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Optics
Fibroptonic
Ref : 202 012
4 Experiment no.13
Fluctuations in the brightness of a light bulb operating on the mains supply.
5 Experiment no. 14
Periodic variations in brightness of a neon indicator operating on mains
ENGLISH
direction does not matter. We will therefore connect the black terminal to the
negative and the green to the positive of the power supply.
•
Connect the oscilloscope to the terminals of the photoresistor: black terminal to the
ground of the oscilloscope and the yellow terminal to channel A (figure 19).
Channel A
Figure 19: Connecting the photoresistor module
•
Illuminate the photoresistor with a light bulb held at about ten centimetres.
•
Set the oscilloscope in the AC position (only AC component) and amplify.
•
We observe a sine curve, whose amplitude can be adjusted with the potentiometer
located on the unit (the amplitude depends on the distance between the bulb and
the photoresistor).
•
The period determined on the oscilloscope is 10 milliseconds (thus a frequency of
100 Hz, i.e. double of the mains frequency because the bulb comes on at every
positive or negative half-wave of the AC voltage of the mains).
•
If we repeat the same experiment with a pocket torch (operating on batteries), there
are no fluctuations.
•
Repeat the previous experiment and place a small neon lamp (orange light)
powered by mains AC voltage, very close to the photoresistor.
•
Adjust the amplitude of the signal observed on the oscilloscope with the
potentiometer (see figure 20). Determine the period, that is 20 ms.
ground
Photoresistor
61
Power supply +
0.2 to 0.8 V
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