Deck 20: Photodetectors

Question 1

Figure 20-05
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The circuit in the accompanying figure shows a phototransistor used to turn on a relay coil. What is the difference when the phototransistor is in the presence of darkness versus light?

Answer 1

When the phototransistor is in darkness, no electrons are emitted by the base junction, and the transistor is turned off. When the phototransistor is in the presence of light, it turns on and permits current to flow through the relay coil.

Question 2

Regardless of the type of photoemissive device used, or the type circuit it is used in, the greatest advantage of the photoemissive device is ____________________.

Answer 2

speed

Question 3

____________________ emit electrons when in the presence of light.

Answer 3

Photoemissive devices

Question 4

Photovoltaic devices are more often called ____________________.

Question 5

The most common photoconductive device is the ____________________.

Question 6

Photovoltaic cells cannot operate electrical equipment without external power.

Question 7

Figure 20-13
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What does the accompanying figure show?

Question 8

Photoconductive devices exhibit a change of resistance due to the presence or absence of light.

Question 9

Photodetectors that have both the transmitter and the receiver units mounted in the same housing depend on a reflector for operation.

Question 10

The amount of voltage produced by a cell is determined by the material of which it is made.

Question 11

Photo-operated devices fall into one of three categories. What are the categories?

Question 12

Figure 20-01

The accompanying figure shows the schematic symbol for a photovoltaic cell. What do the battery and arrow symbols mean?

Question 13

Photodetectors have to make physical contact with the object they are sensing.

Question 14

A(n) ____________________ steals the base current away from some other transistor to keep it turned off.

Question 15

Figure 20-10
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In the circuit shown in the accompanying figure, although the circuit will work if the cad cell is large enough to handle the current, there are still two problems. What are they?