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LAB EXERCISE COTR – L9
ELECTROLUMINESCENCE IN LEDS
Lab format: This lab is performed with the Remote Web-based Science Laboratory (RWSL)
Relationship to theory: This lab exercise explores the wavelengths of light emitted, voltage-current
characteristics, and the energy band levels of LED's.
OBJECTIVES



Determine the wavelengths of light emitted by 4 different LED's using the RWSL Spectrometer;
Determine the Voltage-Current characteristics of the LED's
Determine the energy band levels using Planck's constant.
EQUIPMENT LIST


Lab Kit








RWSL ELVIS oscilloscope with Semi-conductor Curve Tracer
RWSL Spectrometer with LED Display Unit
9V battery clip
330 Ω, 5 W resistor
10 Ω resistor
1 kΩ variable resistor
Simple breadboard
4 LEDs from this list
o LED Red λ = 644 nm (Jameco Part# 152805)
o LED Orange λ = 619 nm (Jameco Part# 152750??)
o LED Yellow λ = 587 nm (Jameco Part# 153146)
o LED Green λ = 560 nm (Jameco Part# 156961)
o LED Blue λ = 515 nm (Jameco Part# 134332)
Voltmeter (low cost, high impedance, 10 V scale, and mV Scale)
Make-shift spectrometer materials from Lab L3
Student Supplied:
 9V battery
 For RWSL Access:
o Computer: PC running Windows Vista or later
o Browser: Internet Explorer 8 or later
o Internet: 5 Mb/s or faster Internet connection
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INTRODUCTION
During the spectroscopic study of ionized atomic gases like hydrogen we used quantum mechanics to
model the relationship between electron energy levels and photon energies emitted. The model can be
extended to explain light energies emitted from solid crystals. In this lab we study light emitted from
semiconductors used in light emitting diodes (LEDs), instead of line-spectra we will see band spectra.
WARNINGS
 Care must be taken to hook the lab kit circuit up as instructed and to follow the testing
procedure or components could be burned out making it impossible to complete Part 1!

There are no special warnings associated with Part 2 of this lab exercise.
THEORY
Energy Levels in Solids: As atoms pack into crystal arrays, their
characteristic energy levels are affected by the regular spacing of
neighbouring atoms. If we look at sodium (see Figure 1) note that
the outer electron 3s1 level in the atom becomes lowered and
broadened into a band of energy levels in a solid. The 3s1 electron
can now travel to other atoms and is said to be in the conduction
zone. The 2s electron is still localized, and is said to be in the valence
level.
Energy Bands: If Schrodinger's equation is solved using Bloch's form
we get an energy Eigenvalue solution (as a function of momentum
vector k) that shows the energy levels are no longer at single discrete Figure 01: Sodium Levels
levels, but can exist in bands. Electrons can transition from anywhere in sodium's conduction band to the lower 2p6 valence level: thus a band of different colours can
be emitted within these energy ranges.
In Figure 2 energy levels are shown in momentum vector space (k) for
gallium arsenide (GaAs). Since momentum is a vector, the [111] and
[100] indicates the [xyz] direction (Bravais angle). Because crystal lattice spacing depends on direction, the energy levels also depend on
direction.
PN-Junctions: As forward bias is applied to the semiconductor pn-junction of the LED, light will be emitted when the energy difference
between two electronic states is great enough for an electron transition
to release one quantum of light at the wavelength of the LED. Unbound
electrons migrate from the n-region across the depletion layer and into Figure 02: GaAs Energy
the p-region, where they combine with majority carriers (holes) where
their energy is transferred into radiation and heat. Simultaneously
holes migrate from the p to the n region where they recombine with electrons, again emitting radiation.
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When an electron moves from a higher energy conduction band to a lower energy valence level when
recombining, the transition energy will be;
U eV
EQ L9.01
where e is the charge of the electron 1.60210-19 Coulombs and V the potential difference between
bands (the band gap energy). The energy U (in joules) will be related to the frequency of light emitted
by the transition according to the relation:
U h f
EQ L9.02
where h is Planck's constant, and f is the frequency of the light.
Wavelengths: The best LED substance, gallium arsenide
(GaAs) (with an energy gap of 1.35eV) emits infrared
radiation at λ = 900nm, while gallium phosphide (GaP)
with an energy gap of 2.25eV emits in the deep red.
Energy gaps in semiconductors are normally small (1eV
or less) and emit in the infrared, but higher frequency
colours can be obtained by widening the gap. This is
Figure 03: Examples of LED Spectrum Bandwidths (These are
done by alloying GaAs with other semiconductors such not necessarily for the LEDs you will be using.)
as indium (In), phosphorus (P) or aluminium arsenide
(AlAs). For example, the AlGaAs alloy emits radiation at 620nm, and GaAsP emits at 660nm.
Efficiency: Light escapes the LED from a window opened in the metallic contact at the n side of the
junction. GaAs has a relatively high index of refraction (n= 3.4) and the radiation making a greater angle
1
of incidence [   arcsin   ] with the surface is reflected back into the diode and lost in the form of
n
heat. Overall efficiency is 10%.
Contact Potential:
At the depletion zone of the pn-junction, an electric field is
produced. The voltage between the junction is related by
the differences in the conduction band energies between
the p and n material:
Vc 
E
cp
 Ecn 
e
20
eg. for Ge, E  0.3V  e  4.8  10 J
Vc 
kBT  N d N a 
ln 
2 
e
 Ni 
EQ L9.03
EQ L9.04
EQ L9.05
Figure 04: PN-Junc
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where kB is Boltzmann's constant ( 1.38  1023
J
), T is temperature in Kelvin, Nd is the concentration of
K
donors, Na is the concentration of acceptors, and Ni is the intrinsic concentration of the semiconductor
(without doping).
k BT
 25.2mV at room temperature.
e
For Germanium, N d  N c  11016 / cm3 ; Ni  2.5 1013 , thus Vc  3.0V .
Typically,
EQ L9.06
EQ L9.07
An oscilloscope can be used to display this characteristic voltage-current curve. Observing the applied
voltage at the "knee" of this curve for several different LEDs and finding the lowest and highest wavelength emitted, will allow the energy band boundaries to be determined within 10%.
The Curve Tracer
This type of simple curve tracer puts a
small AC 60 hertz voltage across the
device. Because alternating current is a
sine wave, the semiconductor is tested
in the forward bias during the positive
part of the sine wave, and in the reverse
bias during the negative part.
The connection from the tester goes to
the vertical and horizontal inputs of the
oscilloscope. The scope is placed in X-Y Figure 05: RWSL Curve Tracer and LED Display Circuit Diagram
mode, where Y voltage is displayed
along the vertical axis and X voltage is displayed along the horizontal axis. Since the X-voltage is the
voltage drop across a 1 kΩ resistor, it represents current in the device. Thus device voltage is shown on
the Y-axis, and device current shown on the X-axis.
Curve Patterns and Using the Tracer:
Voltage Divider and Power to the LED Circuit
A wall transformer rated for 9 VAC at 1000 mA is being used as the power supply. Since the circuit does
not take this much current to operate it is actually forcing the voltage to almost 12 VAC, so the circuit is
receiving about 60 mA through the upper 100 Ω resistor. The two 100 Ω resistors act as a voltage
divider so almost 6 VAC is being applied to the LED part of the circuit between the Scope Vertical and
Scope Horizontal points of the circuit before an LED is connected. (When a yellow LED was connected,
we found the overall voltage being supplied to be 10.74 VAC with 5.26 VAC being supplied to the LED
circuit. Since the specs on each LED will vary somewhat, these exact numbers will vary as well.)
Vertical Line Pattern:
If no LED is connected to the curve tracer, then the voltage varies from the positive to negative
maximum voltage, but current is always zero. This results in a vertical line. The voltage peaks can be
directly read from the scope. The impedance of the LED is the slope of the line. Here that is infinite
ohms.
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Horizontal Line Pattern:
If a wire is connected across the curve tracer device terminals, then the voltage is clamped at zero, but
current varies from -max to +max. This is a flat horizontal line. The slope of the Voltage-Current line is
zero, and the impedance is 0Ω.
Current Limiting Protection: Current is limited to something less than 6 mA by the 1kΩ resistor that is in
series with the LED.
Reading Current on the Scope: To read the current shown on the scope remember that according to
Ohm's law if you are on the 1kΩ range, I= V/R= V/1000. Thus 1 volt of deflection on the scope represents 1mA of current on this scale.
L Pattern:
This is a combination of the high resistance/low resistance patterns. If a diode is connected, then while
it is reverse biased, it will act like an open circuit, resulting in voltage increasing without any current
flow, making a vertical line. But when it is forward biased, around 2V, it will start to conduct current.
Voltage then is clamped at 2V and current increases until it reaches the maximum set by the current 1
kΩ resistor. This is a flat horizontal line for this region. Together, we get the characteristic "L" knee of a
diode.
Slanted Lines:
Since the slope of the line indicates impedance (at 60Hz), you can calculate the impedance of the LED at
 1.9V    2.1V 
V
60 Hz. For instance, in Figure 6 below, the slope of the line is

 187 . This
I
 0.6mA  1.67mA
value is the impedance of the diode at 60 Hz.
Figure 06: Typical Diode ‘Knee’ Curve
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PROCEDURE
Part 1 (Hands-on)
Here you will find the minimum voltage at which each LED will begin to glow. You could also use
the makeshift spectrometer from Lab L3 to estimate the emission band wavelengths of each LED (at
your instructor’s perrogitive).
1) Set up the following circuit using the equipment from your lab kit. See Appendix 4 (Building the
LED Testing Circuit) if you require a step by step procedure for building this circuit. Make sure
the battery, LED, and voltmeter are not connected into the circuit yet, but have them ready to
connect.
Figure 07.1: This is the circuit you should construct.
Figure 07.2: Completed Circuit. (The positive lead of the volt meter is connected to point C jumper
and the common lead (-) is connected to point D Jumper. The point E jumper is trailing off the image
at the bottom and not connected.)
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2) Make sure you have your multi-meter set on the 10 V scale and NOT on the Ohms (Ω) scale.
(The ohms scale includes a battery that could also damage the LED.)
3) Determine when the variable resistor is set at 0 Ω. To do this, connect the multi-meter (set on
the 10 V scale) across the variable resistor as indicated with the positive (+) lead connected to
point C and the negative (–) (common) lead to point D. Attach the battery as shown making
sure to connect the positive (+) terminal to the 330 Ω resistor at point A and the negative (-)
terminal to the variable resistor at point F. Turn the variable resistor dial until the voltmeter
reads 0.0 V. This will be your starting position. In this position there will be no current through
the LED.
4) Connect the Red LED into your circuit and connect the voltmeter to points C and D as shown.
Remember the long lead of the LED is the + lead so it should be connected to point C. Inspect
your circuit one more time to make sure it is correct.
5) Now turn the variable resistor until the LED just begins to light. Record this voltage.
6) Move the positive (+) lead of the voltmeter to point E so you can read the voltage across the 10
Ω resistor that is in series with the LED. (Note – the voltmeter will now read a negative voltage
because it is hooked up backward. This is not a problem, but you can ignore the – sign.) Switch
the voltmeter to the mV scale. You do not want the current passing through the LED to increase
above 20 mA. At 20 mA the voltage drop across the 10 Ω resistor will be 200 mV (0.200 V).
Confirm this yourself using Ohm’s Law. You know the current passing through the 10 Ω resistor
is the same as that passing through the LED because they are in series and part of the same
branch of the circuit.
7) Now continue turning the variable resistor so that the brightness of the LED increases until you
have either 200 mV across the 10 Ω resistor or you cannot increase it any further. At 200 mV
you know there is 20 mA passing through the LED, but if you have less than this; calculate the
current passing through the LED and record it.
8) Switch the voltmeter scale back to 10 V and move the + lead back to point C. What is the
voltage drop across the LED? Record this.
9) Turn the variable resistor down until the voltmeter reads 0.0 V.
10) Repeat steps 4 to 9 replacing the red LED with each of the remaining LEDs in your kit in turn
recording the data requested for each. (orange, yellow, green, and blue.)
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Optional:
Using the makeshift spectrometer from lab L3, attempt to find the range of wavelengths that each LED
emits light at.
Check with your instructor to see if he or she wants you to do this part.
11) Set up the make-shift spectrometer as you did in Lab L3
12) Place the red LED in your circuit. Place the circuit as shown below (see Figures 8.1 and 8.2).
Figure 08.1: LED positioned to shine along the axis of the Make-shift spectrometer template
Figure 08.2: Makeshift Spectrometer with LED shining through the Cornell slit-film
0.13 mm slit and diffraction grating
13) Connect the voltmeter for monitoring LED current (across the 10 Ω resistor at points E and D as
you did in step 6 above).
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14) You’ll need a dark room for this with your eyes dark adapted (preferably). Turn the LED on and
increase the voltage to the LED until the LED glows as brightly as possible without going over 20
mA passing through the LED. (Watch your voltmeter!)
15) Now measure the angles to the ends of each band of colour as accurately as you can. Record
them.
Figure 09.1: LED First Order Spectral Band (Right)
Figure 09.2: LED First Order Spectral Band (Left)
16) Calculate the range of wave lengths in the colour band and record these. You will compare your
results here with the data you will collect with the RWSL spectrometer in Part 2 of this lab.
(Note – The makeshift spectrometer is not a precision instrument so it is expected that your
results here will contain significant inaccuracies, but you will collect much more accurate data
for the orange, yellow, and blue LEDs when you take your data using the RWSL Spectrometer.)
17) Now repeat steps 12 to 16 for each of the remaining LEDs.
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Part 2 (RWSL)
18) Review Appendix 1: The RWSL Electroluminescence in LEDs Lab.
19) Schedule a lab session on the RWSL Electroluminescence Lab. Your instructor will have the
dates when it will be available as well as scheduling instructions.
20) Make sure your computer system is certified for RWSL access as described in Lab Exercise L3
Appendix 1. If it can’t be, find a system at your local educational institution or somewhere else
that can be certified.
21) A few minutes before your scheduled lab session access the RWSL and, if you have lab partners
contact them using the agreed upon back-channel. You will need to coordinate who has actual
control of the RWSL Electroluminescence lab when.
22) At the appointed time of your session, one member of your lab group should request control of
the Electroluminescence in LEDs Lab. Try to give all members of your lab group a chance to
control the electroluminescence lab and collect data. See Appendices 1 and 2 for instructions
on how to do this.
23) Each member of the lab group should take a turn running through the Electroluminescence VI
controls so everyone is familiar with them and can take data.
See the “Experimental Operation” sections of Appendix 1 in
Lab Exercise L3 for a review of these controls and how to
acquire spectrometer data and Appendix 1 below for
instructions on acquiring oscilloscope data.
(Note - The RWSL is connected to the circuit shown in figure 5,
so you are working with a 60 cycle AC power supply. Hence
the LEDs will be turning on 60 times per second unlike your DC
tests in part 1.)
Figure 10: Examples of various
24) Take enough data to plot a voltage versus current curve for
LED turn-on voltages and current
each of the 4 diodes (infrared, amber, yellow, and blue).
graphs. (These do not
25) Determine the turn-on voltage for each diode. Compare this
necessarily match the particular
to the results you had in Part 1 for the orange, yellow, and
LEDs you are using.)
blue LEDs.
26) Use the RWSL spectrometer to find the upper and lower wavelengths of light that each diode
emits.
27) Compare this to your results in Part 1.
28) Make a plot of turn-on voltage versus light frequency for the set of diodes. Include your Part 1
data (with wide error bars to show its relative inaccuracy.)
29) From the graph find the band gap energies using Planck's constant.
ANALYSIS AND/OR QUESTIONS

See the procedure above for what you need to provide in your Lab Report.
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REFERENCES
From Original Lab Exercise:
1. Elementary Solid State Physics by M.Omar, pub Addison-Wesley 1975, Bands and Na p182-188,
GaAs p283, LED analysis p360-361.
2. Tel-atomic Website: http://www.telatomic.com/planck.html
3. Quantum Physics Lab: http://physics.clarku.edu/quantum_lab/led.htm
Original Lab Manual by Rick Nowel, E. Tech, COTR
Adapted for Remote Delivery by Ron Evans, MSc
Under the Remote Science Labs for Second Year Physics Project funded by BCcampus
2012 - 2013
Public domain images in Figures: 01, 02, 03, 04, and 10
were imported from the original lab manual that was produced by COTR.
All other images were produced by Ron Evans
and are covered by the CC license of this document.
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Appendix 1
The RWSL Electroluminescence in LEDs Lab
See Lab Exercise L3 for general operation of the RWSL and the RWSL Spectrometer including system
certification, back-channel communications, how to access the RWSL VI, and other preliminary RWSL
information.
1) When you first access the RWSL Electroluminescence in LEDs Lab you will see something like
this:
Figure A1.1: RWSL Electroluminescence in LEDs VI (power off)
2) Make sure the Spectrometer is selected and then press the ‘A’ selector button to turn on the
Infrared LED. The ‘A’ button will turn orange (see figure A1.3 below) and you will see the rightmost LED light up. (If the fibre optic spectrometer head is not currently lined up with the
infrared LED it will move to the right most position first before lighting the LED.)
NOTE - If you were in the room with it, you would not see the light because your eyes are not
sensitive to infrared light, but the RWSL cameras are sensitive to infrared so you’ll see
this LED light up.
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The shorter wavelength infrared light can be seen by the spectrometer, but the LED emits light
that has longer wavelengths than the spectrometer can detect so the graph that the
spectrometer shows will look something like:
Figure A1.2-1: Infrared LED lit
Figure A1.2-2: Infrared LED spectrum
Notice that the signal strength in this image is not particular strong, because the intensity is not
quite 2,300 counts. A strong signal will be 40,000 counts or more. You can adjust the vertical
scale using the analysis methods described in Appendix 1 of Lab Exercise L3. You can read the
wavelengths where light is being emitted from the horizontal scale. Notice that there is some
signal noise at about 545 nm and 610 nm. These are less than half as strong as the infrared
signal on the right so you can ignore them. Above about 850 nm the infrared signal is definitely
increasing very quickly. You will see the entire band width of the other LEDs like this graph of
the Yellow LED:
Figure A1.2-3: Yellow LED spectrum
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Record the range of wavelengths where the diode is emitting light. Use the spectrometer
analysis tools to help you zoom in on the position where the signal is significantly stronger than
the ‘noise’ at the left of this graph. If you need to ‘nudge’ the spectrometer head left or right a
little to get a stronger signal you can use the green ‘nudge’ buttons to the left or right of the ‘A’,
‘B’, ‘C’, ‘D’ sample selector buttons. See below:
Figure A1.3: Sample Selector Buttons and green ‘nudge’ buttons
NOTE – The table below indicates which LED the selector buttons will correspond to:
Sample Selector Button Corresponds to
A
Infrared LED
B
Orange or Amber LED
C
Yellow LED
D
Blue LED
3) Now switch to the oscilloscope by clicking on the Oscilloscope selection tab:
Also select the Oscilloscope Cursor tab:
this:
. You will see something like
Figure A1.4: Infrared LED lit – Oscilloscope zoomed in.
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If you turn the LED off by clicking on the ‘A’ sample selection button again you will have a graph
that looks like Figure A1.5-1. When you turn the LED back on again you will have a graph that
looks like Figure A1.5-2. See below:
Figure A1.5-1: Oscilloscope graph with LED off
Figure A1.5-2: Oscilloscope graph with LED lit
Use the analysis tools that you used with the spectrometer to get a graph that looks like Figure
A1.4 above.
The RWSL oscilloscope is connected to the curve tracer circuit shown above in Figure 5, but for
easy reference it is repeated below in Figure A1.6. In Figures A1.4, A1.5-1 and A1.5-2 the green
curve represents the voltage drop across Scope Vertical (SV) and Scope Common (SC) and the
blue curve is the voltage across Scope Horizontal (SH) and SC. The RWSL Controlled Relay can
select any LED or none at all. When no LED is selected, no current can flow in the circuit, so we
see the entire voltage drop across SV and SC. Since there is no current flow there is no voltage
drop across SH and SC. Since we are dealing with a 60 Hz signal we see the voltage across SV
and SC oscillating and the blue curve is flat. When a diode is turned on, there is no difference
when the voltage across SV and SC is positive because the diode effectively blocks and current
flow in that direction. However when the voltage goes negative the diode continues to block
current flow until the voltage reaches the turn-on voltage. At this point the LED begins to let
current flow through the LED. We see this as the bottom part of the green curve becomes
blunted and is actually clamped to a specific maximum voltage. Now current is flowing in the
circuit so we see an increase in the voltage drop across SH and SC. The turn-on voltage and the
maximum voltage are unique for each type of LED. Even though the blue curve is actually a
voltage measurement, it is across a very accurate 1 kΩ resistor, so we can easily treat it as a
current measurement using Ohm’s Law. ( I 
V
)
1k
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Figure A1.6: Curve Tracer Circuit
4) Use the analysis tools similarly to the way you used them for the spectrometer readout to zoom
in on the oscilloscope graphs. To collect very accurate readings of the turn-voltage and current
data use the cursor tool. For the turn on voltage you’ll want to zoom in to the point where you
can see the green curve begin to divert from its original path. Below is an example of measuring
the turn-on voltage of the infrared LED. Notice how the cursor is positioned at the point where
the green curve begins to turn. Read the turn-on voltage to 4 decimal places (-0.9729 V) in the
cursor Y box. See below:
Figure A1.7: Infrared LED Turn-on Voltage being measured.
5) Repeat for the remaining 3 LEDs.
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Appendix 2
Supplied LED specifications:
Do not exceed maximum rms current!
Colour
Part Number
(J = Jameco #)
R
Infrared
LVIR333X
J106526
K
Red
LT1873-81-UR J152805
Orange RK LT2kJ3-UR-S14 J153139
AmberARK ??
J??
RK
Yellow
LT2K33-URP-8 J153146
GreenK
LT1823-81-HE J156961
RK
Blue
LT18B3-81UR J134332
uv-a*
-
Brightness (typical)
8mW/cm² @ 20mA
2112mcd @ 20mA
2500mcd @ 20mA
17,000 mcd @ 100mA
9300mcd @ 20ma
500mcd @ 20mA
630mcd @ 20mA
500mcd @ 20mA
max rms
current
50mA
30mA
50mA
100mA
50mA
30mA
30mA
30mA
Peak λ
Material
940nm
660nm
621nm
629 – 574 nm
595nm
567nm
428nm
400nm
GaAs2
GaAlAs4
AlInGaP1
??
AlInGaP1
GaP3
GaP3
-
1
AlInGaP: aluminium indium gallium phosphide
GaAs: gallium arsenide
3
GaP: gallium phosphide
4
GaAlAs: gallium aluminium arsenide
R
These LEDs are used with the RWSL portion of this lab exercise.
K
These LEDs are used with the lab kit portion of this lab exercise.
A Alternate LED
*This LED is not used in this lab exercise.
2
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Appendix 3
Building the LED Testing Circuit
1) Assemble the components you’ll need from your lab kit or another source
Figure A3.1: Required Components.
Simple breadboard
330 Ω, 5 W resistor
10 Ω resistor
1 kΩ variable resistor
9V battery clip and 9V battery
4 LEDs
Voltmeter
4 jumpers
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2) Attach Battery Clip to points A and F as shown:
Figure A3.2: Battery Clip in Place
3) Install the 330 Ω resister to points A and B. Connect the jumper from points E to F.
Figure A3.3: 330 Ω resistor and jumper in Place
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4) Connect the 1 kΩ variable resistor to points B, E, and C as shown.
Figure A3.4: 330 Ω resistor and jumper in Place
5) Install the 10 Ω resistor from point E to D.
Figure A3.5: 10 Ω resistor in Place
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6) Connect the first LED you want to work with to points D and C making sure the short lead
connects to the 10 Ω resistor at point D.
Figure A3.6-1: LED positioning
Figure A3.6-2: LED in Place
7) Add connection jumpers at circuit points C, D, and E
Figure A3.7-1: Jumpers connecting to points D and C Figure A3.7-2: Jumper connecting to point E
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8) Completed Circuit
Figure A3.8: Completed Circuit
9) Inspect your circuit to make sure you have it connected as shown above before you apply power
to it. (Shorting the battery directly across the LED will burn it out!)
10) Remove the LED and follow the instructions in the Lab procedure now.
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