Specifications

71
Charging Circuit
Cost
Size
Complexity
Robustness to power
faults
Simple resistor
network
Very low
Small
Very simple
None
Fast charge
resistor network
(timer)
Very low
medium
Simple
None
Separate
components into
microcontroller
Low to
average
Large
Somewhat
complex;
potential for
failure
Slight
BQ24005
Low ($4.80)
Compact
(smallest)
Simple as a
“black box.”
Very good
Table 2.12.4-1: Comparison of charging circuit solutions
2.13 Airflow Measurement
What happens in the case that a subject under test fails to give a sufficient breath
sample to the hand held unit? Without the ability to quantify the breath sample, it
would be easy enough to just activate the breathalyzer unit, wait for it to
complete its pre-heating period, then deliberately ignore breathing into the unit in
order to produce a false positive result. In order to accurately acquire and
evaluate an automobile operator’s level of intoxication, a means by which to
determine a sufficient amount of breath has been sampled must be established.
2.13.1 Hot Wire Anemometer
A basic anemometer is a device used for measuring the speed of wind passing
by a sampled area. While these devices are used in various forms for weather
evaluation, those designs are far too large for use in a small-scale application. A
variation on this device is a Hot Wire Anemometer. This style of anemometer
uses a very thin wire, heated to a specific temperature over ambient, connected
between a positive and negative terminal to determine air velocity. Because the
electrical resistance of a metal is a function of its temperature, a model can be
established to relate the airflow velocity to the resistance of the wire. A
successful implementation of the hot-wire anemometer can be accomplished by
trying to isolate and maintain a single variable of the application circuit, either a
constant current, a constant voltage across the wire terminals, or a constant
temperature of the wire. As these wires are extremely small and thin, the device
itself can be fairly fragile, but allows for an accurate representation of turbulent
airflow change with a particularly fine resolution.