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4.8 E?®ect of Wireless Reprogramming and Actuation on the
Lifetime
In this section, we analyze the e?®ects of performing frequent wireless reprogramming,
blinking LEDs, and sounding buzzers on the lifetime of ExScal.
One wireless reprogramming consumes 18.14 mA-hr of energy. From (6),
we get that if frepr is reduced from 6 to 5, the lifetime of ExScal will increase
from 871.81 hours to 881.2 hours, an increase of approximately 10 hours.
Today??™s sensors have limited actuation abilities (e.g., blinking LEDs or
sounding a buzzer). In the future, sensor nodes are expected to have more
actuation abilities. Actuations are often a major source of energy drain. To
analyze the e?®ect of actuations on the lifetime, we can use (5) and (6) with a
new term in the denominator to represent the average current draw per hour.
For example, an LED draws a current of 2.2 mA on an XSM(see Table 2).
If one LED is kept active continuously, then the lifetime of ExScal will decrease
from 871.81 hours to 412.4 hours (i.e., reduce it by more than half).
(Substitute isensor = 0.292 mA in (6) and use a new term with a value of 2.2
in the denominator.) If an LED blinks instead of being continuously active,
we can use the following approach. Let f denote the fraction of a second that
the LED is on. Then, it can be assumed that the LED in consideration draws
2.2 mA ?—f amount of current continuously. The remaining analysis will be
similar as in the case when an LED is active continuously.
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