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ENGR201 Circuits Lab
Name: _________________________
Potentiometers, LEDs and Comparators
Fall 2001
OBJECTIVES:
 To learn how to use a poteniometer to create a variable reference voltage.
 To learn how to use an LED as an indicator.
 To learn the operation of the LM311 I.C. voltage comparator.
PART I - POTENTIOMETER
A potentiometer (pot) is a variable resistance. Typically, the device has three terminals (see
figures below), and the resistance between the outer two terminals is fixed. As the pot is
adjusted, the resistance between the inner terminal and either outer terminal varies.
Figure 1 - Physical and Schematic Representation of a Potentiometer
A
B
C
A
B
A
C
RAB
RBC
B
RPOT
RPOT = RAB + RBC
C
0  RAB  RPOT
RPOT  RAB  0
ENGR201 Lab: Potentiometers, LEDs and Comparators
A
12V
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1. For the circuit shown RAC = 10 k, what
should be the value of RBC in order to
make the voltage reference, VREF = 2V?
B
+
C
12V
2. Connect the circuit, adjust the pot until
VREF = 2V, disconnect the pot and
measure RBC and RPOT and compare your
results to the predicted value of RBC in
step #1.
VREF
-
A
3. Calculate the values of RBC needed to
make VREF = +2V, 0V, and -2V.
B
+
-
4. Connect the circuit and verify that
you can adjust VREF to the three
values in step #1.
+
VREF
-
C
+
-12V
A
B
Vs
+
C
VREF
-
RL
5. If the pot is connected to a load
resistor RL as shown, what
should the size of RL be
compared to the size of RBC so
that the value of VREF is not
affected significantly? Explain.
1. RBC = _______________ (calculated)
2. RBC = _______________ (measured)
3. [RBC]+2 V = _____________
[RBC]0 V = _____________
[RBC]-2 V = _____________
5. __________________________________________________________________
ENGR201 Lab: Potentiometers, LEDs and Comparators
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PART II - LEDS
A light emitting diode (LED) dissipates power in the form of light whenever current passes from
anode to cathode (see figure below). If an attempt is made to make current flow from cathode to
anode, the LED acts like an open circuit.
Figure 2 - LED Operation
Anode
Cathode
LED emits light
Anode
Cathode
LED is Dark, I = 0
Typically, LED current should be about 20mA for proper brightness. Current levels significantly
greater than 20mA may damage the diode. Therefore, a limiting resistor is used to limit LED
current.
I
Assuming that the voltage across the LED is zero
when the LED is on, calculate an appropriate value
for Rlimit. Connect the circuit and verify the LED
illuminates. Measure and record the value VLED.
Rlimit
12V
+
VLED
-
Rlimit  12 V/20mA = _______________
VLED = _______________
ENGR201 Lab: Potentiometers, LEDs and Comparators
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PART III - COMPARATORS
An LM311 voltage comparator can be thought of as a voltage-sensitive switch. Whenever (V2 –
V3) = v < 0V (V2 < V3) then the switch is closed (the LED will illuminate in the circuit
below). However, if v > 0V (V2 > V3) then the switch is open (the LED will be off). (Note:
V2 = 12*2.2/49.2  0.5V)
470
47k
8
2
12V
+
V2
-
+
+
v
2.2k
7
LM311
-
1
3
10k pot
8
-
+
V3
-
4
5
8-pin Mini DIP
pin assignments
1
4
1. Connect the circuit shown and adjust the pot until the LED is off.
2. Re-adjust the pot until the LED comes on. Measure and record the value of V3 that causes
the LED to illuminate and compare this to the measured value of V2.
3. Re-adjust the pot until the LED goes off. Measure and record the value of V3 .
4. Is the value of V3 that causes the switch to close the same value as the one that causes it to
open? Is this a good thing? Explain.
ENGR201 Lab: Potentiometers, LEDs and Comparators
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PART IV – A LIGHT SENSITIVE SWITCH
A photocell (photoresistor) is a resistor that is sensitive to the amount of light it receives. The
dark resistance may be several hundred k, while the light resistance is a few k or lower. This
sensitivity to light makes a photocell ideal for many practical applications, including a
photosensitive switch.
1. Measure the light and dark resistance of the photocell provided.
2. Modify the circuit from Part III to use the photocell so that the LED will illuminate in
dark conditions and turn off when the photocell is exposed to light.
3. Show any calculations used to select component values.
(Hint: replace the 47k / 2.2k resistor combination with a photocell and an appropriate
resistor value.)
ENGR201 Lab: Potentiometers, LEDs and Comparators
Photo Cell
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