A photodiode forms part of a light meter used for checking light levels in an office - AQA - A-Level Physics - Question 4 - 2017 - Paper 8
Question 4
A photodiode forms part of a light meter used for checking light levels in an office.
Figure 10 shows the circuit diagram for the light meter.
Figure 10
- 20 kΩ
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Worked Solution & Example Answer:A photodiode forms part of a light meter used for checking light levels in an office - AQA - A-Level Physics - Question 4 - 2017 - Paper 8
Step 1
State the mode in which the photodiode is being used in Figure 10.
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Answer
The photodiode is being used in photoconductive mode, which involves applying a reverse bias to increase its sensitivity to light.
Step 2
In which mode is the operational amplifier being used in Figure 10?
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Answer
The operational amplifier in Figure 10 is operating in comparator mode, as it compares the voltages at its inverting and non-inverting terminals.
Step 3
Estimate, using the information in Figure 11, the light level needed to cause this reverse current through the photodiode.
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Answer
From Figure 11, a reverse current of 0.10 mA corresponds to a light level of approximately 1000 lux (±10%).
Step 4
Calculate the voltage at point X in the circuit shown in Figure 10 for the light level in question 04.3.
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Answer
Using Ohm's law, the voltage at point X can be calculated as follows:
VX=IimesR=0.10extmAimes20extkΩ=2extV
Step 5
Decide whether the output LED would be switched ON or OFF when the current through the photodiode is 0.10 mA.
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Answer
Since the voltage at point Y is set to 1.75 V and the voltage at point X is 2 V, the operational amplifier's output will be high (ON). Hence, the output LED would be switched ON.
