Let’s Get Technical
One problem with
the range of generated
frequencies in Table 1 is
the distribution.
Figure 2. An actual frequency hopping circuit,
wire-wrapped on a breadboard.
Notice that some
frequency changes
between output patterns
are very small (just 1 Hz
between patterns 1 and 2,
for example). The four
frequency resistors need
to be adjusted in this case.
The entire set of generated
frequencies will change,
so a little experimentation
or work with the 555 timer
equation will be necessary
to obtain the desired
frequency spread.
One last point deserves
hopping circuit. In just one afternoon, the student was introduced to
the topic during lecture and the
circuit was breadboarded and tested
in lab.
mention. What happens if the shift
register begins with an initial pattern
of 1111? Through the feedback
circuit, another 1 will get clocked in,
making the next pattern 1111, the
same as the last pattern.
This is an illegal state for the
pseudo-random sequencer to start in
because it can never get out of it. In
addition — due to the inverters —
there will be no resistance to pin 7 of
the second 555 timer, which would
prevent oscillation during the 1111
pattern. A power-on reset or some
other initialization signal is required to
prevent the 1111 pattern from
appearing. NV
About the Author
James Antonakos is a Professor in
the Departments of Electrical
Engineering Technology and Computer
Studies at Broome Community College.
He is also the author of numerous
textbooks on those subjects. You may
visit his website at www.suny
broome.edu/~antonakos_j
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Circle #54 on the Reader Service Card. JANUARY 2005
