The resistor and photocell together behave like a pot. When the light is very bright,
then the resistance of the photocell is very low compared with the fixed value
resistor, and so it is as if the pot were turned to maximum.
When the photocell is in dull light, the resistance becomes greater than the fixed 1
kΩ resistor and it is as if the pot were being turned towards GND.
Load up the sketch given in the next section and try covering the photocell with your
finger, and then holding it near a light source.
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