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Lab 6 - TEXAS A&M UNIVERSITY ENGR 111B Foundations of...

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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 Written by Texas A&M University 1
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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. Written by Texas A&M University 2
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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.
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