TEXAS A&M UNIVERSITY
ENGR 111B:
Foundations of Electrical &
Computer Engineering
Lab 6: Photo-Detection
Team Members: _________________________
_________________________
Section Number: __________
Team Number: __________
This Lab is due by the Beginning of the Next Lab Session.
Written By:
Hank Walker
Lorne Liechty
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Lab 6: Photo-Detection
Time Limit: 1 week
OVERVIEW
In this lab, students will construct a circuit on the protoboard to have the robot
follow a light source. In order to perform this exercise, the following new material will be
introduced:
- Photo-detection
BACKGROUND
To assist students in completing the exercises required by this lab, the following
background information has been provided. It is recommended that each student read all
of the following information as a beneficial review of the topics required.
PHOTO-DETECTOR CIRCUITRY
Photo-detection circuits fall into the category of optical electronics.
Optoelectronic devices are used to detect or emit light as part of their operation. The
photo-detection circuit in this lab will measure two different amounts of light, and
compare the difference between them. In this way, we will be able to determine the
relative direction of the strongest light source. This comparison will be used to control
the motors to turn the robot towards the stronger light source. Conversely, one could have
it turn away from the light.
The method that will be used to measure the light source is a photoresistor (also
termed a photocell). As discussed in lecture, a photoresistor is a resistor whose value
falls with increasing light intensity. Photons knock electrons loose from dopant atoms,
and these additional carriers reduce the resistance. A typical photoresistor is shown in
Figure B1.
Figure B1: Typical photoresistor design.
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In this lab, the resistance of the photocell will be used in a voltage divider circuit,
which will vary the voltage between the photocell and the connected resistance. Figure
B2 shows how this system would work if a photocell were connected to a fixed resistor.
We approximate the photoresistor resistance as falling linearly with increasing light
intensity. As can be seen in Figure B3, this is approximately correct.
Figure B2: Photocell voltage divider and hypothetical model.
Figure B3: Photocell resistance vs. illumination.
As can be seen in Figure B4, the spectral (light frequency) response of a
photoresistor depends on the material. We are using cadmium sulfide (CdS), which
responds primarily in the visible light range, with a peak response to green light. Other
materials include cadmium selenide (CdSe) and cadmium telluride (CdTe), with peak
responses in the infrared light range.
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Figure B4: Photoresistor spectral response characteristics.
The output of the circuit in Figure B2 can be fed to a comparator, along with a
potentiometer-derived voltage reference, as was done with the RC timer in Lab 4. This
converts the photocell output to a logical output, that is, the light intensity is above or
below a threshold determined by the potentiometer setting. We will use this logical
output to turn a motor on or off.
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LAB EXERCISES
BEFORE YOU COME TO LAB: Use the truth table in Table L1 to design the logic so
that the robot turns towards the brightest light source. If both photoresistors receive
similar light, the robot should go “straight” (even if your robot really curves). You will
have photoresistors on the left and right sides of your protoboard.
Table L1: Logic for photoresistor circuit.
Left Sensor Light
Right Sensor Light
More
Less
Same
Same
Less
More
Left Motor On?
Right Motor On?
You will build and design a photo-detection circuit that will use the logic in Table
L1 to steer the robot towards the brightest light source. To begin, assemble the circuit
shown in Figure L1 on your protoboard. You will use your LM393 dual comparator, two
photoresistors, two 25-turn 10 k potentiometers, two 1 k resistors, and two MPSA06
NPN transistors. Place the photocells close to each edge of the protoboard, with no light
obstructions. The circuit will compare the amount of light on each photoresistor, and turn
the motors on or off as necessary to turn towards the strongest light source.
Figure L1: Photo-detection Circuit
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The LM393 functional schematic and pin diagram is shown in Figure L2.
Figure L2: Functional Schematic of LM393 Comparator
The comparator outputs are hooked up to MPSA06 NPN transistors, which in turn
power the motors, as in Lab 4. The comparator outputs are connected to the base inputs
of the MPSA06 transistors, with the emitters connected to ground, the collectors
connected to the motor, and the other motor terminal connected to the battery. As a
reminder, the MPSA06 pin-out is shown in Figure L3.
Figure L3: Packaging Diagram of MPSA06 NPN Transistor. It is housed in a TO-92 package. With the
flat side up, the emitter is pin 1 on the left, base in the middle, and collector on the right. (Most transistors
have the base in the middle and collector on the right).
1. In order to ensure that the circuit functions properly, the reference voltages for
each comparator must be properly set. First, measure the voltage at the node
between the two photocells, termed the “center voltage,” when both are receiving
the same amount of light (e.g. they are directly under a ceiling light). Be careful
not to obstruct the light when making this measurement. Cover one photocell with
your hand (avoid touching the top of the photocell), and observe how the voltage
changes. Repeat this for the other photocell. Record the center voltage in Table
R1 for these three cases.
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2. The reference voltages need to be set so that the motors follow the logic in Table
L1. One reference voltage will be higher than the nominal (equal light on each
photocell) center voltage, which the other reference will be lower. When the
center voltage rises or falls, it will rise above the upper reference or fall below the
lower reference. The closer the references are to the nominal center voltage, the
more sensitive the circuit will be to a difference in the light on each photocell.
Initially, set the voltage of reference 1 (in Figure L1) to be 1V above the nominal
center voltage and reference 2 to be 1V below the center voltage. Record these
reference voltages, and measure both of the comparator outputs as you cover and
uncover the sensors. Remember that the high comparator output will be about
0.8V when connected to the base of the NPN transistor. Record your findings in
Table R2.
3. Place the robot on the floor and see how it functions seeking a bright light source.
You can use laser pointers, flashlights, or hands blocking the light, to vary the
light sources.
4. Adjust the reference voltages until the robot is performing suitably. Record the
operating characteristics of the circuit in Table R3.
5. Reverse the logic of the photo-detection circuitry so that the robot will turn away
from the source of light rather than turn towards it. This may be done by altering
the comparator reference settings and altering the comparator input connections,
or altering the motor connections. The goal is to reverse the motor settings
compared to Table L1.
6. Test the new darkness seeking system to see that it is working properly. Once the
robot is operating successfully, record the operating characteristics in Table R4.
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RESULTS: to be uploaded onto elearning.tamu.edu
1.
MEASURING CENTER VOLTAGES
Both Uncovered Covering Left Covering Right
Table R1
2.
Negative Reference Voltage: ________________
Positive Reference Voltage: ________________
COMPARATOR OUTPUTS
Normal
Covering Left
Covering Right
Low
Reference
High
Reference
Table R2
3.
Negative Reference Voltage: ________________
Positive Reference Voltage: ________________
Right Sensor
Covered
No
No
Yes
Yes
Light-Seeking Photo-Detector Operation
Center
Left
Right
Left Sensor
Voltage Comparator
Comparator
Covered
(V)
Output (0/1) Output (0/1)
No
Yes
No
Yes
Table R3
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Direction of Turn
(Right / Left)
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4.
Negative Reference Voltage: ________________
Positive Reference Voltage: ________________
Right Sensor
Covered
No
No
Yes
Yes
Light-Seeking Photo-Detector Operation
Center
Left
Right
Left Sensor
Voltage Comparator
Comparator
Covered
(V)
Output (0/1) Output (0/1)
No
Yes
No
Yes
Table R4
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Direction of Turn
(Right / Left)
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REPORT: to be uploaded onto elearning.tamu.edu
REPORT REQUIRMENTS
Answer the following questions:
-
Explain the function of the Photo-detection circuit
Explain why the logic needs to be reversed to avoid the light.
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