The first thing that really boiled our asparagus was our mechanical design. Our design needs two main things: a chamber in which the user’s breath sample can stabilize, and a method of preventing saliva from accumulating within said chamber. As lowly aspiring electrical engineers, we didn’t know jack shoot about mechanical design, so we only semi-succeeded on both counts.
Our pressure chamber, which consists of a largish party balloon, mostly holds the breath sample pretty well, but there’s some inevitable leakage at all straw joints and the holes through which the sensors’ leads poke. There are three straw joints (mouthpiece to pressure balloon, and two between the pressure balloon and the ETOH balloon), and eight holes through which leads poke (four for the pressure sensor and four for the ETOH sensor). Of course, we need some pressure leakage (we don’t want the user to actually blow up the balloon and stress all of our flimsy mechanical parts), but we’d prefer if it were under our exclusive control; breath leaking out through sensor holes may throw off the sensors’ readings.
We didn’t realize that it was necessary to prevent saliva accumulation until very late in the game, after we noticed that (WARNING: the rest of this sentence contains a graphic description of bodily fluids) spit was bubbling out the sensor lead holes and contaminating the leads. We elected to use a ripped up shirt as our Saliva Shield, which seems to work as measured by the reduced volume of spit leaking out of the balloons. But we were unable to eliminate the saliva factor entirely, which means we have to periodically clean the ETOH sensor. Of course, had we more time, etc., we would implement a magical Super Saliva Shield that would block all saliva from entering the Breath-o-Matic.
Another thing that we found somewhat lacking in our design was the method in which code was written and the circuit built. Our design process consisted of testing each component on a white board with a snippet of code to ensure that it worked; after testing the component, we added giant, vaguely related blocks of code to our final C file. We had written most of the code (~700 lines worth) without testing it, and we also built the complete circuit on a solderboard without testing it. Then we crammed the two together, and as is predictable, they didn’t work. Debugging took nearly a week. We could have saved ourselves a lot of pain, frustration, stomach churning and nose running if we had built and tested discrete portions of the circuit on the solderboard, eventually building up to the completed thing. “Oops,” is our feeling on this particular topic.
We sampled the TGS 2620 from Figaro Electronics, and we sampled two Mega32s from Atmel distributors for our project. One Mega32 fell out of our solderboard somewhere, never to be seen again. We killed another one with static death discharge. Finally, we got a working processor from the lab, which is the one that is currently in the Breath-o-Matic. In the spirit of this “In Retrospect” section, we kinda wish that we were able to use at least one of our original samples, which would have allowed this report to paint us as being somewhat more competent.
We also, given the opportunity,
wouldn’t have broken our pressure sensor.
As regarding the IEEE code
2. to be honest and realistic
in stating claims or estimates based on available data
6. to maintain and improve
our technical competence and to undertake technological tasks for others
only if qualified by training or experience, or after full disclosure
of pertinent limitations
7. to seek, accept, and
offer honest criticism of technical work, to acknowledge and correct errors,
and to credit properly the contributions of others
8. to treat fairly all
persons regardless of such factors as race, religion, gender, disability,
age, or national origin
Alex tested and and soldered components onto the final solderboard, wrote the keyboard code, helped to test the ETOH sensor and designed the web site. Dan did preliminary design, spoke to parts distributors to get samples, tested and soldered components, built the mechanical structure, performed final ETOH testing, wrote the rest of the code and wrote the writeup.
The Breath-o-Matic is an easy to use, semi-accurate, enjoyable little device, contained in a small (relative to Michigan) little package for Your Convenience. We find it to be informative and fun, and we plan on taking it everywhere we go - just in case. If we had a million trillion dollars, we would buy the world a Breath-o-Matic. Because Lord Knows - it needs it.