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ECE 202 – Experiment 7 – Lab Report
APPLICATION OF OPAMPS AND PHOTORESISTORS
YOUR NAME______________________
GTA’S SIGNATURE_________________
LAB MEETING TIME______________
We have used green letters in parentheses to label questions for which we will expect to see
your responses in the lab report.
EXPLORING PHOTORESISTORS
3. Observe resistance of a photo-resistor as the amount of light changes. Record few values in the
space below. Experiment by using a light source shining directly into the resistor, ambient light,
and different objects to cover the resistor: your finger, different-color paper cups, Sharpie covers,
… Include your findings in the report. (Q.3)
Describe the condition
Resistance measured
BUILDING THE TRIANGLE WAVE GENERATOR
4. Key investigation: generating the triangle wave from a DC voltage.
Assemble the following circuit:
VCC = 4.5 V
R5 = 100 kΩ
R6 = 15 kΩ
R7 = 47 kΩ
C3 = C4 = 0.1 μF
P3 = 100 kΩ potentiometer
OA3 = OA4: LF412CN
Vcc
C4
1
P3
C3
2
Vcc
-OA3
+
R6
3
R5
4
5
Vcc
-OA4
6
+
R7
a) Use oscilloscope to observe and capture v3 and v6. What operation is being performed?
(Q.4.a)
b) Observe v6 (triangle wave output) as P3 is varied. What do you see? Capture a couple of v6
waveforms for different values of P3. (Q.4.b)
ECE 202 – Experiment 7 – Lab Report
c) Change R6 to a much larger value (around 50 MΩ) and describe impact on v6. (Q.4.c)
BUILDING A SINGLE MOTOR COMPONENT
5. Key investigations: how comparator mode of OpAmp and use of PWM signal and transistor
power a geared DC motor.
Assemble the following circuit:
Vcc
C1
M1
M
D1
Vcc
TR1
R1
OA1
7
8
-
11
+
VCC = 4.5 V
R1 = 1 kΩ
C1 = 0.1 μF
Cs = 47 μF (connected between VCC and common)
TR1: BS170
OA1: LF412CN
M1: Velleman MOT1, 3V geared
a) Use two separate DC power supplies to generate v8 and VCC, and waveform generator (set to
triangular signal) to generate v7. Set VCC = 4.5 V. What are the max and min values for v7 and
v8 that you are allowed to set as an input of the OpAmp? (Q.5.a)
b) Vary parameters of DC and triangular signals and capture a few samples for v7, v8 and v11. Set
period of the triangular signal such that you can observe the change in the behavior of your
circuit as v7 changes from being less than to being greater than v8. What period did you
choose? (Q.5.b)
c) What impact does the varying PWM have on the motor speed? (Q.5.c)
d) Measure the current through the motor as it is spinning without any load. Will current change
as you apply load to the motor (you may use your finger to apply pressure / attempt to slow
down the rotation) of the motor. Do not apply too much pressure, as we do not want to
damage the motor; we just want to explore if current will depend on the load. (Q.5.d)
6. Key investigations: how photo-resistor (i.e. varying the resistance) effects the generated pulses
and translation to motor behavior
Assemble the following circuit (you may omit R3 and LED1):
Vcc
P1
M1
M
D1
R3
Vcc
11
TR1
R1
OA1
-
C1
+
VCC = 4.5 V
R1 = 1 kΩ
R3 = 220 Ω
PR1 (photoresistor)
C1 = 0.1 μF
Cs = 47 μF (connected between VCC and common)
P1 = 100 kΩ potentiometer
TR1: BS170
OA1: LF412CN
M1: Velleman MOT1, 3V geared
7
8
PR1
LED1
ECE 202 – Experiment 7 – Lab Report
a) Use DC power supply to generate VCC = 4.5 V, and waveform generator (set to triangular
signal) to generate v7. What are the max and min values for v7 that you are allowed to set as an
input of the OpAmp? (Q.6.a)
b) Vary the potentiometer leading into node 8, while keeping the amount of light constant. How
does the output vary as a result? (Q.6.b)
c) Using a reasonable potentiometer setting, now investigate the role of the photo-resistor. How
does the output at node 8 vary in relation to increasing the amount of light received by the
photo-resistor? (Q.6.c)
d) Once you think you have explored a – c, try changing the period and range of the triangular
signal and explore what happens. (Q.6.d)
CONNECTING THE WHOLE CIRCUIT!
7. Key investigation: how each part combines to produce desired behavior
Connect the whole circuit:
Vcc
C4
1
P3
C3
2
Vcc
-OA3
R6
3
+
4
5
R5
Vcc
-OA4
6
+
R7
Vcc
Cs
C1
P1
M1
M
D1
P2
R3
M2
M
D2
R4
Vcc
R1
OA1
-
TR1
+
11
Vcc
7
8
PR1
VCC = 4.5 V
R1 = R2 = 1 kΩ
R3 = R4 = 220 Ω
R5 = 100 kΩ
R6 = 15 kΩ
R7 = 47 kΩ
C2
LED1
LED2
9
10
+
OA2
-
12
TR2
R2
PR2
PR1 = PR2 (photoresistor)
C1 = C2 = C3 = C4 = 0.1 μF
Cs = 47 μF
P1 = P2 = P3 = 100 kΩ potentiometer
TR1 = TR2: BS170
OA1 = OA2 = OA3 = OA4: LF412CN
M1 = M2: Velleman MOT1, 3V
geared
D1, D2
LED1, LED2
4.5 V battery pack
batteries
ECE 202 – Experiment 7 – Lab Report
a) Tune the circuit by adjusting potentiometers P1, P2 and P3. When finished with the lab and
ready to take your circuit apart, measure values that your potentiometers were set to. (Q.7.a)
b) Use oscilloscope to observe all signals, starting from v1 and following the signal path to one
of the motors. You do not have to capture any of the signals, just write your observations in
the report. (Q.7.b)
c) What was the most challenging part of the lab? What part of the lab did you enjoy the most?
(Q.7.c)
Additional activities: (Q.8)
 Time allowing, setup your circuit to follow the line. Have you succeeded? Were any adjustments
needed?
 How well does your circuit track the line?
 Introduce a small (~100 nF) capacitor to the connection between nodes 1 and 5 on the triangle
wave generator to “clean up” the PWM (why?)
 1k resistor replaced with 10k resistor or potentiometer for gates on MOSFETs to reduce motor
speed
Remember GTA’s signature!