One light amplifier
16
Open door number 16 and take out a
new component. At first glance, it looks
like an LED in a clear housing, but in
fact it is a light sensor; more precisely,
a photo transistor. Install it along with
a resistor and an LED. Make sure you
install it the right way around. Unlike
what you know about an LED, the long
wire has to be connected to the negative
terminal, because that is the emitter. The
red LED lights up brighter when more
light falls on the photo transistor. The
red LED goes out in complete darkness.
Like a normal transistor, the
phototransistor has an emitter (long
wire) and a collector (short wire). There
is also a base, but it has no connection.
The base current is supplied by a
built-in photodiode. If you look at the
transparent case from the front, you
will see a relatively large black area.
That’s the light-sensitive photodiode.
It is significantly larger than the area
of an LED crystal. Therefore, the
phototransistor is much more sensitive
than the LED in your light sensor from
experiment 14.
