User manual

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Besides light-emitting diodes, there are rectifier diodes and
photodiodes made from silicon, the very same material that is
used in the transistors. A photodiode has a particularly large
surface to have as much light as possible enter into the bound-
ary from the outside. There, the light ensures that the isolating
effect of the boundary layer is lifted in part and current flows.
An LED is similar in structure, but its surface is very small.
Thus, the light-dependent current is also very small. However,
for this experiment it will be sufficient thanks to amplification
by the two transistors.
Another test
Both, the red and green LED, can also function as a photodi-
ode. You could check out which LED is the best photodiode.