Lab Photo Detection

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					          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

       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
    - Photo-detection

        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-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.

Written by Texas A&M University                                                               4
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       Left Motor On?      Right Motor On?

              More                  Less

              Same                 Same

              Less                  More

        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
                     MEASURING CENTER VOLTAGES
                Both Uncovered Covering Left Covering Right

                                     Table R1

Negative Reference Voltage: ________________

Positive Reference Voltage: ________________

                          COMPARATOR OUTPUTS
                   Normal       Covering Left Covering Right
                                     Table R2

Negative Reference Voltage: ________________

Positive Reference Voltage: ________________

                       Light-Seeking Photo-Detector Operation
                              Center        Left         Right
Right Sensor   Left Sensor                                         Direction of Turn
                              Voltage Comparator      Comparator
  Covered       Covered                                              (Right / Left)
                                (V)    Output (0/1) Output (0/1)
     No             No
     No            Yes
     Yes            No
     Yes           Yes
                                      Table R3

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Negative Reference Voltage: ________________

Positive Reference Voltage: ________________

                       Light-Seeking Photo-Detector Operation
                              Center        Left         Right
Right Sensor   Left Sensor                                         Direction of Turn
                              Voltage Comparator      Comparator
  Covered       Covered                                              (Right / Left)
                                (V)    Output (0/1) Output (0/1)
    No              No
    No             Yes
    Yes             No
    Yes            Yes
                                      Table R4

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REPORT: to be uploaded onto

   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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