Datasheet

Light-Sensitive Navigation with Phototransistors • Chapter 6
Robotics with the BOE Shield-Bot • 177
Introducing the Phototransistor
A transistor is like a valve that regulates the amount of electric current that passes through
two of its three terminals. The third terminal controls just how much current passes
through the other two. Depending on the type of transistor, the current flow can be
controlled by voltage, current, or in the case of the phototransistor, by light.
The drawing below shows the schematic and part drawing of the phototransistor in your
Robotics Shield Kit. The brightness of the light shining on the phototransistor’s base (B)
terminal determines how much current it will allow to pass into its collector (C) terminal,
and out through its emitter (E) terminal. Brighter light results in more current; less-bright
light results in less current.
The phototransistor looks a little bit like an LED. The two devices do have two similarities.
First, if you connect the phototransistor in the circuit backwards, it won’t work right.
Second, it also has two different length pins and a flat spot on its plastic case for identifying
its terminals. The longer of the two pins indicates the phototransistor’s collector terminal.
The shorter pin indicates the emitter, and it connects closer to a flat spot on the
phototransistor’s clear plastic case.
C
B
E
Light
C
B
E
Current
Collector
Base
Emitter
Flat spot and shorter pin indicate
the emitter (E) terminal