Specifications

convert into these discrete values is defined by its upper and lower voltage

reference. While the integrated ADC has an internal, software selectable voltage

reference of 1.5Vdc and 2.5Vdc, our pressure sensor output has a potential

range between 0Vdc and 5Vdc. In order to allow our ADC to read between these

levels of voltage, a resistive divider network can be utilized to reduce the output

voltage to fall within the tolerable range. Because the Alcohol sensor voltage

response will be in a very small range, 1.5Vdc reference was chosen to provide

the highest degree of accuracy around the sensor’s voltage response.

The step size of each voltage can be calculated by taking the difference between

the upper and lower reference voltages, and dividing by the total number of

discrete values that can be represented. Because 2

= 1024, our discrete step

size with a 1.5v reference would be

[ 1.5Vdc / 1024 steps ] = 1.46mV

meaning each value output from the ADC will represent a 1.46mV increase in the

time-varying analog signal. This information will be used in the software algorithm

to convert the alcohol sensor part-per-million reading into a blood-alcohol content

value, based on sensor profiling with a dry-gas calibration standard.

3.3 Power Supply

Given the hybrid portable and fixed nature of this project, power becomes a

major element of the design. It must be sufficient to drive all the components of

the portable unit, which include several high power requirement devices, such as

the display, heater, and wireless radio. However, the overall package must still

be small and light enough to be considered portable. In addition, a fair amount of

time must be available before the device requires recharging. Given the

additional importance of the portable unit in that the user will require it to be

functional in order to drive, a very short battery life would be unacceptable for a

usable unit.

The overall power draw of the circuit will need to be considered. Even though the

battery is a rechargeable one, there must be useable capacity during which the

device may be used before having to be recharged. Using power draw

requirements of the major components of the portable unit, a reasonable

estimated of the current draw needed at any one time can be calculated. The

optimum way to calculate the required current draw would be to assume a “worst

case” scenario, where all components are drawing their maximum amount of

current. These numbers are summarized in Table 3.3-1.