2 boxxxx :)

[mikebg]

SPICE simulation

The batteries B1 and B2 are large 2V rechargeable cells. Batteries B3 - B8 are small 2V rechargeable cells. The TX coil and inductors are very low ohm. The REMI group set for SPICE simulation the following equivalent circuit diagram. Something in it we missed out because the simulation results are poor.

[aft_72005]

Hi aft,
Your question gives rise to two answers:
1. This not an idea. This is a rediscovery. Metal detector with such a configuration of coils was established more than 100 years ago. It was made from wood with no metal and plastic pipes. It worked two operators. The first one wore headphones and the sensor similar to that shown in the upper figure. The second operator is wearing a heavy box, which had a battery and electric motor to power rotating interrupter for TX coil.
2. I practice about two years as a seeker of meteorites. Two years ago, I modified a device for demining to detect stones stuck in the dust. This machine called my friends "ShingleFinder". So far I found two small meteorites gaunt corroded by corrosion after I dug up more than a thousand stones. My dream is to find meteorites beautiful and expensive to sell it to collectors, jewelers and scientific institutes.
My hobby was design of TXs, RXs and antennas for QRP amateur radio. One group of people with my hobby (called REMI group), chastised me over two years to act as spokesperson for its work in the field of metal detectors. The reasons for this punishment and I have several of them pointed out this forum. What means the abbreviation REMI, also explained to the Forum. During this year my sentence ends and then the author of my posts in the forum will be just me, without editorial interference from others.
But I am not only a spokesman for the group, but reinventor as all its members. My great merit last year is that I rediscovered in a patent the principle for deepest metal detection. The real inventor is Vaino Ronka. During this year, the REMI group will design and made a family of models using this principle, which will be called GLEANER. Now I have for translation in English drafts for postings about circuit diagrams of TX, RX front end, form for coil winder etc.
In that year, the group will make the forum very interesting because are ready revised circuit diagrams for experimening.

Hi mikebg
Thanks for your introduce . I am happy meet you here .
My other hobby is also radio amatory .
Ordinary two box TX and RX coils is orthogonal , but in the plan of you ,
It isn’t orthogonal . I read but cannot understand what advantage behind at this design ?

[mikebg]

Depth!

Aft, RX coils are close and parallel to ground, which means more depth. See the difference:

[aft_72005]

Aft, RX coils are close and parallel to ground, which means more depth. See the difference:

Mikebg , ok, depth is one of the important parameters .
Yes, receiver coil near the ground and main is increase deep , But tow box at garret hands ,
Also TX coil relation to RX coil was 90 degree or other words " orthogonal"

[Esteban]

Esteban, the REMI group has 3 ancient circuits only. I have posted 2 of them because the difference of third one #1902 is in TX and headphones only, but the action of its TX is unsolvable for us. The circuit diagram of strange TX 1902 seems so:

Mikebg, thanks very much. Is interesting to learn how work these ancient gadgets.

[Esteban]

Hi Mikebg and all. Here you can see a variant, TX-RX (2 boxes) type. Sorry the quality because I scanned from photocopy of photocopy! But you can have an idea. The language is Spanish, but seems the original book was in English. I have all the pages (infos), but in Spanish. Connections to both coils are into metalic pipe.

[mikebg]

Aft72005,
Experiments show that when the angle letween RX and TX coil axes is less than 90 deg, depth increases.
The fine adjustment of induction balance also increases depth.
Moreover, the angles of the coils can be adjusted according to the height of the operator or the slope of the terrain so that the RX coil to move always parallel to the ground and be near it.
In a change of operator, the adjustment is made as follows:
1. The operator holds the rod handle 1 and adjusts the length by inserting the tubes 6 and 7 so that the rear part of the carrier rod is horizontal. This principle is not mandatory. The original structure no adjustment of this length. The rear part of rod is a tilted backward or forward depending on the height of the operator.
2. RX coil is fixed so as to be parallel to the ground. Change the angle is necessary and when the operator is climbing or descending on a sloping terrain.
3. TX coil is rotated fine to obtain induction balance. In GEMINI 3 also provides fine tuning of the induction balance.

[mikebg]

Esteban,
This project is a theoretical interest for me.
The theory suggests that for working metal detector with CW (unmodulated TX) is sufficient bandwidth 4Hz. This makes it possible to design narrow-band RX with very low noise and interferences. However, if the TX is modulated with AF for example 440Hz, the RX should work with bandwidth 880Hz.
Please read in the document with what AF frequency is modulated the TX so that it can be heard on headphones.

[GeorgeGW]

2box

HI!! i was made this schematic, and i designed a new pcb, and i have a questions... how much turns.... or inductance must have the coils.. TX and RX??? and in the end with coils.. must have nullled the coils with capacitors... or moving them with one multimeter... and if the meter has 0mV is ok? (multimeter in RX)
thanks George

(sorry for my english)

[porkluvr]

how much turns.... or inductance must have the coils.. TX and RX???

Before you can calculate how many turns, you need to decide on the size of the coil. For the transmitter coil to resonate with .2uF (aka 200nF) you need 30mH.
If you want the receiver coil to be in resonance you would use 15mH.

You should Google and download "MiscEl". It has a versatile coil calculator that can figure rectangular coils, and will tell you the length wire you will need and
what the resistance will be for the parameters that you input. It also will tell you how much the coil will weigh!


Here is a basic tutorial on resonance.
http://www.electronics-tutorials.com.../resonance.htm

Here's another one that has a slightly different approach:
http://www.allaboutcircuits.com/vol_2/chpt_6/2.html

Even though your circuit would be considered a series resonant circuit and most discussions on "tank" circuits refer to parallel resonance, there is not much
practical difference in the formulas. (Unless maybe you are an EE student taking an exam)

Remember that resonance occurs when inductive reactance equals capacitive reactance. If you know the value of either L or C it is easy to calculate the other value
needed for a given resonance frequency by working with formulas given in the tutors.

edit: One thing you should watch out for is that the TX capacitor will need to handle a relatively high voltage that will increase as wire diameter increases.
With the 0.75mm wire in my simulation, LTspice shows a 180Vp-p signal at the LC junction. A Polypropylene capacitor rated at 250VDC might be OK
but 400VDC would be better ESPECIALLY if you plan to use larger wire. I made a mistake with my second circuit simulation diagram after I re-drew it,
left out the 12 ohms series resistance present in the first drawing, and you can see an even higher voltage that could result.
Too late to change that. I hurried to get this "edit" posted and I make mistakes when I get in a hurry.
Use polypropylene, because anything less might heat up under the stress. I do not have conversion formulas for DC and AC capacitor voltages 'safety',
but be aware you could run into trouble if you use an inferior capacitor. Capacitors can explode dangerously.

[GeorgeGW]

Thanks for your help porkluvr
The prices of two capacitors are C3 = 0.4uF and C12 = 0.2uF
We know and the frequency... F=2.048KHz goes in TX from 4520,
and with the formula we calculate the inductance L and next with the program "MiscEl" we calculate turns and wire...
For RX we calculate the same things with the same frequence but with another capacitor... and parallel formula
And the nulling with multimeter? is the same with TGS nulling? or it doesnt matter about them?

[Esteban]

Esteban,
This project is a theoretical interest for me.
The theory suggests that for working metal detector with CW (unmodulated TX) is sufficient bandwidth 4Hz. This makes it possible to design narrow-band RX with very low noise and interferences. However, if the TX is modulated with AF for example 440Hz, the RX should work with bandwidth 880Hz.
Please read in the document with what AF frequency is modulated the TX so that it can be heard on headphones.

The detector works at frequency of 175 Khz and uses intermediates transformers for this frequency.

Different metals show sometimes positive deviation, sometimes negative deviation in mechanical uammeter. Separation between both coils are 66 cm. This was calculated after some tests.

This the approximative translation about modulation frequency:

The transmitter uses an automodulated oscillator circuit. The amplitude modulation frequency, of approximately 250 Hz, is obtained by the smoothing action of the high value of grid resistor (3M3) of R8. This trick saves the complication of an additional step of modulation. See pic.

[aft_72005]

HI!! i was made this schematic, and i designed a new pcb, and i have a questions... how much turns.... or inductance must have the coils.. TX and RX??? and in the end with coils.. must have nullled the coils with capacitors... or moving them with one multimeter... and if the meter has 0mV is ok? (multimeter in RX)
thanks George

(sorry for my english)

Hi GeorgeGW
I cannot seeing picture , Please upload directly your pictures to geotech .thank you

[aft_72005]

Aft72005,
Experiments show that when the angle letween RX and TX coil axes is less than 90 deg, depth increases.
The fine adjustment of induction balance also increases depth.
Moreover, the angles of the coils can be adjusted according to the height of the operator or the slope of the terrain so that the RX coil to move always parallel to the ground and be near it.
In a change of operator, the adjustment is made as follows:
1. The operator holds the rod handle 1 and adjusts the length by inserting the tubes 6 and 7 so that the rear part of the carrier rod is horizontal. This principle is not mandatory. The original structure no adjustment of this length. The rear part of rod is a tilted backward or forward depending on the height of the operator.
2. RX coil is fixed so as to be parallel to the ground. Change the angle is necessary and when the operator is climbing or descending on a sloping terrain.
3. TX coil is rotated fine to obtain induction balance. In GEMINI 3 also provides fine tuning of the induction balance.

Hi mikebg
As I know, when TX relation to RX was orthogonal , at this condition we can see
Minimum amplitude at the receiver in put ( by oscilloscope) , for tow box
this is null position .
but will examine what's you said .
regards.

[porkluvr]

And the nulling with multimeter? is the same with TGS nulling? or it doesnt matter about them?

I was trying to avoid answering that question because I have not built this unit and I might give a wrong answer. That's never stopped me from giving a wrong answer before so here goes:

This detector should probably be nulled it as close to zero as you can get it.

You can use a multimeter for initial setup but after your 2-box has been bounced around a few times it will need to be nulled again. Unless you want to carry a multi-meter around with you, you should be able to use the panel meter for that.