Download Faculty Project Description - IEEE Real World Engineering Projects

Faculty Project Description
Overview
About two billion people in the world’s under-developed nations do not have access to modern
forms of energy. While taken for granted in the world’s developed nations, the lack of safe and
reliable sources of light after dark has a profound negative effect on the socio-economic
condition of people in under-developed nations. Without adequate lighting, education and
economic development programs suffer: it is difficult to improve literacy when there is
insufficient light to read a book or a blackboard.
In an under-developed region where conventional energy sources are either unavailable or
beyond financial grasp, the need for energy-efficient designs is of paramount importance. Aid
efforts that include power-hungry equipment, for instance, will be considered a failure if local
energy resources are insufficient for continued operation. The technical challenge presented here
is to develop an inexpensive, reliable, clean, safe, and efficient source of light for those who
currently do not possess it.
The hands-on project will see groups of students construct circuits that generate light using
LEDs. They will explore how varying a resistance can, within reason, vary the intensity of the
light. A representative measure of light intensity is gathered via the resistance of a CdS cell
(photo-resistor) protected from ambient light. As well, by varying the resistance in series with
the LED, students will discover the design trade-off between battery life and LED intensity.
Once the fundamentals of prototyping and measurement-taking are mastered, students are asked
to explore the effect that different resistance values, LED types, LED configurations, and voltage
sources have upon the emitted light intensity. This exploration is intended to reinforce the
previous discoveries. Finally, students bring together all of the earlier topics to address the main
goal of the project: the creation of a portable lighting system suitable for use in a remote village
in the developing world. As part of the integration, exploration of the effects of another
practical, real-world aspect of the project occurs: case or packaging design. Success here
requires a balancing of trade-offs arising from measures of usability, robustness, weight, volume,
effects on the transmission of light, etc.
Groups of two are the optimal arrangement for these activities.
Learning Objectives
1. Ohm’s Law
a. Learn how to measure voltage and resistance with a multimeter
b. Learn how to determine current using voltage and resistance measurements
c. Learn to determine power consumption based on voltage and current.
d. Method for current-limiting using a resistor
2. Properties of Light
a. The light spectrum: visible and invisible light
b. Devices for measuring light properties
c. Light Intensity vs. Distance: Inverse Square Law
d. Relative vs. absolute light measures: Radiometry vs. Photometry
e. Human eye sensitivity to light
3. Solid-state light emission and absorption
a. Light-dependent resistors (CdS sensor) & photon absorption
b. Diode models, polarity & voltage drops
c. Photon emission from semiconductors (LEDs)
4. Batteries
a. Basic operation
b. Capacity
5. Developing World Design Constraints
a. Socio-economic conditions
b. LED advantages for light in the developing world
c. Packaging
Resources Needed
Hands-on Project: Student Kit
This section outlines the contents and cost of the kit that is issued to each student group. The
contents of the kit are listed in Table 1. A volume-purchase of 100 or more units of the same kit
will reduce the cost of each. It is suggested that the lab stock spare components in the event of
lead breakage or other component failure.
Table 1 Price Estimate for Student Kits
Item
Digikey Part
Number
Prototyping breadboard
923269-ND
1
Cadmium-Sulphide (CdS) cell
PDV-P9002-1-ND
1
#24 gauge solid-core wire
C2003-12-01-ND
2 feet
Roll of black electrical tape
3M33+A-ND
1
Various T1-3/4 diffused LEDs
67-1135-ND, etc.
30
AA NiMH Battery
SY134-ND
4
9V NiMH Battery
N705-ND
1
6V Lead-Acid (low A-h) Battery
P128-ND
1
AA battery holder (wire tail)
139K-ND
3
9V battery clip (wire tail)
BH9V-W-ND
1
6V battery crimp connector
n/a
2
1Ω, ¼ W, 5% resistor
1.0QBK-ND
10
10Ω, ¼ W, 5% resistor
10QBK-ND
10
100Ω, ¼ W, 5% resistor
100QBK-ND
10
1kΩ, ¼ W, 5% resistor
1KQBK-ND
10
Cardboard tube: 4” x 1.5” (dia.)
n/a
1
Packaging options (Jars, etc.)
n/a
n/a
Total:
Estimated Cost
$11.00
$3.00
$0.30
$3.00
$7.00
$20.00
$10.00
$12.00
$1.50
$0.90
$0.50
$0.60
$0.60
$0.60
$0.60
$0.50
$5.00
$77.10 (US)
Some preparation of equipment may be necessary: AA battery holders may require soldering of
stranded wire leads to allow for breadboard connection, and lead-acid battery leads may require
assembly for the same purpose. Additionally, it is imperative that the kit contains information
about the LEDs. Datasheets can be included, but at minimum a summary of maximum forward
DC current, typical forward voltage drop, luminous intensity, viewing angle, and color (for
identification) need to be provided. Likewise, characteristics of the CdS photo-resistor sensors
need to be provided. Finally, including a component price list will assist students in deriving a
cost estimate of their design.
Students should be encouraged to bring in packaging material from outside sources. This will
maximize their ability to be creative, especially in ways that might not be considered by
instructional staff.
Replenishing kits after use should require replacement of only the resistors and wire.
Hands-on Project: Lab Equipment
Table 2 outlines the equipment that is to be provided in the institution’s laboratory environment.
Since most institutions typically have this equipment available for use in undergraduate courses,
it is considered separately from the Student Kits.
The battery chargers could be simple trickle chargers. Student groups could be instructed on
how to use a laboratory supply and a resistor to initiate the charging process.
Table 2- Required Laboratory Equipment
Equipment
Wire cutters/strippers
Needle-nose pliers
Utility Knife
Digital multi-meter (DMM)
NiMH battery chargers
Lead-acid battery chargers
Resistor “decade” box
Test leads (alligator-to-banana)
Number Required
1 / group
1 / group
1 / group
1 / group
Approx. 1 / 5 groups
Approx. 1 / 5 groups
1 / group
2 / group
Lab 1: “Let There Be Light”
Intro: In this lab the students will explore Ohm’s Law using a pair of batteries, a set of resistors
and an LED. Ohm’s Law states that Voltage is the product of Current and Resistance.
Alternatively, Current is the Voltage divided by the Resistance:
I
V
R
Where Voltage is V, Current is I and Resistance is R. Voltage is measured in Volts, Current in
Amperes and Resistance in Ohms. One can think of Voltage as an electric force, Resistance as

the tendency to oppose the force and Current as the flow of electrons once the force is applied to
them.
Figure 1 Experimental setup for Unit 1 (left: schematic; centre: arrangement with bare wires;
right: circuit on a breadboard). A DMM is used to measure voltage across a resistor. The
resistor controls current into the LED. Note in the right-hand breadboard diagram: the light
grey lines demonstrate where the underlying connectors in the breadboard are.
A Light Emitting Diode (LED) is an electrical component that converts electricity into light. If it
is placed in a circuit with a lot of current the light that it emits will be very bright because the
circuit delivers many electrons to it per unit time. A typical circuit for this is shown in Fig. 1.
Strategies: Connect the circuit illustrated in Fig. 1. Use two AA batteries, a 100 Ohm resistor
and a red LED. When the battery is hooked up it should light up. (Make sure that the LED is
oriented the right way; flip it around if you think that you connected it the wrong way).
Please note that breadboards are arranged into columns and rows, with a hidden conductor within
each column (see the light grey lines of the right-most diagram in Fig. 1). No conduction
(current flow) occurs between one column and another. Often, the breadboards are broken up
into top and bottom halves (rows a-c and d-f in Fig. 2) and the conductors only span one half of
the board. So a wire plugged into the e2 hole will conduct to a wire plugged into the f2 hole, but
not the a2 or e3 holes.
A Digital Multimeter (DMM) is used to measure voltage and resistance. Remove the resistor
from the circuit, set the DMM to resistor mode, and measure its resistance. Next, set the DMM
to measure voltage and then measure the voltage across the resistor, as shown in Fig. 1. Then
measure the voltage across the battery pack and the LED. What is the voltage across the resistor?
Is the sum of the voltages across the LED and the resistor equal to the battery? Now, remove the
resistor and replace it with a 500 Ohm resistor. (The LED should dim) The voltage across the
resistor should be lower, indicating via Ohm’s Law that the current has diminished. The students
should repeat the measurements for different resistances (100, 200, 300, 400, 500 Ohms).
Problems: (1) Keeping track of similar-looking LEDs, especially in a large class, can be hard.
We have provided a paper template (in a separate file) that LEDs can be stuck to in order to keep
track of them (see also Fig. 2). (2) This lab is to be conducted with plastic boards with embedded
conductors, called breadboards (see Fig. 1). Because the conductors are hidden inside the plastic
it is not immediately obvious to students how they work. Using a digital multimeter (DMM)
with an audible “conductivity test” feature, and some metal pins stuck in different holes of the
breadboard can help them explore the embedded conductors. (3) Depending on the forward
voltage drop of the LED it may not light up if the batteries are 2 V or less. Make sure that the
students are using a pair of fully charged batteries that put out a combined 2.5 volts (i.e. fullycharged NiCad or NiMH cells) and that they understand how to increase battery voltage by series
connections, if required. (4) Orientation of the LED: the cathode of the LED is normally marked
by a flat cut or a notch on one side of the LED. Make sure that the students orient the cathode
pin (marked “-” in Fig. 1) to be at a more negative part of the circuit (i.e. electrically closer to
ground).
Figure 2 LED organization tags. See the external file LEDtags.pdf for more.
Trade-offs: There are no inherent trade-off relationships in this lab.
Results: In this lab, the students are expected to discover the relationship between a variable load
resistance in the circuit and the resulting current. Exact values for circuit current will vary based
on the type of LED used and the exact battery voltage. However, if you get the students to
measure the voltage across the battery, as well as the resistor and the LED, the latter two will add
up to the battery voltage (between 2.5 and 3 volts). As well, the current at 100 Ohms will be
higher than at 500 Ohms. Finally, while the focus of this lab is an exploration of Ohm’s Law,
the students should notice the relationship between higher current and more intense light
generated by the LED. This will be explored in a subsequent lab. This Unit should take about
two hours.
Lab 2: “How Bright?”
Intro: In this unit students will discover properties of light emission and absorption in
semiconductors. Measuring emitted light intensity (luminance) and comparing it to resistance or
current is a good way to determine how effective a LED is at producing light, or a sensor is at
detecting incident light intensity (illuminance). Here, students will vary the intensity of light
produced by an LED and they will also see how this light is converted to resistance by a
Cadmium Sulfide (CdS) photo-detecting resistor (photoresistor).
The reason for examining these effects is to better understand how the lens and packaging of
LEDs affect intensity measurement. By the end of the lab the students should start having a
feeling for how they might set LEDs up to maximize reading potential at certain distances and
with particular electrical current levels. This will be important for the final project.
Hand out a copy of the “CdS Relation Plot” (or equivalent) found in the leddata_v5.xls file to the
students to help them convert photoresistor resistance to light intensity.
Problems: (1) Keeping track of similar-looking LEDs, especially in a large class, can be hard.
We have provided a paper template (in a separate file) that LEDs can be stuck to in order to keep
track of them. (2) See the “General Remarks: Use of a Cadmium Sulphide (CdS) Cell as a Light
Sensor” section later in this document. As in the first lab, make sure that the LED cathodes (“-”
pin) are located more negatively in the circuit (i.e. closer to ground than the anode “+”).
Strategies: Two tubes should be made to house the photoresistor sensor, shown in Fig. 3. One is
shorter (e.g. toilet paper roll tube; about 10 cm long) and one is longer (e.g. paper towel roll tube;
about 20 – 30 cm long). Most experiments will be done with the shorter roll.
Figure 3 Experimental setup for Unit 2. Note the CdS Photoresistor leads stick out of the cover,
allowing the DMM probes to gain access..
In Fig. 3 you can see the experimental setup for this Unit. It’s similar to the setup of Unit 1,
except that a short cardboard tube will cover the LED. At the far end of the short tube, the
students will install the CdS photoresistor as a light sensor. (The photoresistor manufacturer
provides an equation (see Fig. 4) which relates resistance to light intensity (lux), which can be
used to estimate light intensity in this Unit. See “General Remarks: Use of a Cadmium Sulphide
(CdS) Cell as a Light Sensor” at the end of this document.) The pins should be bent and taped
down so that the ends are exposed, but that the sensor doesn’t move when the DMM leads are
applied to measure the resistance. The tube should mount flush against the breadboard so that
ambient light doesn’t enter the tube.
All provided LEDs can be tested by the students using this setup. The first LED is to be placed
in circuit like that of Fig. 1. Once the LED is lit, the short tube is to be placed over the LED.
The DMM (in resistance mode) will measure the resistance across the two leads of the
photoresistor. With the batteries unplugged from the breadboard (to turn off the LED) and
photoresistor’s resistance can be measured. For each LED, the resistor is to be varied such that
the current is swept (at a reasonable resolution) across its safe operating range. The students will
measure the voltage across the resistor and the resistance of the photoresistor. They will
examine the intensity of the LED every time the resistor value is changed. Students need to
write down all of these values in a table, including a subjective description of the brightness.
Finally, for select LEDs they will perform the same experiment, only using the longer tube.
Trade-offs: The student should note the trade-off between wider light patterns on the LEDs with
narrow lensing. They should also note how increased distance between the LED and the photoresistor results in lower (see the Inverse Square Law) intensity measures.
Results: The students should notice that LEDs with lenses that more narrowly focus the light
result in stronger detection by the photoresistor (i.e. that the resistance decreased with increased
light intensity). Conversely, they should note that LEDs with diffusive covers yield weaker
results. They should also note that the higher the current, the brighter the LED and should be
able to make the connection that more photons are generated by the LED when the number of
electrons passing through it are increased. One of the important points they should discover is
that the photoresistor responds differently to different colours of LED. On a log-log plot of
photoresistor resistance versus illuminance the curves should look linear and each colour group
should have a distinctive trace on the plot. This Unit should take approximately 4 hours to
complete (about 5 to 8 minutes per LED).
Lab 3: “The Flame That Burns Twice as Bright…”
Intro: In this Unit you will examine the relationship between battery life and light intensity.
When you build the portable light in Unit 4 you will want it to be sufficiently bright so that you
can use it to read, but not so bright that the battery will wear out too quickly.
Problems: (1) Keeping track of similar-looking LEDs, especially in a large class, can be hard.
We’ve provided a paper template in the Appendix that LEDs can be stuck to in order to keep
track of them. (2) This is relatively long lab in which the students need to watch a battery drain.
This provides a good opportunity to have the students examine social issues related to lighting in
the developing world.
Strategies: The students will construct two circuits like the one in Fig. 1: one with many LEDs in
parallel and the other just two in parallel. The circuits are driven using fully-charged batteries,
but implemented on the same breadboard (if possible). Current-limiting resistors are selected to
have a reasonably high level of current through each LED, although below the LED’s maximum
DC current. Students connect the battery pack for each circuit and record battery voltage and
light intensity values (using the setup and method of Lab 2) every 5 minutes for a minimum of
two hours.
Trade-offs: Here, the trade-off is between light intensity and battery life.
Results: The students should now be able to determine if the selected batteries will be practical
for creating a reading light that operates over a two hour period. This Unit should take between 3
and 4 hours to complete.
Lab 4: “It Keeps Going, and Going, and Going…”
Intro: This is the final lab, in which the students will be encouraged to free-form design while
incorporating lessons learned from the previous labs. Here, you should have the students
examine issues related to system integration, packaging and the nature of interdisciplinary
design. The final product should be able to produce light for reading over a two-hour period.
The circuit will be based on the one that the students developed earlier, but it will need to be
packaged in a rugged way so that it doesn’t fall apart when used by a regular person in realworld conditions. Warn the students that you or other lab staff will shake it, drop it, and subject
it to other harsh testing conditions.
Problems: This may not be challenging enough for some students. Identify the more advanced
students and have them explore novel ways of charging the batteries. This could, for instance,
involve the use of solar panels that are readily available at hardware stores. While students
should be encouraged to look for interesting parts to use from home or elsewhere, watch to make
sure that they are not simply repackaging a commercial, off-the-shelf part.
Strategies: The students must determine appropriate test conditions for their device during the
design process (heat, cold, humidity, drop heights and drop surfaces, etc.) Have the students
develop test schemes and have them tabulate results of their tests. They will need to write about
their results. The students should be encouraged to bring in parts from home. However, their
system should not be a repackaged commercial LED.
Trade-offs: Primarily, this will result in tradeoffs between usability, durability, effective light
emission, and battery life.
Results: Expect some creative solutions. Test the devices in class, possibly side by side with the
main lab lights turned off. Have the students write reports on their results.
General Remarks: Use of a Cadmium Sulphide (CdS) Cell as a Light Sensor
Cadmium Sulphide cells are semiconductors that exhibit a change in resistance according to the
amount of light incident upon the device. They typically respond to the visible light spectrum
and therefore can be used to provide an indication of the illuminance at the sensing element.
Since this measure is aligned with human perception, it is useful for characterizing light used by
humans.
Calibration of a CdS cell to provide absolute measures of illuminance (often measured in Lux) is
difficult since variations in materials and the manufacturing process yield varied response
characteristics. In the project described here, students are asked to use an uncalibrated CdS cell
to characterize the “amount” of light from different sources. The shortcomings of using an
uncalibrated sensor with non-ideal response characteristics should be emphasized, perhaps after
the project has been completed by the students.
CdS cells, however, have some advantages for their use in this project. They are relatively
inexpensive and simple to use: an Ohm-meter is enough for students to quickly discover the
general principles of operation. These advantages are the motivating factor for their use here.
The underlying complexities associated with the use of a CdS cell as a light meter may, at first
inspection, seem to contradict the objective of having a straight-forward design project for firstyear students. The authors, however, suggest that this underlying complexity forms part of any
design process. By drawing attention to how limited capabilities of test and measurement
equipment affect results, students will quickly learn that it is important to have an understanding
of how the tools of the trade operate and how underlying principles come into play.
Approximating Illuminance Using an Uncalibrated CdS Photocell
Students are to be provided with CdS data in order to perform resistance-to-lux conversions in a
graphical fashion. Alternately, they could be provided with a function that describes this
relation. This section provides the basis for the development of such a function.
CdS cell manufacturers often provide a measure of “typical” sensitivity, sometimes denoted γ
and in units of Ohm/Lux. This measure is defined as
log R100   log R10 

log L*100  log L*10 
where R100 and R10 denote the CdS cell’s resistance under exposure to 100 and 10 Lux of light at
a particular color temperature, respectively. L*100 and L*10 refer to the illuminances at 100 Lux
 L*100 = 100 and L*10 = 10 Lux, although some manufacturers refer
and 10 Lux, respectively. i.e.
to these as luminance rather than illuminance.
If the CdS cell’s response is characterized by a power function
R  KL
where R is the CdS cell resistance (in Ohms), K is a constant, L is the illuminance in Lux and α
is the parameter to be determined, then we can expand the formula for γ,

K 100 

log




log K 100  log K 10 
 K 10 



100 
log 100  log 10
log  
 10 
Therefore, the expression for R can be rewritten as
R  KL ,   0

Conversely, and of more use to the students, is an expression where L = f(R):
1
1
1 


L  1 R  K R .
K
The value of K’ can be determined by using values for R at a particular illumination provided by
the manufacturer. For the PDV-P9002-1 CdS cell, Advanced Photonix, Inc. describes a

minimum of 11 kΩ and a maximum
of 20 kΩ at 10 Lux, 2856°K color temperature. This results
in a value of K’ ranging from 5934964 to 13942112. Additionally, the sensitivity is specified as
γ = 0.7, typical at this same color temperature.
Figure 1 shows the plot of these functions on logarithmic axes. Also shown is data collected
using an illuminance meter while testing white (7500-8500 °K) and blue (470nm) LEDs. The
“characterized” relations are plots of the L=f(R) functions with the parameters derived using a
best-fit power relation curve.
CdS Cell Resistance vs. Illuminance
CdS Cell Resistance
1000000
100000
Ideal Minimum
Ideal Maximum
Characterized White
Measured White
Characterized Blue
Measured Blue
10000
1000
100
0.01
0.1
1
10
100
1000
Illuminance in Lux
Figure 4 - Log:Log plot of the CdS cell resistance vs. illuminance for ideal (typical),
characterized, and measured responses.
The ideal relations plotted here are consistent with the manufacturer’s graphically-depicted
relation. The differences in the measured (and hence characterized) relations for particular LEDs
indicate the weakness of using the CdS cell as a light meter: the response is wavelength- (and
therefore color temperature-) dependent.
Rather than providing students with a wavelength-corrected table for each device they will be
measuring, a simplified summary table for a typical sensitivity and illuminated resistance can be
provided. Additionally, due to manufacturing differences in the CdS cells, wavelengthcorrection is not enough to provide meaningful absolute results: calibration would need to be
performed for each CdS cell. As discussed previously, the instructor is encouraged to share the
limitations of the measurement method with the students.
Additional Online Resources
1. Dr. Dr. Bill’s Optics Stuff web resources:
a. http://www.DrDrBill.com/
2. General Electric Lighting “Students: Math of Light” (GELighting.com)
a. http://www.gelighting.com/na/home_lighting/gela/students/math.htm
b. http://www.gelighting.com/na/home_lighting/gela/students/math_measuring_light
.htm
c. http://www.gelighting.com/na/home_lighting/gela/students/glossary.htm
3. Energy Efficiency Manual by Donald R. Wulfinghof
a. http://www.energybooks.com/pdf/D1150.pdf
4. Light Up the World (Relevant non-governmental organization)
a. http://www.lutw.org
5. Hyperphyics:
a. http://hyperphysics.phy-astr.gsu.edu/hbase/vision/photomcon.html