NerdKits - electronics education for a digital generation

You are not logged in. [log in]

NEW: Learning electronics? Ask your questions on the new Electronics Questions & Answers site hosted by CircuitLab.

Project Help and Ideas » PLEASE HELP WITH STRAIN GAUGE PROJECT

March 30, 2010
by u2slo4me
u2slo4me's Avatar

Hello everyone. I'm a nerdkit newbie and need some help with the strain gauge project. I have completed the temp sensor project with 100% sucsess and now I am struggleing with the strain gauge project. I built my kit exactly as the guide describes. The the strain gauge video says that is basically the standard nerd kit plus the amp. My kit looks diffent than the one in the video. I was able to figure out most of the layout, but still dont understand a few things such as. 1. what is the green component on baord locations 4 and 8? (it looks like a cap to me but not sure if so what rating). 2.what is the cap on the other side of the board located at approx pins 14 and 16 on right side of MCU? 3.I have pins 20 and 21 on the MCU powered from the rail +5vdc and pin 22 on the MCU grounded at the rail as instructed by the temp gauge project. Should I leave them there? 4. I noticed that the voltage regulator has been removed. Is the power source +9vdc or +5vdc and were does it come from? 5.The .1uF cap has also been removed from MCU pins 7 and 8. Is it still needed. 6. would any one share a schematic of the strain gauge set up with me? Not knowing much it would really help me out on this (my second project). 7. One last question, is there and effective way the measure the analog signal coming from the output of the amp with a mulimeter? Not sure if I can post my email? can anyone tell me if thats ok?

Would like to thank everyone in advance for any help. It is greatly appreciated. I love this site and am having a ball!!!!!!

Chuck

March 31, 2010
by hevans
(NerdKits Staff)

hevans's Avatar

Hi Chuck,

You have a lot of questions here. I'm going to try to answer as many as I can, but I think the main takeaway should be that there are many possible ways to accomplish the same thing. In some of our videos and examples we change up the setup sometimes for convenience, and sometimes just to show you its possible. Make sure you have a good grasp of the "bigger picture" of how the project is working, from there you should not have too much trouble recreating it.

The green component you mention is a capacitor. The AD620 amplifier has a voltage reference that we want to set at 2.5V to make that the middle of our output. There is a resistive divider, the capacitor is there as a bypass capacitor to try to keep noise away from that voltage reference.

You don't see the 7805 because we are using the power directly from the USB port. The USB Port gives us a 5V output, so we don't have to regulate it down to 5V.

A normal multimeter is pretty slow, so you are not likely to get a good reading of what is going on in a changing circuit like this one. You might be able to see that there is something coming out of the output of your amplifier and you might see it change if you press on your scale hard enough. You would need an oscilloscope to get a measurement you can really see.

Hope that helps.

Humberto

April 02, 2010
by u2slo4me
u2slo4me's Avatar

Humberto,

Thank you for the help. I think I have the hardware side figured out. Now I'm totally lost on the PC software side (python). Can anyone recommend where to start as far as learning how to use the software?

Once again thanks for all of your help!! Chuck

April 02, 2010
by hevans
(NerdKits Staff)

hevans's Avatar

Hi Chuck,

There are tons of tutorials out there for python. The Dive Into Python lessons are great. The beginners tutorial at python.org is pretty good too, but it gets a bit more technical at the beginning than dive into python.

Hope that gets you started.

Humberto

April 04, 2010
by u2slo4me
u2slo4me's Avatar

Thanks again for all the info Humberto.

April 04, 2010
by u2slo4me
u2slo4me's Avatar

I'm embarrassed to ask this one but here goes. Is python actually running the source code that gives you the graphical display on the laptop in the video? Or is it just used to edit the code? If that is the case can you tell me what software I can use run the code and display the graph.

Thanks Chuck

April 04, 2010
by hevans
(NerdKits Staff)

hevans's Avatar

Hi Chuck,

Don't be embarrassed to ask, its the best way to learn. The graph you see is written in python, and is run by the python interpreter. Like with most languages out there the code can be edited in any text editor. For example, I edit all my code C, Python, HTML or otherwise on the same text editor.

The code we wrote to display the graph on the PC for the weigh scale project uses pyGame (a graphical library for python) to draw the display on the screen. All it does is take numbers coming in over the serial port and make a plot of them over time. You can actually use the same code (with little modifications) to plot other things. We do this in our meat thermometer video

Does answer your question?

Humberto

April 05, 2010
by 87jeepwrangler
87jeepwrangler's Avatar

u2slo4me,

you said you think you have the hardware side of this figured out. specifically, did you figure out...

the rating of the green cap on board locations 4 and 8? the purpose and rating of the cap on pins 14 and 16 of the MCU? the reasoning behind removing the .1uF cap from MCU pins 7 and 8?

and finally, i have been on the lookout for a couple ad620 amps, and there are a few variations. without fully understanding what each of the differences are, i've been unable to decide specifically which part #s will work as i would like them to in this project. will all of the variations of AD620 amps do the job for this type of simple strain gauge project?

thanks, rob

April 06, 2010
by u2slo4me
u2slo4me's Avatar

Hello Rob,

Because I am a newbie I would feel more comfortable if one of the experts on the site answer your question. I am using the AD620ANZ which I think will do the job, however I have not been able to make the project work yet. When I do (and I will) get it running I will post all the details. I'm not giving up! LOL. Sorry I couldn't be more help.

Thanks Chuck

April 06, 2010
by u2slo4me
u2slo4me's Avatar

Hello Humberto,

Can you please tell me the exact model # of the Taylor scale you used.

Thanks Chuck

April 07, 2010
by mrobbins
(NerdKits Staff)

mrobbins's Avatar

Hi Chuck,

It was a Taylor 7322 Lithium Electronic Scale (link to Amazon.com). We found it at our local Target store for $20, if I remember correctly.

However, I will advise you that we can't be 100% sure that the internals haven't changed -- companies sometimes change the guts of a product without changing the model number!

Mike

April 17, 2010
by u2slo4me
u2slo4me's Avatar

Hello all,

Can anyone point me in the right direction? I get the following error message when I attempt to run "pc-weighScale.py". I am running win xp pro 32 bit service pack 3 installed.

Thanks again for all the help

PythonWin 2.6.5 (r265:79063, Mar 20 2010, 14:22:52) [MSC v.1500 32 bit (Intel)] on win32. Portions Copyright 1994-2008 Mark Hammond - see 'Help/About PythonWin' for further copyright information. Traceback (most recent call last): File "C:Python26Libsite-packagespythonwinpywinframeworkscriptutils.py", line 325, in RunScript exec codeObject in main.dict File "C:Python26pc-weighScale.py", line 5, in <module> import wx ImportError: No module named wx

April 18, 2010
by hevans
(NerdKits Staff)

hevans's Avatar

Hi u2slo4me,

You need to install wxPython, which is a graphical library for python we used in this particular scale implementation you are trying to use. While you are at it you probably want to install pySerial, and pyGame. They are all useful libraries to have, and we tend to use them quite often.

Humberto

Post a Reply

Please log in to post a reply.

Did you know that a NerdKit can take control of a remote-controlled car? Learn more...