Data Sheet

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116. Photoresistor
Build the circuit to the left and turn on the switch (62). Whenever light
shines on the photoresistor (68), the lamp (76) will be bright. Cover the
photoresistor (68) with your nger and the lamp (76) will go off.
A photoresistor (68) is designed to have low resistance when there is
light shining on it, and high resistance when no light is shining on it.
117. Photoresistor Basics
Build the circuit shown and turn on the switch (62). Whenever light
shines on the photoresistor (68), the heart LED (69) will be bright.
Cover the photoresistor (68) with your nger and the heart LED (69)
may get a little dimmer, but even the little current entering the Base of
the NPN transistor (50) is enough to allow enough current to ow from
the Collector to the Emitter to light the heart LED (69).
A photoresistor is made of semiconductor material that has high
resistance in the dark. But when light shines on a photoresistor,
electrons in the semiconductor material get enough energy to jump into
the conduction band. The resulting free electrons conduct electricity,
thereby lowering resistance.
118. Resistance of a Photoresistor
Replace the heart LED (69) with the star LED (70) in project #117 and
turn on the switch (62). Whenever light shines on the photoresistor
(68), the star LED (70) will be bright. Cover the photoresistor (68) with
your nger and the star LED (70) may get a little dimmer, but even the
little current entering the Base of the NPN transistor (50) is enough to
allow enough current to ow from the Collector to the Emitter to light
the star LED (70).
The resistance of a photoresistor (68) can vary by device, but typically a
photoresistor (68) can have several MW (pronounced meg-ohms which
is 1 million Ohms) when in the dark and can have as low as 100 Ohms
of resistance or even less in bright light.
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