Variobot varikabi Manual page 97

The Sensors
varikabi's phototransistors (FT) are similar in design to a transistor. The
collector (C) is on the plus side and the emitter (E) is on the minus side.
Instead of a basic connection, however, they have a light-sensitive area.
The incidence of light determines the permeability between the collector and
the emitter.
The circuit diagram shows that the three phototransistors FT
all connected. They say they're in series. This series connection results in a so-
called voltage divider, which divides the voltage of the 9 V battery depending
on the illumination of the sensors.
Examples:
With exactly the same amount of light, the voltages at the sensors are
•
three volts each, regardless of the brightness: U
If, for example, the middle sensor FT
•
than FT and FT , a voltage four times lower would drop at FT
2 1
supply voltage would be divided as follows: U
Note:
The two variable voltages between the three sensors control the speed of
the two motors.
= U = U
2 3
were illuminated four times brighter
3
= 4 V, U
2 3
, FT and FT are
1 3 2
= 3 V
1
and the
3
= 1 V, U = 4 V
1
FT U
2 2
FT U
3 3
FT U
1 1