Baracuda Build - Not Off to a Good Start

[Nevets]

Hi All,

I have just started a build of a Geotech Baracuda (Rev-A) Kit from Silverdog and have struck problems at Step 1. I have fitted PL1, C4 (220uF), D3(IN4001) and U3 (78L05) as per instructions. The voltage check at at U7 pin 8 gives +7.6V , not +5V. I am using 8x1.5V AA batteries in a pack as the power source that reads 12.7V as an input voltage. I have quadruple checked the component orientations, soldering connections and wiring and it is all ok. The 7.6V o/p is out of spec according the the documentation for the 78L05 so I am going to change it and see if fixes things. Does anyone have an opinion on what might be wrong apart from a faulty regulator ?

Thanks for any advice.

cheers
Steve

[homefire]

Are you checking between Pins 1 and 2 on the 78L05 ? If so it sounds like it went Dead Bug.

[kt315]

impulse character of a signal on DC range of modern digital tester can give you 7,6 instead 5v you could wait.

just cause IMPULSE (that is certainly abnormal, you have to get clear constant level) and just cause DC TESTER RANGE.

set o-scope on this pin and give us a pic.

[kt315]

check

[Qiaozhi]

Hi All,

I have just started a build of a Geotech Baracuda (Rev-A) Kit from Silverdog and have struck problems at Step 1. I have fitted PL1, C4 (220uF), D3(IN4001) and U3 (78L05) as per instructions. The voltage check at at U7 pin 8 gives +7.6V , not +5V. I am using 8x1.5V AA batteries in a pack as the power source that reads 12.7V as an input voltage. I have quadruple checked the component orientations, soldering connections and wiring and it is all ok. The 7.6V o/p is out of spec according the the documentation for the 78L05 so I am going to change it and see if fixes things. Does anyone have an opinion on what might be wrong apart from a faulty regulator ?

Thanks for any advice.

cheers
Steve

Are you measuring the +5V supply voltage as shown in Fig-2?
Remember, on the Baracuda, the 0V line is referenced to the negative battery terminal.

[Nevets]

Thank-you all for your responses.


I just realised it was rather rude of me for my first post on this great forum to be a problem. I should have introduced myself first. So I will do it now.
I am Steve from Australia want to take up the hobby of metal detecting. Over 40+ years ago as a kid I saved up and bought a toy detector via mail order. For a toy it actually worked quite well down to about an inch, maybe a little more. I found mostly bits of rusty iron, cans, tent pegs and on occasion a coin or two. Finding a coin back then was a huge deal for a kid and I was hooked. But my fun was short lived as the detector broke after a month or so and I couldn't fix it so it eventually got thrown out.


Fast forward 40 years, my kids have left home, I have more time on my hands and I want to try something new. So why not revisit metal detecting. After a bit of research, I found this forum, and always being keen for a challenge I decided to go down the DIY path. I may buy off the shelf later on, but I like the satisfaction to be gained from building something yourself and really learning how it works. I decided on the Baracuda as the board was supplied as a kit and it made starting a bit easier. I have also bought ITMD Ed 2 which is a fantastic resource to learn from. I also hope to contribute to this forum as much I can as well as learn more from this community.


Which brings me to now and my first little hurdle. I will go back and test and check as per all the advice from your replies. I am at work today so I will do it tonight.
Cheers

[Nevets]

Are you measuring the +5V supply voltage as shown in Fig-2?
Remember, on the Baracuda, the 0V line is referenced to the negative battery terminal.

Don't I feel like a dill. I got home from work, tested and voila +5.1V. I must have misread the diagram and been testing from the battery +ve to U7 pin 8 which thinking about is ~ 7.6V given my input voltage reads ~12.6V. I suppose it was late at night and I was tired but still I can't believe I did that. I will be more careful next time. Thanks for your assistance.

[Qiaozhi]

Don't I feel like a dill. I got home from work, tested and voila +5.1V. I must have misread the diagram and been testing from the battery +ve to U7 pin 8 which thinking about is ~ 7.6V given my input voltage reads ~12.6V. I suppose it was late at night and I was tired but still I can't believe I did that. I will be more careful next time. Thanks for your assistance.

Firstly, welcome to the forum.

Secondly, I suspected that might be the case. Not the part about you were being a dill , but that you'd used the +ve battery terminal as the 0V reference.

[homefire]

Good Stuff. On with the Build.

[Nevets]

Hi All,

Haven't had a chance to get back to this until this week-end. Just finished Step 3 - TX Oscillator and wanted to confirm my measurements. I am getting ~910 Hz (1.1 ms between pulses) with a 60 uS pulse width at ~ 4.8Vpp.
Is that ok? NB. I only have an old CRO which hasn't been calibrated for years.

I started on my coil following the instructions in a document by Joseph Rogowski. ~ 8 inches using 23 turns of 30 AWG telflon coated wire I had lying around (old wire wrap wire). Not sure if I should have done a turn or two less. It measures ~450 uH with 1 layer of coil-wrap holding it together, no shielding yet. Capacitance is ~180pF across the leads which I assume will become more when I add shielding and ground it to one of the leads.

Cheers
Steve

[eclipse]

Steve you can't measure capacitance of the coil directly. You need to measure the resonanse frequency and do the maths.

[Qiaozhi]

Here's a circuit you could try for measuring SRF ->
http://www.robkalmeijer.nl/techniek/...e48/index.html

and there's a handy spreadsheet calculator here ->
http://www.geotech1.com/forums/showt...265#post231265

[homefire]

Or you can be lazy like me and use a Chines Toy. https://www.banggood.com/GeekTeches-...r_warehouse=CN

[Nevets]

Steve you can't measure capacitance of the coil directly. You need to measure the resonanse frequency and do the maths.

Yes of course how stupid of me. It has been a quite a while since I have done any electronics theory and it is slowly coming back to me.


Before starting on the Baracuda I bought one of these kits (http://tronixstuff.com/2011/04/28/ki...racy-lc-meter/) from a local electronic store to i) help me learn to solder again; and ii) provide a means to measure inductance. It is based on the design of the 'LC Meter IIB' by Neil Heckt of the US. Operation is described here. (http://my.integritynet.com.au/purdic...er-project.htm)


I should have done this right at the beginning, but I finally took the time to study how it works.
After initially making a goose of myself again, I think I can actually use it to get a C measurement of my coil, as well a get a more accurate reading of Lcoil. It has a handy feature where you can read the oscillator frequency. So I took two frequency measurements with no coil connected and two with the coil connected in parallel to the internal LC circuit. Each of the pair of frequency measurements was done with the internal C of the LC circuit being C1 only; and then C1+C2. [Where C1 and C2 = ~1nF +/- 5% and L1 ~= 100uH


Using these frequency measurements and the value of C2 as the only known input I can calculate C1, L1, Lcoil and Ccoil.


After several measurement runs I get the following average calculation outcomes:


Lcoil ~= 425uH
Ccoil ~= 60pF


My calculated Lcoil is almost spot on with Qiaozhi's calculator: Lcoil calc = 426uH [23 turns, 0.5mm, 130mm radius] (NB it is a touch over 10 inches coil not 8 inches as previously stated above.)

Just using my LC meter to measure the L as per normal usage I get L = ~450uH. I am guessing this is because the calculations it uses does not take into account the internal capacitance of the Coil which will affect the resonant frequency of the internal LC circuit.


Cheers
Steve

[Nevets]

Hi All,

I am needing some assistance to fault find. I got to Step 9 of the build having passed, or at least thinking I had, each previous step.
Initially after completing step 9, the response was quite poor. eg it required a metal target such as a heavy pair of pliers to be about 3 -4 cm away before
the volume changed. After several attempts at fault finding over several days the response is now nil.

I have gone back and re-done all the measurements and settings. One thing I noticed during my fault finding is that at step 7, I can no longer get U7 pin 1 voltage to increase when a metal object is placed near the coil. (I could at least get a weak response before during the build when I reached step 7)

In step 6, I set the sampling delay via R14 at 20us but at step 7, U7 pin 1 is 4.8V rather than the -2.1 v that is specified if no oscilloscope is available. Adjusting R14 indicates 0v is the lowest it can go, so -2.1v is not possible.

The main and secondary sample pulses and delays measured at U4 are within spec so it seems the problem lies within the integrator area.

NB my coil required 1K R to damp the response.

I am looking for some assistance to fault find from here. Thanks in advance

cheers Steve