Variobot varikabo Instructions Manual page 43

5) Phototransistors
varikabo's sensors are so-called phototransistors
(FT). Just think of them as variable resistors whose
resistance value decreases with increasing
brightness.
However, their structure is more like a transistor.
Instead of a base connection, a phototransistor has
a light-sensitive area.
The intensity of the light determines the
conductivity between collector (C) and emitter (E).
In the illustration, it can be seen that varikabo's
phototransistors FT , FT
2
nected. Such a series connection of (variable)
resistors forms a so-called voltage divider.
This divider splits the supply voltage of the 9 V
battery depending on the lighting conditions of the
respective phototransistors.
The variable voltages U
the transistors) the speeds of varikabo's
motors.
, and FT are all intercon-
3 1
and U control (via
1 2
C
Two examples:
FT
•
If all three sensors are lit up
equally, regardless of the
E
overall brightness, the
supply voltage divides
evenly:
U = U
2
FT U
2 2
•
If the middle sensor FT
up four times brighter than
FT and FT
2
times smaller would drop
across FT
FT U
voltage would divide as
3 3
follows:
U = 4 V, U
2
U , U , and U
1 2
FT U
1 1
proportional to their res-
pective lighting conditions.
= U = 3 V
3 1
is lit
3
, a voltage four
1
, and the supply
3
= 1 V, U = 4 V
3 1
are inversely
3