Let's made a PC-base metal detector with usb interface !!!

[simonbaker]

Hi friends,

I observed a very critical situation: when noise or distortion caused by other devices nearby the resonant frequency are detected by the LC resonant circuit (RX coil), this has tremendous effects on the detector. The normal operation will be disturbed causing false signals, when the distortion is not continuous (e.g. time-domain pulsed, not linear, etc.).
I had nearby my operating frequency of 19875 Hz irregular activity on the frequency ca. 19650 Hz. The RX resonant tank has lower Q to shield this distortion out. It caused 10-20 µV of signal change on my operating frequency. I am detecting just few µV chances.
I have deactivated the transmit part and took an AL foil to shield the coils and it disappears then. So some EMF is catched through the RX coil despite of the coil shielding (high impedance graphite shield).

The solution:
A very narrow bandwidth FFT-Filter (I took 50 Hz bandwidth) on the signal does the job very good. So the FFT-Filtering should be part of the detector, which should be activated when there is some noise nearby operating frequency. It will need more processing power and therefore more battery power. But it makes the detector very stable.

Implementation of the fast fourier transform (FFT) for forward and backward transformation is recommended. Other digital filters have probably not the short bandwidth filtering capability. If yes, then let me know.

Have a good day.

Aziz

I am surprised that FFT-Filter is more narrow band than lock-in amp algorithm. Lock-in should be very narrow band except for harmonics, isn't it?

[Aziz]

Hi Tinkerer,

Aziz,

I am using about 30 to 50 microns, with embedded drain wire of graphite shield with good results.
What kind of graphite shield do you use?

Tinkerer

I am using graphite shielding shown below. It is produced from a graphite powder with rim varnish (for car tyres). After brushing the paste to the coil and drying, it is then polished to reduce the resistance (smear effect of soft graphite). I could lower the resistance with a wire going to the half of the coil ends (not short circuited, ending on the gap). I have to try this one day for better shielding.

It is working quite good and has lower eddy current effects.

Aziz

[Aziz]

I am surprised that FFT-Filter is more narrow band than lock-in amp algorithm. Lock-in should be very narrow band except for harmonics, isn't it?

Yes, lock-in has very narrow band. The influence of the distortion is very low but detectable (µV). As I am detecting on µV level, I can see the influence.

My FFT-filter took only the resonant frequency response and converted the signal back. Just using one or some harmonics of the frequency response (FFT).

Aziz

[Berti]

hi

New detection distance record (air-test):

1 Euro coin at 55-56 cm distance!

Without any external amplifier! Just the coil and laptop.

Aziz

How do you think to give us coil details.

I am ready to try it.

Thnx

[Aziz]

Berti,

How do you think to give us coil details.

I am ready to try it.

Thnx

to achieve such results, you have to use high Q coils:
- lower resistance
- lower eddy current wire

The coil and the cables must be very tight and must not change its geometrics and other properties. Capacitors should be polyprene or styrene type.
Not all sound-card are able to give such results. I am using a very good sound-card. Even my last experiment was very difficult to achieve. So typically, 40-50 cm for 1 Euro coin is possible. Also the art of software coding matters tremendously. The software should be able to compensate much disturbing effects. Lock-in amplifier is a must have criteria and must be implemented correct.

The more sensitive the detector is, the more demands it has. I will drop at least 5-10 cm sensitivity and will gain more stability.

Aziz

[simonbaker]

Hi Tinkerer,



I am using graphite shielding shown below. It is produced from a graphite powder with rim varnish (for car tyres). After brushing the paste to the coil and drying, it is then polished to reduce the resistance (smear effect of soft graphite). I could lower the resistance with a wire going to the half of the coil ends (not short circuited, ending on the gap). I have to try this one day for better shielding.

It is working quite good and has lower eddy current effects.

Aziz

Can you give us some resistance measurements of your graphite coating?

What effect have you found on the depth of shielding the coil with different conductive coatings? Graphite vs. Foil for example?

Regards,

-SB

[Aziz]

Hi Simon,

Can you give us some resistance measurements of your graphite coating?

What effect have you found on the depth of shielding the coil with different conductive coatings? Graphite vs. Foil for example?

Regards,

-SB

The graphite resistance depends on many factors:
- Coil diameter
- Coil windings diameter
- graphite shield thickness
- polished/not polished
- varnish type (bounding agent for graphite powder)

Here is some values for my 20 cm coil:
Before polishing: ~10 kOhm (measured from one point to another between shielding gap)
After polishing: ~2-3 kOhm

Graphite shielding causes less eddy current effects. This has more detection sensitivity to targets. The coil can be balanced more exactly to zero (no residual voltage)!

AL foil has better faraday shield. But causes detection depth losses.

Aziz

[Aziz]

To see the difference between polished and not polished, just look below:

[simonbaker]

Hi Simon,




The graphite resistance depends on many factors:
- Coil diameter
- Coil windings diameter
- graphite shield thickness
- polished/not polished
- varnish type (bounding agent for graphite powder)

Here is some values for my 20 cm coil:
Before polishing: ~10 kOhm (measured from one point to another between shielding gap)
After polishing: ~2-3 kOhm

Graphite shielding causes less eddy current effects. This has more detection sensitivity to targets. The coil can be balanced more exactly to zero (no residual voltage)!

AL foil has better faraday shield. But causes detection depth losses.

Aziz

Thanks Aziz, your information is very useful. Your graphite shielding sounds very good, in a good range of resistance. Do you have a chemical formula for us ?

I have only tried balancing unshielded coils so far as experiments. I notice that the RX signal goes small but not to zero - instead it shifts phase and then increases as you pass the null position. I wonder if this is capacitive coupling of the coils touching each other and if shielding will eliminate that, and what minimum resistance shielding is needed to eliminate capacitive coupling (assuming it is bad - maybe it's not). It seemed if I separated the coils by an inch or more, then balanced, a better null is obtained, some evidence that capacitive coupling is involved when touching. But maybe it is something else I don't understand when coils very close. Your thoughts always appreciated.

Cheers,

-SB

[Tinkerer]

Graphite shielding

Thanks Aziz, your information is very useful. Your graphite shielding sounds very good, in a good range of resistance. Do you have a chemical formula for us ?

I have only tried balancing unshielded coils so far as experiments. I notice that the RX signal goes small but not to zero - instead it shifts phase and then increases as you pass the null position. I wonder if this is capacitive coupling of the coils touching each other and if shielding will eliminate that, and what minimum resistance shielding is needed to eliminate capacitive coupling (assuming it is bad - maybe it's not). It seemed if I separated the coils by an inch or more, then balanced, a better null is obtained, some evidence that capacitive coupling is involved when touching. But maybe it is something else I don't understand when coils very close. Your thoughts always appreciated.

Cheers,

-SB

Here is a formula for making graphite shielding:

One volume part of 2 component epoxy resin, mix well.
Add 5 volume parts of graphite powder, mix well, it gets very thick so start adding the solvent.
5 volume parts of epoxy paint solvent.
1 volume part of colloidal or fumed silica.

Mix well and apply evenly with a paint brush.

Before application, attach a very thin, bare (no insulation or varnish) copper wire to the coil with a few thin strips of tape.
Cover the copper drain wire with shielding paint.
Later solder the drain wire to the cable shield.

The silica prevents the paint from being runny and gives it some strength, but has little influence on the conductivity of the coating.

At first apply a thin coat. Test after drying. Then apply more if needed. 30 to 50 microns is usually good.

Make sure to leave a gap, or sand off a gap after drying.

Tinkerer

[simonbaker]

Thanks Tinkerer, excellent!

[Aziz]

Here is my secret and formula for excellent graphite shieldings:

I am using a very fast drying resin (has more solvent) and keeps the graphite more conductive. I have to work quick. So I have to spray more solvent from the spray can if the mixture gets thick-flowing. First the resin, then the graphite powder. Not the other way! Otherwise, you would blow the powder out of the glass!

Here some photos of my equipment and corresponding steps:

Aziz

[simonbaker]

Thanks also Aziz for valuable lessons on your graphite shielding. Do you just add solvent by "feel" until it is thick enough?

I am always impressed by your (and others, Max, Ivconic) craftsmanship, very neat construction and setup. Thanks for sharing it.

-SB

[Aziz]

Hi Simon and others,

Thanks also Aziz for valuable lessons on your graphite shielding. Do you just add solvent by "feel" until it is thick enough?

I am always impressed by your (and others, Max, Ivconic) craftsmanship, very neat construction and setup. Thanks for sharing it.

-SB

I am mixing only small portions of graphite with the shown varnish spray. The varnish drys very quickly and if the mixture gets thick, I add some into the mixture (in the small glass). The varnish spray contains lots of solvent and therefore drying quickly. You have to try a bit to find out the best viscousity for painting. Normally one layer of graphite is thick enough. But there is no limit. If you like, you can make the shielding thicker.
Not all varnish spray types may be appropriate. I have tried some of them.
This shown one binds the graphite particles with less varnish and enabling a much higher conductivity.
Polishing is quite easy: A metal brush or any other metal stick is moved gently over the coil to smear the graphite. Then the conductivity of the shield increases dramatically. The graphite shield should be protected with some tape finally. Also between two different coils, the shieldings should not touch each other!

Aziz

[Aziz]

I have recognized the most difficult part of this project: the search coil

Initial balancing and phase shift is very important. To get the right properties, it is very difficult to meet these requirements. Additional critical point is the mechanical stability with inherent temperature coefficient (mechanical shock, tension, temperature drift). Further are the coil cables. Shielding is also critical but can be realized easy (double layer graphite shielding: coils itself and the coil housing).

Due to varying sound-card techical spec's regarding the output impedance and voltage, input voltage sensitivity, I will use an external low noise amplifier for the TX and RX part. TX will be driven on one channel (amplified). RX coil will have additional independent winding for right balance and phase shift adjustments fed from the other output channel (not amplified). I will discard the balance adjusment circuit part. Balancing is made purely with magnetic field coupling.

Using an external amplifier gives more flexibility and depth performance.
Now focusing on how to make a portable and tight search coil.

Aziz