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Optoelectronic Devices
10 questions By Tony R. Kuphaldt
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Question 7 of 10
What happens when external light strikes the PN junction of an LED? Design and operate an experiment to verify your hypothesis.
Reveal answerDid you really think I would reveal the answer to this question?
Notes:This makes an excellent in-class experiment, and demonstrates a useful property of LEDs that not many people know about.
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Question 8 of 10
Photodiodes may be operated in either “photovoltaic” mode or in “photoconductive” mode. Describe the difference between these two modes, drawing schematic diagrams showing how a photodiode would be used in each mode.
Reveal answer
Notes:Ask your students whether a regular LED could be used as a photodiode in each of these modes. How would they set up an experiment to test an LED’s ability to act as a photodiode in each mode?
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Question 9 of 10
Describe the function of a phototransistor. What typical applications do phototransistors have in circuits? What is the schematic symbol for a phototransistor, and how is it properly biased?
Reveal answerPhototransistors are used as light sensors, and also as electrically isolated switching elements when illuminated by LEDs.
Notes:Ask your students what point there would be in illuminating a phototransistor with an LED. Why not just dispense with the LED and just use a normal transistor to allow one electrical signal to control another?
