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The light sensor
14
There is a yellow LED behind
door number 14. You could
install it in your circuit instead
of the red or green LED and
try out another colour. But it
can also perform a completely
different task. In this experiment,
the yellow LED is used as a light
sensor. When illuminated, it
delivers a very small current,
similar to a solar cell. This is then
amplified by two transistors and
causes the other two LEDs to
shine. Do not install the capacitor
first. Test the experiment using
a flashlight. The stronger you
illuminate the yellow LED, the
brighter the other two LEDs will
shine.
In addition, a capacitor is then
inserted into this circuit, which
serves to greatly slow down the
switching on and off. Only when
you have illuminated your photo
diode long enough, the red and
green LEDs will light up. After
switching off, they then continue
to glow for a long time and
only go out slowly. In addition,
the push button switch is also
installed. You can turn on the
light quickly and let it go out
slowly over half an hour.
Photodiode
Each diode consists of a semiconductor with
a PN barrier layer, which conducts the current
in one direction and does not let any current
through in the other, i.e. blocks the current.
In addition to light emitting diodes, there are
also rectifier diodes and photodiodes made
of silicon - the same material your transistors
are made of. A photodiode uses a particularly
large area so that a lot of light from outside
can penetrate into the barrier layer. There,
the light ensures that an electrical voltage is
generated and that the current can flow. An
LED has a similar structure, but only a very
small area. Therefore, the light-dependent
current is rather weak. However, after a large
amplification by the two transistors, it will be
sufficient for this experiment.
The red or green LED can also work as a
photodiode. Replace the LED in your circuit
and make sure you insert it in the right
direction. This way, you can explore which
LED is the best photo diode.
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