Need help - Matchless Metal Detector

[adiputra]

It is not theory, just nonlinearity. Crystal headphones of the olden days of radio were made with a more solid piece of piezo salt crystal that was mechanically coupled with a membrane. Linearity-wise that contraption sucked big time, but also provided some level of demodulation. So when you look at some ancient schematic and see some of these, you'll be in trouble explaining WHY that particular thing works at all.
Nowadays piezo's are much better sound sources, yet their equivalent in electronic sense is - a capacitor of a few hundred nF. They lack the most of the nonlinearities that made the olden headphones work their magic. Some nonlinearities must be present so I guess that might work - as worse piezo as better sound
So far I was not inclined to make a circuit that would mimic the behaviour of the olden headphones, mostly because I don't have any to compare the results with. You could try connecting a capacitor of ~100nF in series with a combination of diode and a 2k resistor to gnd i.e. a simple detector. Some amplification and you have a sort of crystal headphones equivalent. I have no inclination of doing it myself because of the vast choice of better solutions.

oh my, its sound like complicated to me (noob guy)

[jap]

Hi adiputra
can u put schematic.
j.

[adiputra]

Hi adiputra
can u put schematic.
j.

hi Jap,

This schematic taken in geotech website, the IC and the piezo sounder I can't sure I can find it here, but as you guys said try other type like NE556 etc. Also, the coil position almost make me confuse, do I need follow exactly as the author show (in picture, in the project article)? or do I need some calibration/reposition it? Any advice?

[jap]

Hi Gwil,

So, can NE556 IC be used this project? Also, piezo tranducers - is this looks like in WM6 picture? Can this be replace by a normal speaker, say 8 ohm mini speaker?

You can NE556,Also, piezo tranducers in WM6 picture.
Can this be replace by a normal speaker-.....NO ,Can use headphones up 32 ohm
best time
j.

[adiputra]

You can NE556,Also, piezo tranducers in WM6 picture.
Can this be replace by a normal speaker-.....NO ,Can use headphones up 32 ohm
best time
j.

Hi Jap,

normal/standard headphone? stereo or mono type?

[Gwil]

Hi adiputra

I haven't built this detector so I don't know for sure, but I can see one possible problem with using an ordinary 556 timer instead of the CMOS low power 7556. The input impedance of the comparators in the 7556 is many times higher than in the 556, and this may be necessary for successful operation, at least in the IC1b section.

Gwil

[Davor]

I must admit my memory did something funny. There are actually a few "Matchless" concepts, of which the most interesting to me was a circuit that appeared in February 2012 EPE. That particular one depends highly upon the crystal headphones. I even made a LTspice simulation of it.
I guess the other builds are less dependent upon the "quality" of the headphones. Unfortunately they didn't catch that much of my attention.

Of all Matchless flavours the June 2006 EPE is the most unholy of all. It works by forcing the op amp input JFET gates to conduction (!)
Version in October 2003 Elektor Electronics is OK, also June 2002 Silicon Chip version seem OK (here in post #18 )

Regarding the difference between CMOS and bipolar 555s, the CMOS is much easier on the power supply rails because it generates considerably less of a spike when switching. So when in doubt - go for CMOS.

[WM6]

Hi WM6,

Nice picture you have, is this the matchless metal detector project? or it it just another project? And, do you use a screen wire to attach the search coil?

Yes adiputra, it is photo of "Matchless metal locator" (as on schematic you posted).

As coil cable I use screened double microphone cable connected directly (without connector).

My piezo element is relative big of about 5cm in diameter with nice tone, not to high tone as on small one. Hot glued on housing.

According author TTL version of NE556 will not work - but I did not check this. Rod is out of bamboo.

This is an old construction and don't remember all details any-more, except that two different CMOS are working in the same way and that depth in soil was very poor (not to mean that you cannot achieve better results).

[adiputra]

Yes adiputra, it is photo of "Matchless metal locator" (as on schematic you posted).

As coil cable I use screened double microphone cable connected directly (without connector).

My piezo element is relative big of about 5cm in diameter with nice tone, not to high tone as on small one. Hot glued on housing.

According author TTL version of NE556 will not work - but I did not check this. Rod is out of bamboo.

This is an old construction and don't remember all details any-more, except that two different CMOS are working in the same way and that depth in soil was very poor (not to mean that you cannot achieve better results).

hi Wm6,

Ok, so in conclusion, do you think i should build this project or not - any advice/suggestion? and if I still want to continue to build this one, what are the suitable IC i should use? can you give some example of the IC - (as you use in your "matchless metal locator" in your picture)? According to your picture, may I know the depth it can detect metal in the soil?

[WM6]

hi Wm6,

Ok, so in conclusion, do you think i should build this project or not - any advice/suggestion? and if I still want to continue to build this one, what are the suitable IC i should use? can you give some example of the IC - (as you use in your "matchless metal locator" in your picture)? According to your picture, may I know the depth it can detect metal in the soil?

I think it is worth to try to get some experience in MD design.

It is extremely well documented, which is very important (you see - even so, you have a lot of questions).

It is very cheap (you can burn one IC only in worst case).

It is good experience how to make and tune DD coil.

Yes, on depth it is shallow (at least mine - maybe yours perform better), but a lot of stuff you can find will lying shallow 1" or 2".

As IC use any CMOS version of 556, how your MD will perform, will be mostly on your coil construction and on quality of used earphone and not on IC brand name.

Try with smaller coil than mine first, say no more than 12cm in diameter (or as per author directions).

[adiputra]

I think it is worth to try to get some experience in MD design.

It is extremely well documented, which is very important (you see - even so, you have a lot of questions).

It is very cheap (you can burn one IC only in worst case).



It is good experience how to make and tune DD coil.

Yes, on depth it is shallow (at least mine - maybe yours perform better), but a lot of stuff you can find will lying shallow 1" or 2".

As IC use any CMOS version of 556, how your MD will perform, will be mostly on your coil construction and on quality of used earphone and not on IC brand name.

Try with smaller coil than mine first, say no more than 12cm in diameter (or as per author directions).

Hi WM6,

Thanks for the reply. Sorry for a lot of question, I just want to know what i'm trying to do, well its useless if I try to build something that I don't know how its working or maybe why its not working etc. That's why I need some solid answers from you guys (mostly guru titled in this forum - I assume most of you guys are above senior member, have a lots of experience right?). I'll try build this project using NE556 IC or other type as mention above and try to see how it will works. By the way, to all people that help me in this issue, many thanks to all you guys.