Elenco Snap Circuits SOUND &reg - Manual (Page 44)

Photo Brightness
Adjuster
Project 56
Amplified Photo
Brightness
Adjuster
Project 57
Amplified Big
Brightness Adjuster
Project 58
Vary the brightness
of the color LED
(D8) by varying the
amount of light
shining on the
photoresistor (RP).
Vary the brightness of the color LED
(D8) by varying the amount of light
shining on the photoresistor (RP). Notice
that you have to cover the photoresistor
to make the color LED dim.
Some materials, such as Cadmium Sulfide, change their
resistance when light shines on them. Electronic parts made
with these light-sensitive materials are called photoresistors.
Their resistance decreases as the light becomes brighter.
The resistance of your Snap Circuits ® photoresistor changes
from nearly infinite in total darkness to about 1k W when bright
light shines directly on it. Note that a black plastic case
partially shields the Cadmium Sulfide part.
Photoresistors are used in applications such as streetlamps,
which come on as it gets dark due to night or a severe storm.
In the preceding circuit the photoresistor
directly controlled the current through the
color LED. In this circuit the current
through the photoresistor is amplified by
the NPN transistor (Q2), so the light on
the photoresistor must get very dark
before the color LED brightness is
reduced.
Vary the brightness of the
color LED (D8) using the
500k
W adjustable resistor
(RV3). The brightness
won’t change a lot; you
may need to view it in a
dark room to notice the
difference. Placing the egg
attachment on the color
LED may help to notice the
brightness difference.
Compare this circuit to project 55 (Big
Brightness Adjuster). In project 55 the
color LED was dark for most of RV3’s
range. In this circuit the NPN transistor
(Q2) amplifies the current through
RV3, so the color LED is bright for
most of RV3’s range.
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