Download Science and Engineering Saturday Seminars What Electrical

Science and Engineering Saturday Seminars
What Electrical Engineering Can Do For You
January 23, 2010
Lab Activity: Working with electronic components in circuit and system design
Goals:
1. Demonstrate how R and C components can be used to control
the frequency of a pulse generating (clock) circuit;
2. Demonstrate how environmental quantities (light, pressure,
temperature) can be converted into an electrical signal;
3. Understand how a circuit may be a stand-alone system, or a
building block for a more complex system.
Investigations:
1. Define, observe and measure the frequency (displayed by a
LED) of a pulse generator circuit.
2. Identify circuit components; relate component markings (i.e.
color code bands) with component values; recognize polarity
markings for appropriate components.
3. Modify the frequency of a pulse generator by varying the values
of the timing resistor(s) and capacitor.
4. Use a loudspeaker to produce an audible output from the
pulse generator.
5. Use a light-sensitive resistor (photoresistor) as a timing
component; observe how varying light intensities affect the
output of the pulse generator.
6. Use an aluminum-foil capacitor as a timing component; observe
how varying the spacing of the plates (applying pressure)
affects the output of the pulse generator.
7. Recognize and identify the subsystems that comprise a singledigit clock.
Circuit/System Block Diagram:
(Input)
timing
components
R and C
R and C
(Process/Control)
555 timer IC
(Output)
LED and
loudspeaker
Activity Notes:
1. Frequency (symbol = f) is defined as “number of events per unit time”. With
this circuit, an “event” is one on-off cycle for the LED. Frequency is a measure of
cycles per second and the units (“old system” = cps) are Hz (pronounced like
“hurts”) Related to this is the period (T) of a repeating waveform which is the
time for one cycle to occur. It is measured in seconds (s). Frequency and period
are reciprocals: f = 1/T, and T = 1/f.
2. There are a number of electronic components associated with this circuit
including:
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Resistors: A resistor (symbol = R) is typically made of a small “rod” of
carbon-based material that looks like the lead in a mechanical pencil. The
resistance value on the component is indicated by a color-code scheme.
Resistance is measured in units of ohms ().
Capacitors: A capacitor (symbol = C) consists of two conducting surfaces
separated by an insulator (dielectric material). The capacitor we will make in
this workshop is a good “model”, but actual circuit components are made
very small by using very close plate spacing, and special dielectric materials.
Some capacitors are polarized (have + and - leads); some are not. Typically
the value of the capacitor is indicated, although some “decoding” may be
needed. Capacitance is measured in units of farads (F). One farad is a very
large amount of capacitance; typical circuit components have values of
microfarads (F), 10-6 F, and picofarads (pF), 10-12 F.
Integrated Circuit: The 555 IC was “invented” in the early 1970’s, but
remains a low cost (~35 cents) and “fun” device to work with. It consists of
30+ transistor functions along with resistor and diode functions. Its output
consists of one or more square wave pulses with the frequency controlled by
external R and C components. It can serve as a relatively precise “clock
pulse” circuit for use in more complex systems.
LED: Led’s (light emitting diodes) are also a device that became available in
the early ’70’s. They convert electricity directly into light (no filament) in a
semiconductor material. They consume (relatively) small amounts of electric
power, produce small amounts of heat and have very long lifetimes. Although
they have been used, historically, as indicator (power-on) lights, they are
becoming increasingly common in our lives as light sources for various
applications. LED’s operate on DC current and are polarized -- must be
connected properly, + and -, to work.
Loudspeaker: The loudspeaker we are using is referred to as a “permanent
magnet” loudspeaker. It consists of a small cylindrical magnet with a coil of
very fine wire located near one end. The coil is physically attached to the
paper cone of the speaker, and free to move near the magnet. When an AC
(time varying) current flows through the coil, its (electro-) magnetic field
interacts with the field of the permanent magnet causing the paper cone to
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move back and forth. The air, or other medium, between the speaker cone
and our ear provides the mechanism to transfer/propagate the mechanical
energy of these vibrations.
Thermistor: The thermistor (thermal resistor) is really a resistor that changes
its value quite significantly as a function of temperature. They are typically
made of semiconductor/oxide rather than carbon-based materials. They
should not be confused with a thermostat, which is an on/off switch typically
using the thermal expansion/contraction of metals as a basis of operation.
Photocell or Photoresistor: Photoresistors (sometimes referred to as lightsensitive resistors) are typically made of oxide materials that conduct (or
resist) electricity in a way that is proportional to the amount of light striking
them. Many household “nightlights” incorporate a photocell to “sense”
darkness and cause the nightlight to turn-on under this condition. Photocells
are very different from photovoltaic (“solar”) cells which convert light directly
into useful DC electric current.
3. The design of the 555 IC allows its output signal (square wave pulses) to be
varied in frequency as the timing components (two resistors and a capacitor) are
changed in value. I’ve selected (using “design” equations) values that will
produce low frequency (LED / visible light) and high frequency (loudspeaker
producing a tone) outputs. With the LED activity, you should be able to
determine the pattern relating output frequency and timing capacitor value.
What is the relationship between the
value of “C” and the frequency of
LED blinking?
4. - 6. With the loudspeaker connected (must use a capacitor of 10-100 F in
series. Also, by keeping the resistor from the LED-output circuit in place, the
loudness -- amplitude -- is reasonable) to the output of the 555, and by changing
the value of one of the timing R’s and/or C, an audible tone can be produced.
This is really just a series of square wave pulses (signal) that has a frequency in
the audible range. These pulses cause the speaker’s paper cone to vibrate at a
high rate. Different frequencies of this tone indicate changing amounts of heat
(thermistor), light (photocell) or pressure/distance (spacing of capacitor plates).
What are some possible uses for a circuit with sensors such
as the ones we’ve experimented with? What other
“environmental quantities” might be important to sense and
measure?
7. Assume that a 555 circuit is designed to produce one pulse per second (f = 1
Hz) with a high degree of precision. How could this be used as the foundation
(subsystem) for a time-keeping clock? What would the additional circuitry have
to do?
How does a “grandfather” clock work ? What is
the role of the pendulum? How does a battery
operated clock or watch work? What makes it
“keep time”? Does a clock that operates from
120V (wall outlet) ac current use the same
circuit design as one that is powered by a
battery?