User manual

ECE 477 Final Report Spring 2004
5.0 Reliability and Safety Analysis
For your consideration, our project has been called the “Wireless Ordering Device,” with
the unfortunate acronym “WOrD.” This device will not only perform the same function as
current restaurant seating devices, which light up when a waiting customer’s table is ready, but
adds the functionality of being able to order drinks and appetizers while waiting, so that they can
be ready when the table is. The device itself will be a low-powered battery operated box about
the size of a coaster, with an LCD screen and a few small buttons. The coaster will communicate
via RF with a small device hooked up to the serial port of a computer that will display the
appetizer and drink orders. Since the coaster is battery powered and the batteries are in a
separate sealed compartment, there is little to no risk of a malfunction physically harming
anyone. The base module likewise is not much of a threat due to it never needing to be in any
contact with people except during install, and there are so few components that the chance of
malfunction is small.
Reliability Analysis
As with many designs, the most vulnerable part of the device’s design is in the power
supply. Voltages and current tend to run higher here, as well as temperatures, and parts tend to
wear down faster under those conditions. Even though power consumption has been minimized
to run on a battery, there are places where smaller pieces handle larger current. There are also
some delicate portions in the RF that, although not especially vulnerable, could cause nearly
untraceable malfunctions if something like frequency drift happened. The components that I will
look at are the 1N5822 Zener diode, the Si9435 transistor, our rather high-valued 150uF
polarized capacitor, and our .07 ohm resistor.
1N5822 Zener Diode
According to the reference documentation, the equation for the failure rate of a diode is:
λ
p
= λ
b
π
T
π
S
π
C
π
Q
π
E
-13-