I didn't need another project -

[Pipeliner]

Hi Eric, Battery check is good just below 9v. Only reading below scale at white capacitor, with red positive lead on metal frame and black ones wire then white terminal, got 2v reading. Thanks again for taking time with suggestions...Rob

[Ferric Toes]

Hi Pipeliner,
Yes, I am enjoying the XL500 and have done a few modifications (upgrades) to the electronics which, IMHO, greatly improves the performance. It is now a 'motion' type detector with a smooth, sharp response. I have yet to take it to the beach, but hope to in the near future.
First thing is to get yours working. On first switching on, the meter should read battery volts for a few seconds and then revert to the signal reading. The meter is scaled 0-10 and this relates to battery voltage when switching on. If there is no reading,the next thing is to see if the battery voltage is reaching the circuit board. On the component side under the meter is a large cylindrical capacitor and the battery volts should appear across this. All you need is a d.c. voltage test meter to do this. To make things easier, unscrew the four red cap retainers on the solder side and swing the circuit board sideways on the leads so as to have the component side up. This makes it easier to connect to this capacitor.

Do this first and report back.

Eric.

[Pipeliner]

Hello Ferric Toes, I registered to this site and your post came up first with the search. I'm recently retired and hoping to spend time detecting. I have the same story as yours with the xl500. Picked up off Ebay about 2 weeks, same price as you out of Jacksonville Fl. Had the identical experience as you. Factory batteries have been replaced with 8 sub C installed at batteries plus. 9volts at the batteries. Coil attached and no activity on the gauge or headphones. Any direction/guidance/opinions would be appreciated! Hope your rebuild has gone well and you're already enjoying it!

[Ferric Toes]

Sometimes a picture helps. Charted saltwater slope -5/2, ground slope -1 and a nickel TC=10. The decay slope for the nickel is the same when delay equals nickel TC times power slope. Nickel decays faster at longer delay time.

Thanks Green. Yes, a picture does make things clearer. I have a sea water graph plotted at the time and compared to that of an underwater gas pipeline, which is not very relevant to hobby use. My problem would be finding the plot amongst all the papers going back the the 60's and 70's.

Eric.

[green]

Sometimes a picture helps. Charted saltwater slope -5/2, ground slope -1 and a nickel TC=10. The decay slope for the nickel is the same when delay equals nickel TC times power slope. Nickel decays faster at longer delay time.

[Carl-NC]

In geophysical prospecting a salt wet sandy beach would be regarded as a 'conductive half space', the background decay of which obeys a t^-5/2 law. Tests done back in 1968, by lowering a coil into the sea from a boat, indicated the same decay law.

Eric.

Good to know, thanks.

[Ferric Toes]

In wet salt sand I believe the decay is reasonably exponential. I assume the tau depends on salinity but probably on the order of a couple of microseconds. In full submersion it may behave differently, I've never investigated. I would also assume that moving the coil away from wet salt sand will require more distance than a coin to cut the signal in half because the salt "target" is so much larger.

Yes, a Figure-8 will cancel salt effect.

In geophysical prospecting a salt wet sandy beach would be regarded as a 'conductive half space', the background decay of which obeys a t^-5/2 law. Tests done back in 1968, by lowering a coil into the sea from a boat, indicated the same decay law.

Eric.

[kt315]

Ask a bunch of nuclear physicists for a bread recipe, they might include a particle collider on the list, just after wheat flour and before yeast.

How about glow in the dark.

nothing new. such technology exists 200 years. we call that nuclear magnetic resonance (NMR).
NMR is an abbreviation for Nuclear Magnetic Resonance. An NMR instrument allows the molecular structure of a material to be analyzed by observing and measuring the interaction of nuclear spins when placed in a powerful magnetic field.

For the analysis of molecular structure at the atomic level, electron microscopes and X-ray diffraction instruments can also be used, but the advantages of NMR are that sample measurements are non-destructive and there is less sample preparation required.

Fields of application include bio, foods, and chemistry, as well as new fields such as battery films and organic EL, which are improving and developing at remarkable speed. NMR has become an indispensable analysis tool in cutting-edge science and technology fields.

[ivconic]

Thanks both.
Despite (needle) resonance way is possible, as we know from from resonant small pole WiFi antennas,
we are here with about 3 - 5cm needle length in couple of GHz band to get resonance , so searching dipole
antennas become very small and unpractical to search some school yards, cause antenna should be put as
close as possible to hidden object (to exclude as much amount as possible of disturbing surrounding medium).
UHF dipole antenna can be increased in stacked antenna way (this add more questions and a lot of research)
Differentiating way (as described in present article) is in fact based on eddy current principle. At very high
frequencies different medium (soil par example) act as ferrite core load with very different characteristic
(par example eddy currents free or absence substances act as hole in surrounding medium). Main problem
here is amount of detecting medium (needle) which we are trying to differentiate from surrounding medium.
This amount is very small, usually to small to get useful distinct for indication.
Regular sewing needles are easily to grab with magnetic broom but hypodermic needles are made from
high grade medical steel and this way immune to magnet.
Probably we need here another approach and start to produce ferromagnetic medical injection needles.

You don't need to worry much on antenna design. Been there. Done that.
You'll have to figure out "only" the rest of circuit.

[dbanner]

Ask a bunch of nuclear physicists for a bread recipe, they might include a particle collider on the list, just after wheat flour and before yeast.

How about glow in the dark.

[WM6]

An RFID chip embedded in the base solves the problem.

Don't say, someone will realize this post-mechanical approach,
and then we face with a lot of interference with human RFID.

[kt315]

like a maniac. have to inform. winter theme again? we had have huge thread on needles & pins. just reminder to you.

https://www.geotech1.com/forums/show...dermic-needles

[6666]

Here in Australia we currently have some maniac putting sewing pins about one inch long, into strawberry's and people are finding them after they have bought them in super markets.
Also happened in 2018 but they caught that nut case.

[Carl-NC]

I seem to remember that in the '40's -50's, hypodermic needles had a larger metal fitting at the end to connect it to the syringe. Re-introducing that would solve the detection problem.

I recently consulted for a company who was designing a medical metal detector. As you can imagine, they wanted to detect all sorts of difficult targets including titanium which was impressively hard to detect. Mostly instruments though, and the topic came up of tagging everything with RFID chips. They are super-tiny and completely passive. The needles I see lately have a stainless shaft but a plastic base. An RFID chip embedded in the base solves the problem.

[WM6]

I seem to remember that in the '40's -50's, hypodermic needles had a larger
metal fitting at the end to connect it to the syringe. Re-introducing that would solve the detection problem.

Yes, nice idea and probably most simple solution, Ferromagnetic ring or bead at syringe connection part
of hypodermic needle.