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Offline adnimoTopic starter
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« on: August 11, 2009, 05:08:38 am »
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Im willing to start my first metal detector project. From what I could gather a BFO would be the best alternative at least to start with. Should I work on the

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http://www.easytreasure.co.uk/bfo.htm
? If not, please point me towards the right direction.

One thing Im uncertain of is the actual coil. I have to match both in total length right?, if I use very thin insulated copper wire (the kind youd find in an old buzzer or a guitars pickup just to mention a few) will I have too much loss?, Im more after a probe that I can use to find metal behind 1-2CM of wall.

I could buy one of the commercial ones but there is no fun in that!

Any help will be greatly appreciated, thanks.

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« Last Edit: August 11, 2009, 05:10:34 am by adnimo »
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« Reply #1 on: August 11, 2009, 07:07:14 am »
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adnimo;

Are you electronics literate? (To adapt a computer phrase)

I can give you a formula for  winding coils, I could tell you hown to use a small frequency counter, I can't tell you an easy way to deal with the distributed capacitance of the winding.

It has little to do with the length of the wire, in the coil, it has everything to do with the inductance of the coil.... the diameter and the number of turns. Does that answer your questions or make it worse? If you are not electronics literate, it would be better to state needs and then build on the calculations of somebody else.

Have you got any kind of schematic and/or directions?

BFO's need a Faraday shield, what do you have for a coil form? A friend of mine made one by
using 1/2 inch soft copper tubing bent into a circle, then a slot sawed around the outside. That saw cut has to be de-burred so it doesn't cut insulation. If you want 6" diameter, make the inside 6", etc. I can draw you a simple sketch on how to mount stuff on this copper circle, but it is hard to describe, clearly.

The BFO constant tone needs to be around 800 to 850 Hz, this is a long established comfort zone for radio telegraphy and directly applicable to BFO use.

How are you doing?

goldigger


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« Reply #2 on: August 12, 2009, 01:24:15 am »
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Hi, thanks for the info.
I'm starting in electronics, so inductance is something I've yet to fully grasp. It's certainly a taboo for me.

This is the schematic I'd like to follow, although I'm unsure whether it's the right choice: {alt}
I already have all the materials (from my junk box!)

I'd like to avoid ICs because they are hard to find in my country. If there is a need for a counter (555, etc) I could find those since they're relatively common but I'd still like to avoid them as much as possible.

One of my biggest problems is that I don't have an LC meter...

Ideally I would like to reproduce the detector that the original author created at the aforementioned link, however I'm open to better alternatives if any.

From a scale of 1 to 10, 1 being someone who doesn't know the difference between AC and DC and 10 being someone who built an entire PC from scratch (see

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) I would dare say I'm a 2, 3 tops. (Being generous). I can read most schematics without a problem and understand the majority of them, unless it involves inductance which is my Achilles heel. I've built PC interfaces on serial and parallel (ir receivers, transmitters, etc) but that's about as much as I've done so far.


edit: I'm not sure why the forum escaped some characters, I'm certain ASCII 39 is very common in English...


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« Reply #3 on: August 12, 2009, 01:34:42 am »
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One of the most wonderful things about LC circuits is their predictability. You rarely need a meter as the circuits behave just like theory. Unlike other active components they are quite tame.

Good luck with your studies and welcome to the forums.

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« Reply #4 on: August 12, 2009, 02:40:37 am »
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adnimo;

Do I dare ask what country? Try US English on your computer as UK, or European anything, has strange reactions with some characters.

I should rework the circuit and make a small change that will make an improvement in its performance also I can send you a small diagram of a simple bridge for measuring capacitance and inductance but you will have to do paper and pencil OR calculator work.

So I will give you some informal formulae (how is your Latin? Mine is rotten.)

The circuit, you have shown, has a small problem, not serious but can be improved. I noticed it does not list the transistor types.... can you get the common ones, like 2N2222, 2N3904, C1815, the BCx equivalent to 2N2222? (like BC108??)

I can redesign, slightly, so all transistors are the same... I just need to know which you can get, there. Here, mail order to the US, will get me most of the general purpose ones, such as 2N2222 or 2N3904 for 6 cents US, or less.

If you are in Iran, I can understand the scarcity, but can not help, with that. I can hep with the electronics, though. (I think.)

About inductance, think of it as a coil's equivalent rating to a capacitor's Farads, nano-Farads, micro-Farads, if that helps.

Capacitors and coils have ac resistance, and it is called 'impedance', for either, but the impedance changes with the ac frequency and the inductance does not nor does capacitance.

When I learned electronics, things were a bit different, for instance, I had to re-learn capacitor designations, from micromicro-Farads (uufd) to nano-Farads or .001 ufd for 1 nanof-Farad that I knew as 1 uufd.

Now I use micro-Farads, uf (forget the nano-Farads, just use decimal micro-Farads,) and I use pico-Farads (pf) I find it easier.  A lot of the old formulas that took forever by pencil, I redesigned, for calculator. Its quicker, what took a whole sheet of paper, once, I can now do with a couple of finger pokes.

A prime example is capacitors in series and resistance in parallel. The inversion capability, on any calculator, does it effortlessly, example: .01 uf in series with .047 uf

invert .01 and put it in thhe memory;

invert .047 and add it to that memory;

recall the memory and invert it, you should have .00824561403 ufd.... or if you dont have that many decimal places, maybe you have .00824561 ?

Resistance in parallel: 4700 ohms parallel 5600 ohms:

invert 4700 and add to memory;

invert 5600 and add to memory;

recall, invert = 2555.3399 ohms.

I like to work with kilaohms (k) because I used to only have a 6 digit calculator (long ago) now I have a 12 digit with two independent memories.

Hey! a short lesson on sneaky tricks... how about frequency:

1) L in micro Henry's (uHy) X C in picoFarads (pf);
2) find the square root of this;
3) divide it INTO 159.159159159 or as many digits as your calculator can hold;
4) the answer is in megaHertz

AND:

1) L in uHys X C in microFarads (uf);
2) find square root;
3) divide INTO 159.159159 etc;
4) answer is in kilaHertz (kHz)

159.159159159 is derived from pi from as many decimal places as is practical and is more accurate than the formulai I learned... and easier to remember.

Next time.... and I will redo the circuit

goldigger


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« Reply #5 on: August 12, 2009, 05:13:57 am »
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Those are some handy tricks!

I'm in Argentina, theres only two places I can get most electronics from and they're at 60 minutes by train, so it's not ideal. Needless to say you can't find Arduinos or almost any IC for that matter. int-shipping is not an option since they delay everything in customs and, again, it's a long trip to the post office and a long wait as well. I don't know why they do this because a) I never imported any hazards so I shouldn't be on any list and b) it only puts a stop in progress. Then again it's the third world, what can I do...

I was thinking about using "603" transistors (2sd603 -

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) because I have plenty of them. They're old japanese low-frequency low-current NPN transistors. I don't have polyester caps so I'd be using ceramic disk which I have in excess.

The original circuit calls for "BC184B".

What influence does the gauge of the wire have on the inductance of a coil?

I'm quite interested in building a simple LC meter, I don't mind doing calculations; you can't live without maths anyway!.

Thanks

PS: My keyboard is always set to US English because I program a lot and it's the only way of doing it properly, Spanish setup requires a lot more movement of your hands mainly because most programming languages were designed with English keyboard layout in mind. It's also the first time I encounter this issue in a forum. I'll stop using apostrophes if not...

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« Reply #6 on: August 12, 2009, 05:30:57 am »
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From the circuit webpage:

Search coil: "10 turns of . 25 mm  enameled copper wire" -- 15cm diameter
Reference coil: "125 turns . 25 enameled copper wire" -- 10/12mm diameter and about 50mm long

I reckon this are just estimates, though. I couldn't find anywhere in the page the amount of μH it calls for... It just mentions: "Ideally This coil will be oscillating at about 104khz, with an amplitude of about .5v p to p". I did a rough estimate but I'm unsure about the results.




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« Reply #7 on: August 13, 2009, 07:51:11 am »
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adnimo;

OK, I thought Argentina was fairly progressive, especially after the Falk.... er.... Malvinas Kerfuffle? There are a lot of Canadians, there, to day, involved in the petroleum industry and some in forestry, also wine making....

Customs! Yes the blighties, they used to delay things, coming into Canada, for up to 3 months, for no reason. Then I discovered, if you get the order AIR mailed, to you, customs did not even open the package: boom, right through and on its way!

What would happen if I mailed you a half dozen 2N2222s and a half dozen 2N3906s, I would have to make  a customs declaration of a GIFT and no duty should be required....Huh??

Are you Spanish speaking? Obviously. I have some Mexican Spanish (Indio oriented, mostly)

I have struggled with the French terminology, as Can. is bi-lingual, but I think I can say I can decode the terminology, but Spanish I have yet to see much of.

Tell me what you have for tools, like soldering tools, etc. Can you make printed circuit boards?
If you cant, terminal strips work, a bit messy but functional.

I have altered the circuit, which you posted... its an old one on this forum.... and I have changed the oscillators to a much, much more stable design and included a simple 2 diode mixer, rather than using the base/emitter junction as a mixer. The diodes, I have listed as 1N914: can be replaced by 1N4148, it will be the same and you could even use a pair of 1N34, 1N64, etc, as long as they are the high frequency, low capacitance type, for general radio use. They do NOT have to be matched. No diodes? use one of those 2SD...., the base (equivalent to the two anodes) goes to the center 100k and the amplifier and the collector (a diode cathode) goes to either of the other 100 k, while the emitter (another diode cathode) goes to the last 100k. You can not use a transistor, like this, in any circuit that will exceed the 5v or so, reverse volt rating of the base to emitter junction.

I have no clue to your  age, like, if you are living at home, or?? I would suggest salvaging parts from old, dead TVs, but if you are still at home or in a school dorm, it may make it hard to do and to dispose of the non-usable remains. TVs have all kinds of semiconductors, as long as they are not Motorola, which have in-house markings, even so, one should be able to Google the in-house codes.

I am going to upload the changed circuit, to the forum.... unfortunately, I worked on it with my lap top, and it has degraded seriously? Print it out, if you can.

OK, go online and get yourself a free copy of a program called pad2pad and I can send you some printed circuit files. If you have a printer, that will print on laser transparencies, or even inkjet transparencies, you can make PC boards. There are all kinds of materials that you can use for the etchant mask, also called RESIST, since it would be hard for you to get a photo etch kit from Jameco. The resist just has to protect the copper that you want on the board.

There is a 15 post requirement for stuff like this, but you sent ME the circuit, so it does not apply, I am just showing you how to improve it and not supplying totally new stuff.

The apostrophe problem is supposed to get fixed... I hope.

OK, normally I am used to wire rateing in AWG (American Wire Gauge) and I used to have a cheat-sheet on wraps per inch = wire gauge, so if you did not have a gauge, you could cheat.

 Funny

I see. The frequency to  which I redesigned the BFO schematic, is 30 kHz... you will need 127 turns for a 6 inch (15.25 cm) coil, using half inch tubing; 104 turns for an 8 inch ( 20 to 21 cm) coil, using half inch tubing; 88 turns for a 12 Inch (30.5cm) using 3/8 inch tubing, and 76 turns. These instructions will not make exact values as they are calculations and when you wind  the coils, they develope distributed capacitance and this throws the values off.

You can salvage wire from the primary of auto ignition coils, the old type auto battery regulators, old transformers, auto relays, or buy wire wrap wire, which is 28 to 32 gauge, probably less than 0.25mm. The ignition coil wire will be about 0.5 or 0.75 mm, Im guessing. Moderate insulation is OK, it reduces distributed capacitance, but potting the coil with shellac or epoxy will increase this capacitance.

I have to go to my other computer to do a design sketch, for a simple LCR bridge, which is only as accurate as your hearing, but basically, you need a tiny transformer, with reasonably equal primary and secondary windings; the primary side makes a one transistor oscillator... 10 kHz is a good frequency (use one of the 2SD...). The tap goes to + v, one side goes to the collector, the other side of the primary winding, still, goes to a resistor, say 47k which you can bridge with .1 or .01 ufd. Place different size caps across the whole primary, until you get 5 or 10 kHz.

OK, on the secondary, ground the center tap; one side, attach a red binding post, with a black binding post for center, not  ground, do the same with the other side but use a green binding post for that side. Install your earphone jack and connect the two black posts to the ear phone jack, and the other earphone jack lead goes  to ground (center of the secondary winding.) make sure those 2 black posts do not get grounded. The tiny phone transformers, 600 ohm CT : 600 ohm CT are ideal, and .022 ufd can be padded to bring it to 10 kHz.... whatever works.

TO USE: attach a potentiometer to one set of posts, the green and its black; attach a known capacitor to the other set of binding posts (I am assuming you achieved 10 kHz and can hear 10 kHz) Plug a crystal type hi-impedance earphone, into the phone jack, turn on the osc, and you should hear the tone. Turn the potentiometer, until the tone is zero! The bridge is balanced. Measure the potentiometer, with one end free (dont turn it anymore, of course.)

The value of the resistance equals the capacitor,s impedance at 10 kHz! Go to your calculator and figure the value. If the printed value was 0.01 ufd, for instance:

1 / 2 X pi X mHz X cap in ufd   or  1 / 6.283 X .01mHz X .01   /10 kHz is .01 mHz/ 
(result = 1591.5915 ohms)

So to reverse this, for your process, you first invert the resistance:

 (1 / resistance in ohms) / (2 X pi X frequency in mHz) and  (1 / 1591.5915) / (6.283 X .01)

gives 0.01000003055, on my calculator, which is 0.01 allowing for decimal error. On an 8 digit calculator, the error may be worse.

So, you can see how to turn resistance into capacitance and the same works for an inductance. However, your useful range is about 10 or 100 ohms to about 9,900 ohms and 10 kHz, will give ranges:

caps : 9,900 ohms = 0.0016 ufd to 10 ohms and 1.5915 ufd .... not great;

inductances: 100 ohms at 10 kHz = 1591.5915 uHys to 9900 ohms and 157,568 uHys or 157.568 milliHys.

So I will leave you with that, for now and upload the circuit.

goldigger (Brian)

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« Reply #8 on: August 14, 2009, 03:25:23 am »
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Hi Brian, thanks again for your support, I really appreciate it!

I began to draw your diagram in an electronics simulator (so I can later on create a pcb design) when I noticed some resistors are missing their value. I'm also unsure about the electrolytic at the very right corner of the diagram. There is also a confusion on my side regarding the coils (Perhaps I'm not used to the European DIN nomenclature -- or should I say symbolism?) Are the coils those circles with a dot?. Is Coil B supposed to be a variable inductor? (The way I've learnt inductors is with a spiral and two straight ends, when it regards a variable one a diagonal arrow pointing up is placed on top - However I understand that DIN signifies resistors with a rectangle and what you've drawn would be a variable resistor, but I'm unsure since no value is described either).

After taking another look, the electrolytics at the right appear to be 100 micro-farads, but I'm still unsure about the resistors with missing value. Are those supposed to be 1k?

At the "circle with a dot" on the right (coil b??) the number 16 is placed on top, is this the inductor's value?. I'm clueless.

- Ok I set the simulator to "European DIN" and the inductors I had already placed did not change to the aforementioned symbol, so I'm truly out of guesses!

I hope you can clarify it and I also hope it isn't too much of a hassle since you've already helped a lot.

Thanks again!


Posted on: August 14, 2009, 03:50:33 AM
Update:
{alt}

This is the "ported" schematic so far, the resistors with "?" and "1ohm" are the unknown ones, theres also the issue with the coils and the variable resistor (which in case it really is one of them, I don't know the value).

Once that's clarified I could run a simulation / tidy up the diagram / share the file / create a pcb / hope it works.

I never actually made a PCB before, but I've seen my brother work on them (although back in the day he used a sharpie and nothing else, now almost anyone can create great PCBs with low cost, which is great!) - I basically plan to use the sponge method with ferric chloride (or hydrochloric acid / hydrogen peroxide if I can't find the bottle at home). I've read about regenerating old ferric chloride but I never tried it. needless to say it should all be tried outdoors and with proper protection (I did learn my lesson when I was younger but with other chemicals) - For the resist I plan to use a laser printer and high-gloss paper (it seems to work well, at least most people agreed on this).

Cheers

PS: Regarding the LC tester, what kind of phone coils do you mean? I have a couple of old analog phone boards that happen to have a small inductor, although this ones have 10 leads - Any idea?.


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« Last Edit: August 14, 2009, 03:31:28 am by adnimo »
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« Reply #9 on: August 14, 2009, 08:10:20 am »
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adnimo;

I was working in paint and only had the symbols, already there, to use so I used a very old symbol for a variable coil.... its not a resistor, that would be a 10,000 uHy coil.... actually, I dont use European, being in Canada.... its old fashioned and the spiral drawing disappeared in the 40's, here. I did the LCR bridge in paint, also.  Exhausted

In fact, I hate the rectangles. for resistors, they can be mistaken for other things, also, I dont like the capacitor symbols (2 bars??) I did a simple LCR bridge, it will work for components in the VLF detector range.... the 2SD603 has an hFE of 360, according to the chart, that is pretty high, also I substituted 2N3904, for the oscillators, in the BFO (with an altered circuit) and 2N2222 OR 2N3904 for the others. What I really dont like is there is no volume control.  Violent

The electrolytic is there to prevent interference leaking in, from the oscillators,a filter, (nice clean-up) the things you thought were coils are jacks, put them back, the one on the right was an audio output jack, not a coil; the one on the left is a coaxial jack for the search oscillator coil....  No no!

C7 and C8, at  .1 is may be too big and may cut the AF too much, I have included R6 to filter  out the higher frequencies, but can be 3.3 k, see below about a capacitor across R8. You want a  tone about 800 to 850 Hz or a bit lower, otherwise your ears get fatigue. I downloaded  your image and will correct, here.

Replace VR1 with a 10mHy variable (slug tuned) coil, it has a ferrite slug, that turns in and out; R8 is 470 k (I put it on there...where did it go); remove L1 and L2 and replace L1 with a coaxial jack, for coil A, which is off the circuit sheet; then replace L2 with an open circuit phone jack, or a symbol for a Motorola phono jack.... this is for earphones,, which are also off the sheet and are what you call "understood", because they do not appear in the schematic.

I thought you were doing every thing by hand or I would have totally redone the diagram. I use sub-standard North American style..... that is, what I have adapted for quick hand sketching.

What (name) is your simulator and what files can it read?? I have pad2pad for doing boards... its simple and I simulate in my head.  If it can read pad2pad files.... but not likely. Pad2pad is a free download... easy way to do boards, from someone else.... can you revers engineer with that simulator....?

Q6 is an emitter follower, and is low impedance, note that R15 is a 100 ohm resistor, no mistake, and change C16 back to 220 ufd, it needs good low frequency response; the C16 connects to the jack, not a coil; R4 is 1k; R5 is 10k, never changed; try bridging R8, the 470k that was missing the value, with a .001 ufd, it will tend to reduce the 30 kHz remnant.

After you make the changes, upload another copy, so I can check for any remaining errors.... It would never work, the way it is now.

Not too bad, but you were second-guessing me and so you misinterpreted a few things, not unusual. If you feel we  are cluttering the forum, too much, try emailing me, just click on the little square, where it say online or offline. If you think schematics are weird now, you should have seen them in 1952... far less standard, for one thing, and all vacuum tubes. (Thank Heaven for little semiconductors....)

There is a way to get a volume control into the circuit, replace R5 with a 10k potentiometer, then connect C6 to the wiper.... C6 could be bigger, even 1 ufd..... I just discovered a fatal error, the emitter end of R4 has to have a wire, directly to the junction of C9,C10, and C11, without it, the circuit would never oscillate, because there will not be feedback, look at Q1, it will be the same; also the junction of D! an D2 should not connect directly to the base of Q4, but R8 should.... (this was something I forgot to fix) place  a .1 ufd cap between the diodes and the R8 and base connection, that is , isolate the dc volts, of the diodes,  from the base. Then run a 22k resistor from the diode cathodes, to ground, so they have a dc return.

goldigger

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