Geotech Baracuda - REV-A - Coils

[chuubaba]

I am not new to the passion for MDs but only felt dire need of it with knowledge only of trade and commerce in electronics. I can understand from my high school lab experiments and use of radios how human flesh. polyester, plastic and nylon etc can adversely effect magnetic waves. I have been making MD in my dreams since past few weeks and after reading some posts here I have decided that i will coat my first MD circuit and coil with stealth coating material that i received as sample for one of my customers. I am sure it will reduce all interference in magnetic waves other than those coming from the ground to improve the performance. Lets separate fact from fiction on this.

[BKY1337]

Thanks a lot!

[Qiaozhi]

Hello, i have been asked at ebay if the barracuda schematic has a copyright, because i sold PCBs from it there.
Can someone help me?

The original Baracuda was designed by John Pugh (lead designer and owner of Anchor Electronics) in the early 1980s. Each detector was hand-built and personally tested by John before leaving the factory. There have been many back-engineered copies made of the design over the years.

The Geotech Baracuda REV-A is an improved version of the original design, and is an open-source project. You can do with it whatever you like, including selling PCBs on eBay.

[Chiv]

not sure about that but Qiazohi is the one to ask

[BKY1337]

Hello, i have been asked at ebay if the barracuda schematic has a copyright, because i sold PCBs from it there.
Can someone help me?

[Chiv]

I haven't had a response to any of my coils other than a double spiral pancake coil I made out of 3mm twinflex just to see what I did but it was running at 11us(if scope was right) @350uh .3ohm made a few thing get hot, so did not use this coil for log but got good distance with it on my air tests.

[waltr]

tried this 10ohm in series added it in coil head, gave me a real nice square pulse, also cleaned up some of the emi I was seeing too. Thanks for that Waltr, I'm still reading the Inside of the detector book so still lots to learn and try and make it stick.

That's great news.
I was still running on proto-boards when I tried a series resistor. This does two things:
1- it decreases the Coils Tau (L/R) allowing the magnet field in the coil to reach a peak much sooner.
It seems that allowing the coil current to level out before switching off works better.
2- It decreases to maximum coil current and also decreases to fly-back Voltage preventing the MOSFET going into Avalanche.
A lower coil current can also help maintain the battery Voltage preventing Voltage drops that can affect other circuits.

I got to adding a connector to my 10 inch Mono coil last night. I am not getting a response to my hand.
Cold (0°C) again today but planning to get out in my yard for some 'real' testing.

ITMD book is pretty good but does not go into lots of details on the PI detector. Much more info within the threads here.

[Chiv]

Possibly it is when the two samples are taken? Or op-amp offset adjust or something else different in my PCB, I did run a full Ground (VB+) on the bottom.
Have you played with TX Pulse width and adding a series R (10 Ohm) between TX MOSFET and Coil? I found this really helped sensitivity and stability.

tried this 10ohm in series added it in coil head, gave me a real nice square pulse, also cleaned up some of the emi I was seeing too. Thanks for that Waltr, I'm still reading the Inside of the detector book so still lots to learn and try and make it stick.

[waltr]

6us sounds too short for any mono coil.
The first sample (delay) should start after the pre-amp's output signal is raising and is fine starting the sample when the signal it nearing the rolling off to level.
Have you looked at the pre-amp out and first sample on a scope?

The mono coils I've tried needed at least a 15us delay.
With nulled DD coil sampling delay can be 8us and maybe a little sooner.

Try increasing the delay a bit.

Check Figure 26 & 29 of the Hammer Head article. This is about as short a delay as can be used. Note that the sampling takes place on the rising wave form (pre-amp output).
If there are noise, stability of other issue increase the delay.
http://www.geotech1.com/pages/metdet...ead/HHv1p5.pdf

[green]

in my projects hand become detected when delay is at minimum (don't know exactly, around 10us).

I've been using a 6usec delay, maybe that is the reason I need shield. Next time I make a coil I'll test if a longer delay doesn't need a shield.

[ionut_mtb]

With no shield I get an indication when my hand comes close or touches the coil with my PI. Interested why yours doesn't.

in my projects hand become detected when delay is at minimum (don't know exactly, around 10us).

[waltr]

Just check the Baracuda schematics and instructions:

Check that the main sample and EFE sample times are very close to the same.
This is noted in the instructions (Other Points: #2) that they may not be exact. Try matching C12-C13 and R15-R21 if the sample times are not the same.

Also look with your scope at the integrator output (U7a pin 1). Look for the two samples, the first should show as a drop , the second as a raise.
Are the R's and C's (R26-R27, R19-R20, C10-C11) in the integrator stage matched?
Is the second (EFE) sample working?

Really just taking Guesses as to why there is a 'hand' response. A 'hand' would be just an electrostatic field (which is what the shielding blocks) unless you have a ring on the hand.
So my guess is that the EFE sample should remove this effect.

[green]

Yes, seems odd.
Even with a mono coil I didn't get a response to my hand but did get response to Ferrite, a Brick and a magnet.

Possibly it is when the two samples are taken? Or op-amp offset adjust or something else different in my PCB, I did run a full Ground (VB+) on the bottom.
Have you played with TX Pulse width and adding a series R (10 Ohm) between TX MOSFET and Coil? I found this really helped sensitivity and stability.

Just got more connectors for coils so will be getting the Mono coil wired. I'll double check this.

I've have played with different pulse width, delay and sample timings for other reasons. Don't know if they would help the hand indication, had the problem when I first started. Shielded the coil, problem went away. Haven't tried adding a 10 ohm resistor in series. Add a MUR460 in series to allow earlier sampling with mono coils. Haven't decided if the diode helps with separate Tx and Rx coils.

[waltr]

With no shield I get an indication when my hand comes close or touches the coil with my PI. Interested why yours doesn't.

Yes, seems odd.
Even with a mono coil I didn't get a response to my hand but did get response to Ferrite, a Brick and a magnet.

Possibly it is when the two samples are taken? Or op-amp offset adjust or something else different in my PCB, I did run a full Ground (VB+) on the bottom.
Have you played with TX Pulse width and adding a series R (10 Ohm) between TX MOSFET and Coil? I found this really helped sensitivity and stability.

Just got more connectors for coils so will be getting the Mono coil wired. I'll double check this.

[green]

Thanks,
I currently am running non-shielded for several reasons:
1- I didn't have good shielding material
2- Read some that PI detector coils really do not need shield (been wondering if shielding is just a hold over from VLF coils). Tinker's threads are also very interesting.
3- In the KISS principle- start with the minimum needed then test. My coils are not glued so easy to pull apart to change (add shield, etc).

When weather is warm enough to get outside I'll be testing and reporting in my thread.

With no shield I get an indication when my hand comes close or touches the coil with my PI. Interested why yours doesn't.