Data Sheet

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121. Advantages of Photoresistors
Build the circuit shown on the left and turn on the switch (62).
Whenever light shines on the photoresistor (68), the bi-directional LED
(71) will be bright red. Cover the photoresistor (68) with your nger
and the bi-directional LED (71) 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 bi-directional LED (71).
The advantages of photoresistors are that they are small in size and
thus can be carried or placed in just about anywhere easily, and they
are low cost.
122. Disadvantages of Photoresistors
Reverse the direction of the bi-directional LED (71) in project #121.
Whenever light shines on the photoresistor (68), the bi-directional LED
(71) will be bright blue. Cover the photoresistor (68) with your nger
and the bi-directional LED (71) 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 bi-directional LED (71).
The main disadvantage of photoresistors are their accuracy. The
instantaneous resistance of a photoresistor is dependent on wavelength
of the light and can vary based on temperature. So photoresistors are
typically not used to make accurate light measurements.