large bulk capacitor as well. In the low power (lower voltage) Pi’s I’d say the resistance of the coil itself is the current limiter.(?)
It is strange to me that the high power tx techniques are not discussed much. The most I see is about boosting battery voltage to get a little more depth - which should be true with the DF as well, to a point. How different are the DF and TDI BH TX circuits?