Understand Surf PI working

[nebulos]

Hi everybody!

I spent time on many other posts about the Surf PI 1.2 detector, but I hadn't found description about some part works (well of course the circuit on my breadboard is still not working and my oscillator is pretty bad so it's not helping...).

I don't understand the part with the 4093. I'm expecting that this MC close switches of 4066 just after the pulse to listen only the answer of the coil. From what I understood U4C, U4D and U4B are doing that, but the result of U4C and U4A are doing the contrary. I was expecting U7A and U7B to switch simultaneously!

Ok, on my circuit I think my 4093 is dead (waiting for new ones, and a logical analyzer to understand better).
I don't understand also the rôle of a resistor is between both entries like U4A: finally does it compares pin 1 and 2 or pin 2 and V+ ? Also why the resistors are different (51k and 240k)?

Another question about this part, what is the rôle of P3? I don't see any effect on my circuit...

Anthony

[kt315]

ITMD is nice book to read first. INSIDE THE METAL DETECTOR (inside this b-sht bear that does me crazy)

[Qiaozhi]

Hi everybody!

I spent time on many other posts about the Surf PI 1.2 detector, but I hadn't found description about some part works (well of course the circuit on my breadboard is still not working and my oscillator is pretty bad so it's not helping...).

I don't understand the part with the 4093. I'm expecting that this MC close switches of 4066 just after the pulse to listen only the answer of the coil. From what I understood U4C, U4D and U4B are doing that, but the result of U4C and U4A are doing the contrary. I was expecting U7A and U7B to switch simultaneously!

U4C generates the main sample delay.
U4A generates the main sample pulse.
U4D generates the secondary pulse delay.
U4B generates the secondary sample pulse.
The secondary (later) sample is subtracted from the main (early) sample to remove the effects of moving the coil in the Earth's magnetic field, and to eliminate (to some extent) external EMI.

Ok, on my circuit I think my 4093 is dead (waiting for new ones, and a logical analyzer to understand better).
I don't understand also the rôle of a resistor is between both entries like U4A: finally does it compares pin 1 and 2 or pin 2 and V+ ? Also why the resistors are different (51k and 240k)?

C10/R26 define the main sample pulse width, and C11/R28 defines the secondary sample delay. R28 is a larger value than R26 because the delay needs to be longer.

Another question about this part, what is the rôle of P3? I don't see any effect on my circuit...

Anthony

P3 is used to adjust the main sample delay. Trigger channel 1 of your scope on U1 pin 3 (TX OSC), and connect channel 2 to U7A pin 13. Then it should be become clear.

[nebulos]

Thanks for your answers.
This book looks indeed like a masterpiece on the subject.
Just found one new on eBay in UK. Ordered!

I'm newbie in electronic, but I like to learn on the job (most interesting).
I was confuse about the core part of the circuit (CD4066 and MC14093B), and thanks to your answer Qiaozhi I realize it's complexity (my point of vue).

I hope I will see better how plays U4 when I will have receive my digital analyzer (my oscilloscop is really a half-broken sh**).
I understand better it's role and the delaying between each parts. I will spend just more time on a paper to be clear...

I still don't know why my circuit is not working, but really enjoy to understand better instead of having just a solution!

[Sean_Goddard]

I'm not blowing my own trumpet but I stuggle to see why so many have problems with the Surf PI.

I bought Silverdogs kit, built it and it worked first time (I spotted the crossed over resistors).

I get around 14" on a £1 with a nickel spray screened 16AWG magnet wire coil and coax.

The ONLY thing I added was I changed the Tx frequency to 888Hz (change the 22nF near the 555 to 15nF) and added a snubber so the FET is NOT avalanching all the time (makes the machine very quiet) and thus getting red hot.

If ANYONE has a problem and would like me to get their project working for them let me know. I suspect the main problem is the quality (or lack of) soldering and failure to clean the board PROPERLY after finishing it. Also, the 4066 and the 4093 would benefit from 100nF decoupling caps across their supply terminals (nice miss Whites).

Finally, adjust the DC offset on the 5534 with the machine running as NORMAL, NOT without the coil attached, again this makes a quieter machine for some strange reason.

Looking at the input to the final stage, I'm convinced more integration is required to get it even smoother and I plan on playing with the sample pulse widths as I think big gains in stability can be made there.

Have fun all.

[Qiaozhi]

If ANYONE has a problem and would like me to get their project working for them let me know. I suspect the main problem is the quality (or lack of) soldering and failure to clean the board PROPERLY after finishing it. Also, the 4066 and the 4093 would benefit from 100nF decoupling caps across their supply terminals (nice miss Whites).

There does seem to be an inordinate number of members having problems getting the kit working. This is usually down to inexperience, a lack of understanding of the design, and/or fitting components in the wrong place or the wrong way round.

I looked at a failed Surf-PI project for someone a year or two ago, and there were numerous issues. If I remember correctly, at least one of the ICs had died (possibly due to getting too hot during soldering) and the 100R and 100k resistors had all been inserted in the wrong positions. Unfortunately, many constructors simply populate the whole board and expect it to work first time. Those of us with more experience know that this is a recipe for disaster.

[bbsailor]

There does seem to be an inordinate number of members having problems getting the kit working. This is usually down to inexperience, a lack of understanding of the design, and/or fitting components in the wrong place or the wrong way round.

I looked at a failed Surf-PI project for someone a year or two ago, and there were numerous issues. If I remember correctly, at least one of the ICs had died (possibly due to getting too hot during soldering) and the 100R and 100k resistors had all been inserted in the wrong positions. Unfortunately, many constructors simply populate the whole board and expect it to work first time. Those of us with more experience know that this is a recipe for disaster.

I always find that a PI circuit should be constructed in stages. Stage 1 is always the power supply stage that provides main power to the TX coil through the MOSFET driver. Then, the 5V +/- supplies that feed the other circuits is critical to get working before further populating the board. If Qiaozhi could decompose the specific metal detector circuit and board population into logical stages that need to be checked and verified before proceeding, there would be a higher degree of success. For example, if the PI circuit uses a 555 timer then Stage 2 would be testing the 555 timer output using a 16 to 32 ohm headphone with the ear buds wired in series and attached to the 555 timer through a few uF cap and possibly a small resistor to hear the timer work without needing a scope. Specifying each testing Stage would depend on the nature of the circuit and the logical dependencies of each circuit design but if done properly during the assembly instructions would ensure a higher success rate. However, nothing beats having a scope to see what is going on throughout the entire circuit.

This is just my 3 cents, with 1 cent added for inflation.

Joseph J. Rogowski

[Qiaozhi]

If Qiaozhi could decompose the specific metal detector circuit and board population into logical stages that need to be checked and verified before proceeding, there would be a higher degree of success.

If I only had the time.

That's what I did with the MiniPulse Plus Build Document. Perhaps someone else could produce something similar for the Surf-PI.

[mschmahl]

The ONLY thing I added was I changed the Tx frequency to 888Hz (change the 22nF near the 555 to 15nF) and added a snubber so the FET is NOT avalanching all the time (makes the machine very quiet) and thus getting red hot.

Finally, adjust the DC offset on the 5534 with the machine running as NORMAL, NOT without the coil attached, again this makes a quieter machine for some strange reason.

Hi Sean_Goddard!
Can you say or show something more about your snubber-solution?
And if i get it right you set the output of 5534 to 0V with coil connected? Or do you set it to an other value?

Greetings from Saxony
Michael

[Polymer]

Hello,
I finished the surf pi longboard (SURF 1.2 "ApB") a week ago and dreaded switching it on
after reading about all the problems people were having with it.

Thought I might as well give some of my insights and experiences from the SURF PI and the Baracuda project from my perspective.

I very much appreciate the layouter's work on the PCB's, Silverdog's work and that of all others involved.

I found the SURF PI to be better documented than the Baracuda. I read oodles of info on the forum about the
Baracuda, but had a hard time discerning which information was the real deal. Sorry, I've digressed.

Back to the SURF PI: It worked straight off the bat! The sound is loud and clear.

Perhaps people who have it working straight off don't post this as often as people who are having problems!?

I DO measure EVERY resistor with the multimeter before inserting it and double check where it goes.
I once had a batch of falsely color coded ones and it caused many problems. Since then, see above.

It may look sleek, but the blackness of the top laquer prevents me from seeing/following the traces on the board
as orientation. That's a personal quirk of mine.

I find it interesting that one would adjust offset WITHOUT the coil connected.
That idea never crossed my mind and it intrigues me how different human perception
of the "right" way to do things can be. That's why I am interested in "bullet proof"
step-by-step instructions to do things.

The one thing that impacted my simple testing/adjusting of SURF/BARA the most was my work table - of all things

My work table is very old (fitted and glued) and made of solid wood without any metal parts in it.
So I thought. As it turned out, there are some random bits of iron in it and that caused quite some confusion when my test
coil was lying on one of these areas.

Take-away: hang the coil in the air and beware of steel girders/tanks in walls/floor/ceiling.
They will skew any adjustments you make. Testing outside is usually better depending on if you know whats below.

I find that stating the definite operating voltage eg. 10.6-12v is very important as I was confused by reading too many posts.

My 7660S in the Surf PI is rated at 12V. My battery pack of 8x1.5v alkaline batteries has a voltage of 12.6 volt at a load of 60mA.
I found no information on upward tolerance in the data sheets. I did switch it on after a prayer, nothing went broke. I let it run for a few hours.

I found that giving the nominal power consumption eg. 55mA with and xxmA without coil is a good pointer of knowing things are headed in the right direction.

@ Qiaozhi: Thank you for the clear information on what does what!

@ Sean_Goddard: I agree with you on nearly all points except the snubber of FET, because my FET does not get hot
This has me thinking - Is something wrong with my SURF PI? Of course I'm interested in less noise!
What is the crossed over resistor? I didn't find it ???? What's wrong with my SURF PI now?

@ bbsailor Great idea to make the 555 timer audible for testing with simple methods! Not everyone has an oscilloscope, like me so far.
I used SoundcardScope to "look" at things on the board and see if I could adjust damping with it. Definitely NOT.
The sawtooth signal from the 555 timer is clearly depicted in this software though.

What else had me scratching my head building the SURF PI?
Ah yes, the bridge above R16. What could it be meant for if I leave it out?

Enough ramblin on and on ... Have a great day