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An Interval Timer
(Designing a Printed Circuit Board)
Intro to Eng. II - Module 2
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Part 3: Designing for a Specified Time Interval
A Graph for Designing Interval Timers
Suppose your job title is" Interval Timer
Designer" (a simple job indeed) . You would probably find
a quick way to pick a capacitor and resistor to meet the required
time interval. Perhaps you would find a graphical way to do this,
or maybe you would design a computer program which selects the
appropriate values.
In this part of the project, you will make
a plot which allows you to easily pick sets of values for the
capacitor and resistor given the specified time interval. You
will then check it by constructing an assortment of interval timers
with a supplied set of components.
Starting with the equation for the interval
time in terms of the controlling resistor and capacitor that was
derived in Appendix A:
take the base 10 log of both sides and use
it to generate a plot having the form shown in Figure 30. The
plot should be on 4 cycle loglog graph paper, and you should have
traces for C = 1uf, 4uf,10uf, 40uf, 100uf, 400uf, and 1000uf.
Also add traces for C = 22uf and 470 uf. They will be useful
later.
When you are done change the numbers on the
abscissa and ordinate to be in ohms and seconds respectively.
Checking Your Design Plot
Using the 10K, 100K, and 1M resistors and the
100uf and 470uf capacitors provided (also the 22uf capacitor you
used in Part 1), use the interval timer prototype, shown in Figure
31, to check the design graph for the 9 possible combinations
of these elements.
To do this, use the sets of resistors and capacitors
for R and C in your interval timer. You can arrange the data
in the form of the following table where 'xxxx' represents the
time you measured:
| 10k | 100k | 1M | |
| 22uF | xxxx | xxxx | xxxx |
| 100uF | xxxx | xxxx | xxxx |
| 470uF | xxxx | xxxx | xxxx |
Part 3 Exercise: Nominal
Value Graph and Measurments
(HINT: First take the base 10 logarithm of
the equation T=1.098RC)
