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MSP430 WiFi Smart Scale


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Hey all,

 

*Quick Reflow Oven Update

 

Sorry you haven't heard much from me in awhile, work has been keeping me very busy. First off, I apologize for not being able to keep up with the reflow oven kit demand. My new job has been abit demanding and the displays I was using appeared to have gone off the market so it would require a redesign. In addition, I ran out of code size on the msp430 while many features still needed to be implemented. I'm working on a new one but it won't involve a msp430.

 

*Onto the project

 

So with my new job and all i've spent abit too much time sitting around. Consequently I needed a little motivator to get out to the gym. Basically I wanted to log my weight data and view it in a graphical form. Apparently these already exist in the form of a "smart scale". Anyhow, didn't really know that going into the project nor would I feel like paying 100USD+ for one.

 

Step 1:

First I bought the worlds crappiest weight scale at walmart for less than 20usd

post-664-0-01434300-1392641055_thumb.jpg

http://www.walmart.com/ip/Healthometer-LED-Split-Mat-Bath-Scale/10264542

 

Step 2:

Then I opened it up and attempted to hack the internal control board. Acourse it was one of those black blobs which meant it was a system on chip. Sadly the analog lines were feeding directly into the micro which presumably had its own differential amplifier. Sadly there were no outgoing lines from this micro that had a voltage or pulse width proportional to the weight. Long story short, there was no easy way to get the weight from the prebuilt electronics so I scraped it.

post-664-0-18015800-1392641770_thumb.jpg

 

Step 3:

So next step was to determine how to build the electronics required for the weight scale. Weight scales use a sensor called a load cell which are generally just strain gauges. Strain gauges are sensors that have a very very very small change in resistance with respect to weight. Consequently, a wheat stone bridge configuration is generally used to read them. In addition, one uses multiple strain gauges in order to avoid temperature fluctuation and drift. This weight scale used load cells similar to ones on sparkfun:

post-664-0-12964300-1392642165_thumb.jpg

https://www.sparkfun.com/products/10245

 

So in the scale, there are 4 load cells. Each load cell is a half wheat stone bridge. With clever wiring, you can make 1 wheat stone bridge consisting of 4 half wheat stone bridges, see picture below:

post-664-0-34334000-1392642961_thumb.jpg

 

Step 4:

...Time to go to work so i'll post more on this later. Basically I used a instrumentation op amp to read the Wheatstone bridge. Then I built a board with dorkbot pcb and populated it. As for the 802.11 support, ages ago I bought a wizfi210 from bluehash so I used that. Also used one of my few remaining 2.2" lcd displays.

post-664-0-73584000-1392643141_thumb.jpg

 

post-664-0-24948000-1392643164_thumb.jpg

 

Step 5:

write the code stuff and make it upload to xivley:

https://xively.com/feeds/1844651192

 

Step 6: (ongoing)

WIP is getting this to not drift and give consistant repeatable measurements.... i'm getting there.

 

 

and yea.. i'm off to work. I'll get more of a writeup on this latter.

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