Houston...I have a problem! (bandido coil)

[Max]

Hi guys,
I have a problem...with Bandido-coil
I've realized the Bandido circuit and I think something is wrong in the coil...

Let's explain:
I've used Mike's instructions to make Tesoro concentric starting from (30AWG):
- 110 turns for TX
- 175 turns for RX
- 22 turns for RX1
First I have to decrease TX turns to get 10KHz in tx osc. --> I've done using 105 turns , OK
Second I decreased RX and RX1 turns to obtain 10.5 KHz wiring RX+Rx1 to tx osc. terminals:
- used 156 turns for RX and 20 for RX1
Then I have to null the coil adjusting RX1:
- I have connected TX coil to tx osc. terminals
- I have connected 47nF ceramic cap to RX+RX1 to have a resonance of about 10KHz-->Ok

then connected channel 1 of the scope to tx osc. (to have the reference signal) and
channel 2 to RX+RX1+47nFcap

In few words I have:
- good looking 10KHz on channel 1 --> TX say 5Vpp (peak to peak)
- 2Vpp signal on channel 2 !!!

I think is too hi for RX to have 2Vpp with no metal near ...so I think I have a nulling
problem. I've tried to minimize this voltage decreasing RX1 turns to 18...but things go
bad...too signal again...
then I rewounded rx1 with 35 turns...then decreasing --> no way !

I've tested RX-Rx1 using the bandido circuitry...and seems something works...
but I have poor detection depth (1 200gr hammer at only 10cm ! 1 eur coin at 3cm !)

What's wrong ??? Seems problems originates from the coil...
What's the good nulling voltage peak to peak I've to expect ?

I've used a fixed rigid homemade coil former: pvc, abs plastic, some glue...that seems
good and leave me the freedom of wounding coils and then remove easy some turns.

Outer coils are on 20cm former, Rx is on 12cm - very well centered !

I have mc33078 and everything else like in schematic. I'm awaiting for delivery of
mc33178s but seems impossible that 33078 cause the problems of above. Negative rail is
at -3.9 volt...that seems ok too... I have the 3 seconds battery check beep...as expected...
etc... but things aren't so good about performance !

Any idea ?
Any suggestion welcome.

Best regards,
Max

[Carl-NC]

You should be able to null down to a few mV. Are you sure the RX1 coil is reversed?

- Carl

[Max]

You should be able to null down to a few mV. Are you sure the RX1 coil is reversed?

- Carl

Hi Carl,
yes I'me sure is reversed because if I reverse leads have much more voltage peak to peak (5Vpp and more), like what I have on tx stage and less this way.
I don't understand why I have so big voltage expecting just about some mV.
Maybe the problem is the coil former ? I've realized in plastic...but seems ok - well centered etc to easy decrease turns if required without making on the hooks.
Seems very strange. And very strange also that TX osc. freq I have I get with only a 5% difference from originals MIke's instructions...then I have to decrease from 170 to 156 turn RX to get 10.5 KHz that is more (about 9%)- don't know why. Think that problem may be on RX1 that I've wounded on top of TX coil to easy adjust...could this be a problem ?

Best regards,
Max

[Geo]

I had a similar problem with my DD coil. Reg said me to connect parallel with Rx coil a load resistor 1K....10K. I connected a 2k2 resistor (only for the test) and i took a 15mv signal. I fixed the coil and now it work perfect. Ohh take out the 47nf from RX when you try to null the coil. You may have self oscillations.
You must have a output signal 5...30 mv
Regards

[Max]

I had a similar problem with my DD coil. Reg said me to connect parallel with Rx coil a load resistor 1K....10K. I connected a 2k2 resistor (only for the test) and i took a 15mv signal. I fixed the coil and now it work perfect. Ohh take out the 47nf from RX when you try to null the coil. You may have self oscillations.
You must have a output signal 5...30 mv
Regards

Hi Geo,
thanks for the help...I expect also some mV ! I have to test with a load to avoid self resonance ! yes...just forget that I need have a load on this RX!
I was energizing a free tuned circuit...!
But why disconnecting the cap ? There is a cap too in the RX section...

Best regards,
Max

[Geo]

Hi Geo,
But why disconnecting the cap ? There is a cap too in the RX section...
Best regards,
Max

Hi Max.
No problem if the 47n capacitor is connected. Simple... sometimes if there is a good (tight) tuning then it is more easy for self-oscilations.
Btw who is the sensitivity of Bandido. I have some tests and the sensitivity is a little lower then Classic III.
Regards

[Max]

Hi all,
I've modified the RX1 to get a good null with a 1Kohm load , with 25 turns now I get:
- 35mV peak to peak that seems ok (at least for testing)

Well, there is another problem...now
I've noticed that at the output pin 7 of IC7, that for me is at now mc33078, i get a
strange not perfectly sinusoidal wave. So i've tested at inputs pins 6,5 and there is
a perfect sin wave !
On output pin I have an half of the wave that have a kind og "bounce" like in overshoots
down to the other. It's a small "bounce" but I think this is a sign that something go wrong
on mc33078. I get STmicro devices, then checking the datasheet I found that these work on
+2.5/-2.5 volts at minimum but I have more +5/-3.9 ! So I think that I need don't trust
datasheets anymore...because I've done another test (hey!) putting an LM358 in the IC7
socket and having a PERFECT sine wave !!!
So, I think that my 33078s are ready to launch...or to be destroied by hammer...
LM358 could work from +1.5/-1.5V for real (as datasheet say) but seems that my 33078s
don't work fine with +5/-3.9V ...what can I say ??? Better don't say anything about...

Another issue is performance:
- in all metal I get now a 2 eur coin at 8cm ! (wow)
- in disc mode it never sound... (wow)
Seems that my bandido isn't so happy...
There is a performance table or something out there???
I don't know...

Now I'm awaiting for delivery of real 33178s...I've ordered...but untill they arrive...
I'm very furstrated-depressed !

Any idea ??? Any suggestion ??? What I have to expect as depth in air for 1 or 2 eur coins ???

Best regards,
Max

[Max]

Hi Max.
No problem if the 47n capacitor is connected. Simple... sometimes if there is a good (tight) tuning then it is more easy for self-oscilations.
Btw who is the sensitivity of Bandido. I have some tests and the sensitivity is a little lower then Classic III.
Regards

Hi Geo,
I know ClassicIII and used it but don't know Bandido...never used it...I can't say what's the best between.
I decided to make bandido to have a good disc to avoid problems with false detection due to bricks and ceramics in general.

At now I only know that my Bandido doeasn't work well...

Best regards,
Max

[Geo]

Hi all,
What I have to expect as depth in air for 1 or 2 eur coins ???

Best regards,
Max

Hi Max. Sorry but i am not at good condition ( i am very tired) to look for Bandido bad operation. But 2 Euro you must detect at 22 cm on air with 20cm coil. I saw it at a detectors test. I believe that at "all metal" mode you must detect the coin at 28....30cm .
Bye

Last edit.....
Minelab ( Sovereign XS, Sovereign XS-2)
All MetalPomiary dla $25DETEKTORCEWKAAIRGROUNDGarrett GTI 20009.5" 37 cm 25 cm Garrett Treasure Ace 300 8.5" 17 cm 13 cm Garrett GMH CXII 8.5" 24 cm 18 cm Garrett GTA X 1000 8.5" 25 cm 17 cm White's Spectrum XLT 9.5"Blue Max 30 cm 21 cm White's 6000 Di Pro 9.5"Blue Max 35 cm 22 cm White's Quantum XT9.5"Blue Max 26 cm 20 cm White's Classic SL III 9.5"Blue Max 24 cm 15 cm White's Classic SL II 8" Blue Max 19 cm 14 cm Fisher 1225 X5" 17 cm 12 cm Fisher 1235 X8" Spider 21 cm 16 cm Fisher 1265 X8" Spider25 cm 17 cm Fisher 1266 X10.5" Spider 29 cm 19 cm Fisher CZ5 8"Spider 32 cm 21 cm Fisher CZ 20 10.5" Spider 36 cm 24 cm Tesoro Lobo Super Traq 10" x 5" elipsa 28 cm 20 cm Tesoro Toltec 10.5" 26 cm 18 cm Tesoro Golden Sabre8" 21 cm 16 cm Tesoro Bandido8" 22 cm 14 cm Tesoro Silver Sabre10.5" 24 cm 17 cm Tesoro Sidewinter8" 20 cm 16 cm Tesoro Cutlass8" 19 cm 14 cm Tesoro Amigo7" 17 cm1 3 cm Discovery Baron Module base 8" 32 cm 23 cm Minelab Sovereign XS8" 24 cm 18 cm Minelab Sovereign XS7.5" 26 cm 19 cm Minelab Sovereign XS-2 10" 33 cm 22 cm

[Max]

Hi Max. Sorry but i am not at good condition ( i am very tired) to look for Bandido bad operation. But 2 Euro you must detect at 22 cm on air with 20cm coil. I saw it at a detectors test. I believe that at "all metal" mode you must detect the coin at 28....30cm .
Bye

Last edit.....
Minelab ( Sovereign XS, Sovereign XS-2)
All MetalPomiary dla $25DETEKTORCEWKAAIRGROUNDGarrett GTI 20009.5" 37 cm 25 cm Garrett Treasure Ace 300 8.5" 17 cm 13 cm Garrett GMH CXII 8.5" 24 cm 18 cm Garrett GTA X 1000 8.5" 25 cm 17 cm White's Spectrum XLT 9.5"Blue Max 30 cm 21 cm White's 6000 Di Pro 9.5"Blue Max 35 cm 22 cm White's Quantum XT9.5"Blue Max 26 cm 20 cm White's Classic SL III 9.5"Blue Max 24 cm 15 cm White's Classic SL II 8" Blue Max 19 cm 14 cm Fisher 1225 X5" 17 cm 12 cm Fisher 1235 X8" Spider 21 cm 16 cm Fisher 1265 X8" Spider25 cm 17 cm Fisher 1266 X10.5" Spider 29 cm 19 cm Fisher CZ5 8"Spider 32 cm 21 cm Fisher CZ 20 10.5" Spider 36 cm 24 cm Tesoro Lobo Super Traq 10" x 5" elipsa 28 cm 20 cm Tesoro Toltec 10.5" 26 cm 18 cm Tesoro Golden Sabre8" 21 cm 16 cm Tesoro Bandido8" 22 cm 14 cm Tesoro Silver Sabre10.5" 24 cm 17 cm Tesoro Sidewinter8" 20 cm 16 cm Tesoro Cutlass8" 19 cm 14 cm Tesoro Amigo7" 17 cm1 3 cm Discovery Baron Module base 8" 32 cm 23 cm Minelab Sovereign XS8" 24 cm 18 cm Minelab Sovereign XS7.5" 26 cm 19 cm Minelab Sovereign XS-2 10" 33 cm 22 cm

Hi Geo,
thank you for the useful infos.

Best regards,
Max

[Max]

Hi all,
I've made some modifications on RX1 coil obtaining a very good null:

- 2mV peak to peak at Rx-Rx1 leads when no metal present

I can see on the scope when no metals 2mVp-p, and when big metal plate present about
1Vp-p or little more, that seems ok.

So coil now seems good nulled. But circuit still doesn't works well...unstable...
sometimes I hear cracking sound.

Performance is sometimes better than previous RX1...but still too weak!

I connected first the two leads of RX-Rx1 to RX pads on
PCB ALONE (mean with no GND connection) ---> doesn't work

Then like in Mike's instructions I connected leads to RX but having one connected also
to GND --> doesn't work too like expected but here I can see at output IC7 a different
output...meaning referred to GND and then increasing a lot when a metal is present.

Question is : what's the correct wiring for RX leads to circuit ??? I think one must have
GND connection...near zero for op-amps, away from saturation voltages...anyone can confirm ???

I can see also phase shifts when metal is near...but disc seems doesn't work like
described in manual.

Any idea ? Any suggestion welcome.

Best regards,
Max

[chemelec]

Hi all,
I've made some modifications on RX1 coil obtaining a very good null:

- 2mV peak to peak at Rx-Rx1 leads when no metal present

I can see on the scope when no metals 2mVp-p, and when big metal plate present about
1Vp-p or little more, that seems ok.

So coil now seems good nulled. But circuit still doesn't works well...unstable...
sometimes I hear cracking sound.

Performance is sometimes better than previous RX1...but still too weak!

I connected first the two leads of RX-Rx1 to RX pads on
PCB ALONE (mean with no GND connection) ---> doesn't work

Then like in Mike's instructions I connected leads to RX but having one connected also
to GND --> doesn't work too like expected but here I can see at output IC7 a different
output...meaning referred to GND and then increasing a lot when a metal is present.

Question is : what's the correct wiring for RX leads to circuit ??? I think one must have
GND connection...near zero for op-amps, away from saturation voltages...anyone can confirm ???

I can see also phase shifts when metal is near...but disc seems doesn't work like
described in manual.

Any idea ? Any suggestion welcome.

Best regards,
Max

Phase Shifts are Normal.

In My pinion the RX Coil connects to R26 and R27 as Shown on the schematic. Not to Ground.

This works good using my coil on my Split board design.
(However I have only used my unit for bench tests, Nothing outside on the Ground)

One other thing I recommend doing while Nulling the coils, is to slightly adjust the value of C16 for best Peak detection of the signal.
If your coils or frequency are Not Perfect, (Which I Doubt they ever are) This will Increase the Sensitivity by obtaining a better resonance to the actual transmit frequency.

Take care...Gary

[Max]

Phase Shifts are Normal.

In My pinion the RX Coil connects to R26 and R27 as Shown on the schematic. Not to Ground.

This works good using my coil on my Split board design.
(However I have only used my unit for bench tests, Nothing outside on the Ground)

One other thing I recommend doing while Nulling the coils, is to slightly adjust the value of C16 for best Peak detection of the signal.
If your coils or frequency are Not Perfect, (Which I Doubt they ever are) This will Increase the Sensitivity by obtaining a better resonance to the actual transmit frequency.

Take care...Gary

Hi Gary,
thanks for the help.
Maybe you mean C15 (parallel to the RX coil) not C16. In Mike's instructions there is something about + or - 4.7nF to try on the 33nF cap (C15).
My actual frequencies are:
TX = 10.155 KHz
RX = 10.530 KHz

TX was exactly 10.002 Hz before wounding RX1 then increased. Don't know why - maybe due to force applied by RX1 coil wounded on it.

I can see on another german post that other frequencies could be:
TX 10.15 KHz
RX 11.13 KHz

Are these fine too ? Anyone knows ?
How much critic are frequency ratio and values for TX and RX ???

Best regards,
Max

[Max]

Hi all,
I get 80mV peak to peak at 20Kohm resistors (output pin 7 of IC7) after C17.
Seems good considering that gain of IC7/a is about 20 --> so I have only about 4 mV
peak to peak on Rx coil or less when I connect to the circuit.

When measured with 1Kohm lad it was 2mVpp.

But I have these results (80mVpp at pin7) only connecting a GND wire to one lead (in my
case the one that goes to pin6 of IC7). Tried different reversing for phase...
but disc doeasn't works in any case.
In all metal I get a 1 eur coin at about 10cm in air test. Again too poor detection.
I've read something about non motion in all metal, but I have always a motion
behaviour ::confused:: when hold a coin fixed to the coil...it stops sounding.
Using auto switch timing are different but even motion detection occours.

Don't know why performance is so small...I think I have to experiment with rx capacitor
C15 adding or removing some nF. But seems strange that I have so few cms detection.

My actual frequencies are now (using Tx OSCILLATOR):
TX coil ---> 10.040 KHz
RX+RX1 ---> 10.432 KHz

So I could try to remove some turns from Rx/Rx1 to get 10.50 Khz ...think that 1 turn on
Rx will be ok to add about 70 Hz. It's so critical ?

Best regards,
Max

[Leto]

Frequency Vs Amplitude

Hi Max,

I have no experience with uMax or concentric coil making.
-> but anyway here are my thoughts:
*I would start testing the circuit with DD coil - you can achieve null easyier.
and you will have reference to that original coil you are making now.
If both coils will show bad results then should be problem on board.
Maybe you have already some appropiate scrap coils; hook them to the circuit:
1) null them in DD (OO) configuration.
2) with changing caps on Tx get appropiate frequency.
3) with changing caps on Rx search for highest amplitude.

*I found out that searching for highest amplitude on Rx coil works better than trying to get same resonant frequency (even if this should lead to same result)
Amplitude was measured on input pin of preamplifier (ic removed).
Rx coil driven by circuit.