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Topic: I hate when this happens (Read 1 time) previous topic - next topic


So I'm building a new shield, I've got the whole thing breadboarded and think "OK time to order the prototype boards and put the first one together".  So I finally get the boards from Itead after a couple of weeks and sit down to assemble the board. Oops...messed up one of the footprints for a component on Eagle, no big deal I've got that part on a breakout board and I have pretty steady hands...problem solved.  I solder everything together, give the board a once over and mount it on my Arduino.  Turn on the power, open up serial monitor and sure enough everything works great, it's spitting out data just as it's supposed too.  Then I decided to pull out my scope and start checking some signals (it uses I2C).  That's when I started to see the noise...everywhere.  At first I thought maybe it was the thin wires I used to hook up the breakout board for the logic converter picking up noise.  I moved the wires around but didn't see a change so I clipped the wires and took the board completely out of the equation and the noise was still present.  Then I started probing around and found the noise was also present at the power supply both on the input (5v from Arduino) and output. 

I then thought I would try a new USB cable because it's kind of a cheap cable and maybe it's picking up some noise...nope still there.  Then I thought maybe I should try another laptop because the one I'm using has a broken hinge and it's being temporarily held together with duct tape.  So I pull out my brand new laptop hook everything back up and what do you know the noise is gone!  So I walk away to go crack open a well deserved beer and when I get back sure enough the noise is back and now it's worse.  Not only is there noise now it looks like the voltage levels are "rolling around". 

So now I decided to take the shield off and give the whole thing a thorough examination because I wasn't totally impressed with how well the drill hole alignments came out so maybe there were other issues as well.  Everything as far as board layout looked good. I pulled out my ESR meter to start checking the resistors and caps (had to use the meter because this was the first time I've used 0402 components and let's be honest...they all look the same) and everything looked good.  By this time my desk is overloaded with tools and stuff and I have no more room to work so I start to move stuff around and clean things up.  That's when I noticed the noise disappeared again.  I started jiggling around the scope lead and what do you know...the problem was a loose connection in the scope lead all along! Sorry for the long rant but maybe this learning experience will help the next person.


Did the BNC jack on the scope come loose or just between the jack and the plug on the cable?

That was long but worth it though.
Serial LCD keypad panel,phi_prompt user interface library,SDI-12 USB Adapter


Actually it was the opposite end, right where the cable meets the probe. 


I see. Need new probes?

When I was going through your long post and I caught iTead in the middle I almost had a heart attach, thinking that maybe iTead was to be blamed! I just dished out a couple hundred bucks for their PCB service :smiley-roll-sweat:. I felt relieved when I read a few lines down about the connector. XD
Serial LCD keypad panel,phi_prompt user interface library,SDI-12 USB Adapter


Rob Gray aka the GRAYnomad www.robgray.com


I was having similar problems with multimeter probe - my +lead keeps breaking right at the base of the squeeze base. Have soldered it several times. Must be crap wire as is just keeps breaking after not much use. I have some other wire & new clip leads, will make up a new set of probes one of these days.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.


For those in Australia, it's like the Super Cheap Auto ad.   :)

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