Design Results

Problems

Over the last days we ran into some problems with our PC board. Initially the entire device would be self-contained and be able to be mounted on a wall. A pressure sensor we installed on it was broken off when it was knocked to the floor. During the course of fixing it, more problems were introduced. The layout turned into a disaster after that, and we eventually had to move everything onto a STK500 and a breadboard. Eventually we would like to rewire the PC board when there is time so that there can be a finished product.

Testing

The main portions of our system that required testing were the pressure and gas sensors. Testing the pressure sensor simply required blowing on the tube and observing the deviation of the returned ADC levels. Additionally, we observed the normal value on a few separate days to avoid setting the threshold at a level where the barometric pressure could activate the circuit. To test the gas sensor, we first observed the ADC level returned by the sensor during normal operation by a sober person. Then, we used a bit of commercial breath freshening spray that contained alcohol to simulate the presence of alcohol on the breath of the user. This allowed us to calibrate the unit without having to work with electronics while under the influence.

Results

The interactivity of the program worked excellently. However, the device is not very consumer friendly. With only the small screen, it was hard to have much information displayed, and the device would definitely have to come with a manual. The actual pressure detection was somewhat fussy, but after a good deal of tuning we got it to work well enough for reliable use. The gas sensor turned out to be quite sensitive, and was quite easy to calibrate and use. However, a persistence of gas in the piping caused hysteresis in the BAC readings that required flushing out the air in the system occasionally

The operation of the unit for end users was quite simple. If a user code exists, the door may be opened by simply punching those four numbers into the keypad. As for the breathalyzer functionality, the door can be opened very easily by just blowing on the tube. This was an important design consideration for us because of the possibility that our users might not be in a terribly logical state of mind. The blend of high end functionality and simplicity works very well. There is almost no delay with the exception of the BAC reading due to the sensor’s lagging. The administrative functions are somewhat unintuitive, however they all work without flicker and extremely quickly while also allowing for the door to be opened by breath.

Accuracy

We would like the device to give an accurate BAC reading someday, but it was not necessary for our objectives. Both the gas sensor and pressure sensor only needed to output binary signals, indicating if someone was using the device and if they were too intoxicated respectively. As far as we’ve been able to test, the sensors have been very precise in their measurements with very little noise.

Safety

While we didn’t expect any possible safety problems, we included the two foot rubber hose anyway just in case of some freak short circuit. However, this is not the biggest safety concern. Alcohol can be very dangerous, and it might be tempting for some users to use this system as a toy to measure their state of drunkenness. This project was created to curb the destruction caused by alcoholism, and so we have taken precautions to ensure that this device cannot be used as a toy or a game. We did not include the BAC of the user or output voltage level of the vapor sensor in the output reading, to avoid quantifying drunkenness beyond the simple threshold level. If this device was to ever be used in a commercial setting, it must have the strongest possible language against alcohol abuse because of the device.