Specifications

52
determines if the user is capable of maneuvering a vehicle safely. Therefore, due
to the nature of this project, the accuracy of the sensor output has become the
single utmost important element of this project. In addition to the need of
accuracy, a few more expectations are also considered important, such as the
size of the sensor, start up time of the unit; start up time after use, the ease of
calibration, its sensitivity to ambient temperature, which all will be summarized
below.
Detecting blood alcohol content is not a new technology, although many newer
technological breakthroughs have aided it in increasing its accuracy significantly.
Most of the sensors in the current market belonged to two categories, semi-
conductor based such as silicon oxide or fuel cell based sensors, which is a new
technology. Both technology embodies advantages and disadvantages, therefore
extensive research are required and comparisons are needed.
Semi-Conductor based sensors currently occupies the majority of the consumer
market due to its low price, ease in implementation and most importantly it’s a
matured technology and have been around for decades, as opposed to the new
comer, Fuel Cell technology based, it is found to be more accurate but at the
same time can cost up to thirty times more expensive than the silicon oxide
sensors. As a result, fuel cell alcohol gas sensors are currently used only in high
end devices utilized by the government law enforcements and research labs.
The list below is our expectations and requirements of our design:
High sensitivity (Detection Range):
10- 1000ppm Alcohol (Acceptable) or
0-1000ppm Alcohol (Desirable)
Working Voltage: 3.3V or 5V
Size: <18mm Diameter
Cost:
Semi-Conductor: <$8
Fuel Cell: <$40
Working Temperature: -10C (20F) to 50C(120F)
Semi-Conductor Sensors (Silicon Oxide)
MQ-3
There are three selections of semi-conductor sensors being considered and each
with its own advantage and disadvantages. The MQ3 Gas Sensor is by far the
most commonly used sensor in the market, it is low in price and yet still delivers
desirable functionally and meets majority of the requirement for most uses. The
dimension of MQ-3 is provided below in figure 2.10-1 followed by an example
application circuit in figure 2.10.2.