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Results 1831 - 1860 of 1917
It's very difficult to explain in a non-technical way, and many non-technical explanations I have read are horribly wrong and misleading.
Different manufacturers/machines work differently, analogue does things differently to digital. For Minelab-specific (and general) info, take a look on Minelabs' website, there's a couple of .pdf's by Bruce Candy that are quite informative.
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Pimento
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Thomas Dankowski Metal Detecting Forum
This is the weighty pdf detailing electrical properties of alloys. It contains measurements taken at a variety of temperatures, so makes for hard reading, the graphs are particularly cluttered. Page 33 of the pdf is the Au-Cu data, cupro-nickels are on page 44.
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Pimento
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Thomas Dankowski Metal Detecting Forum
I can offer a fairly good insight into the behaviour of the different gold alloys, the science equally applies to bronze/brass alloys and cupro-nickels too.
Alloying two metals can produce hugely different characteristics to the individual elements, the ratio of the two is very critical. Ditto for 3 elements.The idea that the resulting alloy behaves like an 'average' of its constituent
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Pimento
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Thomas Dankowski Metal Detecting Forum
If you fancy a bit of hacking, Yeasty, you can get Hall sensors out of brushless DC motors, such as 12V cooling fans in PC's, floppy disk drive motors, CDROM drives, VCR motors and more. They are usually 4 leaded SOT-23 surface mounted, fiddly, but they're 'free', so a few can be destroyed in the name of science! They need a DC voltage applied to them, and then the differentia
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Pimento
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Thomas Dankowski Metal Detecting Forum
Yeasty - using a second coil the way you have done basically makes a transformer, so your pickup loop voltage mimics the drive voltage to the main transmit-coil. To actually measure the current, you would need to insert a low-value series resistor inline with the coil drive (one Ohm, maybe?), then measure the voltage accross that. Or use a non-differential magnetic sensor near the coil, such as a
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Pimento
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Thomas Dankowski Metal Detecting Forum
Based on the timing details above, and the scope traces posted, channel 1 has a high-freq. burst of 22560 Hz, (fundamental = 2820 Hz) and channel 11 has hf burst of 27600 Hz (fundamental = 3450 Hz). The ratio of these extreme values is 1.22 :1. The centre-frequency (chan 6) would appear to be about 25 KHz, as previously suggested.
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Thomas Dankowski Metal Detecting Forum
I have made a minor error in my description of the make-up of the pulse-train.
On closer examination, I realised the 25KHz burst is assymetrical, it consists of 7 1/2 cycles, not 8. As good engineering design dictates the whole pulse train should be symmetrical/balanced, there is more to the timing than I first thought. In fact, the missing half-cycle is added in before and after the 25kHz burst
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Thomas Dankowski Metal Detecting Forum
I was going to suggest you try that, but now Carl has said it too - the current is the important thing, that's what generates the magnetic field. Current and voltage aren't in phase on a single-frequency machine, they're certainly not going to be so at 'two frequencies'. Don't forget the receive coil, either. Coils are differential transducers, that is they respond
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Pimento
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Thomas Dankowski Metal Detecting Forum
Regarding the way in which the frequencies vary with channel no. : The waveform you can see on the oscilloscope traces is digitally generated, from software timing methods (though it could be digital logic chips). The timings are precise multiples of the 'basic' block - the time for a half-cycle of the 25KHz wave. All the other timings are multiples of this. The burst of 3KHz is made fr
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Pimento
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Thomas Dankowski Metal Detecting Forum
This thread states the '28 frequencies' claim is more advertising than technically useful reality. There will be lots of harmonics generated by a square-wave signal. What you do with them is the important thing.
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Pimento
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Thomas Dankowski Metal Detecting Forum
It does look like chan 11 is about 20 - 25% higher in frequency than channel 1. Equivalent to a change from 13 KHz to 16 KHz, or from 5 to 6 KHz, for example. The difference is subtle, but probably enough to make a difference on those on-the-limit signals. Worth considering, though, is the question - Is the detector optimised for the middle channel 6 frequency, and might operating at 1 or 11 be s
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Pimento
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Thomas Dankowski Metal Detecting Forum
The detector isn't designed to work with the coil disconnected, so what it does under these conditions is irrelevant. If you seriously want to see if the control unit is picking up EMI, you need to short the two receive-coil pins together on the control unit, or maybe use a 43 Ohm resistor to simulate the coil resistance.
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Pimento
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Thomas Dankowski Metal Detecting Forum
Another sound-card scope/spectrum-analyser program you might try is a free German one called SpecOszi. On one of my laptops, I was able to run it a 192000 samples/second. This should allow you to see more high frequency detail.
Edit: I only have it on a Packard-Bell at the moment, the soundcard max. sampling rate on this is 96000 samples/sec, the spectrum analyser display goes up to half this, i
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Pimento
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Thomas Dankowski Metal Detecting Forum
When this was discussed recently on Geotech's forum, I'm sure someone (Qiaoshi?) reckoned that this WAS a good thing. A win-win situation?
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Pimento
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Thomas Dankowski Metal Detecting Forum
The power put into the transmit-coil drive on a modern machine is still a modest proportion of the total consumed (amplifiers ,comparators, A to D converters, microcontrollers, LCD drivers, etc all take power) so TX power could be increased and still give presentable battery life. Eg. if an F75 ran for 10 hours instead of 40 hours, that would be OK with most folk. But as Carl says, there's l
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Pimento
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Thomas Dankowski Metal Detecting Forum
I suspect there is some confusion over what constitutes 'Coil Power' here. Several possibilities exist: A: How much power is consumed from the batteries as a result of driving the TX coil, B: How much total power the coil itself dissipates (heat and RF transmission) and C:How much power is actually transmitted as RF/magnetic field. Clearly case A is largest, B next, then the smallest by
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Thomas Dankowski Metal Detecting Forum
I thought it would be worth explaining why I would like the iron-tone the way I described.
Having got used to the F75's non-obtrusive iron tone, I appreciate the benefits of being informed about what is under the search-coil, even if I don't want to dig it. Example: If I get a 90's TID iron-false, I can easily find the iron, and explain the false. But when searching in woods for i
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Thomas Dankowski Metal Detecting Forum
Having just spent some time trying to find targets in a nail/iron pit, I've been experimenting with single-tone and all-metal modes on my F75.
As a result, I would like to add to my 'dual-freq F75' wishlist (2 posts ago):
In single-tone disc mode '1', can we have the adjustable audio pitch, as set in all-metal mode?
New dual-tone mode,'2': anything disc-ed ou
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Thomas Dankowski Metal Detecting Forum
I wouldn't recommend trying, it would need some design skill.
The master oscillator is controlled by a 60 KHz mechanical resonator. To shift the frequency, this would need to be changed to something in the 59 - 61 Khz region. Resonators are not available at these frequencies, so you would need to design your own precision oscillator and inject its signal into the existing master oscillator.
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Pimento
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Thomas Dankowski Metal Detecting Forum
I would assume the detector electronics would be calibrated with a dummy coil, signal generators and other custom test gear. Coils are probably tested on a dedicated rig, too. The coil and electronics together would just be given a go/no-go test at the end. It's generally not a good idea to match coil and electronics, because when you put a different coil on, it might not work properly.
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Pimento
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Thomas Dankowski Metal Detecting Forum
I knew I'd seen 100mW mentioned before, I just didn't pay much attention to the fine-print, as I knew detectors typically ran nearer 10mW. Up to 250mW would be a viable coil power whilst still giving adequate battery life, but it would be of limited use - high EMI locations being one possible. Lowering TX power, as per the original posters question would have minimal benefits re. batter
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Pimento
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Thomas Dankowski Metal Detecting Forum
Welcome to the Forum.
I would like to correct you regarding 'all LCD's can easily be backlit'. This is absolutely not true. Displays that are DESIGNED to be backlit are sometimes used without, and thus adding the feature afterwards is possible. But a great many (simpler/cheaper ones) are only intended to be front-lit, and adding a proper backlight is near-impossible.
The internal
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Thomas Dankowski Metal Detecting Forum
The power put into the coil is rather low. It doesn't need to be high, you're only trying to find things 10 inches away, not 100 yards away. Government regulations (FCC, EC, etc) limit the maximum power too. Reducing transmit power doesn't severely affect depth, but it does start to worsen existing issues like circuit noise, electrical interference pickup etc. It's generally c
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Thomas Dankowski Metal Detecting Forum
CZ machines combine the 5KHz demod signal and the 15 kHz demod signal in a 'factory set' way (I think the 'Salt' setting changes this, but most adjustment is fixed internally). The Whites machines look at the demod signals independantly, and thus have more options, in terms of - responding individually to each freq; responding to the 'best' one at any given time; res
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Thomas Dankowski Metal Detecting Forum
Just picking up on a minor point re. lower stem length. It is possible to drill ( with care/skill) a new hole closer to the top end of the lower shaft, and relocate the spring-button. This could gain you 1/2 inch / 15mm in length. The joint overlap still seems adequate.
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Thomas Dankowski Metal Detecting Forum
Re: F80 - 12 years ago
Johnny,
the F75 runs at 13KHz. Hence Tom's earlier wish for a 5 kHz-ish F75 for his nail-pit.
If they weren't so expensive, that would be one project I'd have considered having a go at....
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Thomas Dankowski Metal Detecting Forum
I hope it will be able to do both frequencies independently, in the F75 way, as well as simultaneously.
If you're reading this Mr. J, can it have some of these features, please: reasonably water-resistant. A thick-ish screen to protect the LCD. Breaks down into three equal-length short-as-possible parts (with coil still attached to lower rod) for optimum compactness. Has a depth-gauge going
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Pimento
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Thomas Dankowski Metal Detecting Forum