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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