Signal Processing - May 2017 - 76

of these optical illusions. The offering is
motivated with real-world examples,
and students learn that our perception of
a rapidly flickering light source being
constantly illuminated is called the persistence of vision. We take advantage of
this property to display the time and
other text using a single row of LEDs.
■ The foundation of the project is a
small microcontroller that we custom
program in assembly language. This
microcontroller is capable of executing millions of instructions each second and is responsible for flashing
the LEDs at the required speed. The
LEDs are moved across our field of
vision leaving a trail of flashes that
appear as text floating in space.
■ The project is based on a custom
printed circuit board and requires
soldering skills in its assembly. Most
of the computer code is prewritten,
and the student makes changes to
customize the unit to display the
desired text.

Experiment 1-Understanding persistence of vision
Students use function generators and
LEDs to demonstrate the phenomenon
of persistence. We detect the lowest
flashing rate that appears constant to
each student, we move the LED and
observe the "trail" that the flickering
LED leaves, and we observe the effect
of duty cycle on apparent brightness.

Experiment 2-Building the project and coding
Students solder to assemble the project
and test the board. They learn enough
assembly language programming to
make simple changes to the microcontroller program. This enables the unit
to display an arbitrary string of text that
the student chooses [see Figure 4(a)].
In addition, the unit is capable of displaying the time of day.

Experiment 3-Strobe effects
The project has a mode that flashes the
LEDs at an adjustable rate. Students use
this feature to observe rotating objects
and measure their corresponding rotational speeds. They also demonstrate
effects related to sampling at speeds
greater than the Nyquist frequency.

Discovering the radio
In this laboratory exercise, students
learn the basic theory of amplitude
modulation and detection as used in the
transmission and reception of AM radio
signals. They build a tuned-radio-frequency (TRF) one-chip AM radio from
a dedicated kit.
■ In the process of building the radio
kit, students become familiar with
circuit components such as variable
capacitors, air-wound inductors,
electrolytic capacitors, resistors,
and, of course, the single integrated
circuit chip used for detection. They

(a)

■

also learn about transistor audio
amplifier stages and become acquainted with the notion that the job of
engineers is to design and build
properly functioning circuits. Students
learn the processes of AM tuning,
detection, and audio amplification as
they complete the various stages of
the kit.
Familiarity with small hand tools is
useful but not required, as the skill
can be rapidly acquired in this experiment. Soldering is required; however,
students quickly learn the necessary
techniques even with limited prior
experience [see Figure 4(b)].

A night-light
This is a simple project that allows
novice engineers to apply basic electrical engineering concepts to daily
life. The students are given materials to
create an optical switch-activated LED
module or, in layman's terms, a nightlight. Concepts related to voltage dividers, photo-resistors, transistor functioning,
and the handling of a prototype breadboard are introduced. With the completion of this project, the students
have the introductory skills necessary
to design their own electrical engineering projects.
■ This project uses a straightforward
dc-analog design. A single 9-V battery powers the circuit. The voltage

(b)

Figure 4. Activities from the 2015 After-School Engineering Program for students in grades 9-11: (a) persistence of vision clock and (b) discovering
the radio.

IEEE Signal Processing Magazine

May 2017

Table of Contents for the Digital Edition of Signal Processing - May 2017