Datasheet

Light-Sensitive Navigation with Phototransistors • Chapter 6
Robotics with the BOE Shield-Bot • 189
Introducing the Capacitor
A capacitor is a device that stores charge, and it is a fundamental building block of many
circuits. Batteries are also devices that store charge, and for these activities it will be
convenient to think of capacitors as tiny batteries that can be charged, discharged, and
recharged.
How much charge a capacitor can store is measured in farads (F). A farad is a very large
value that’s not practical for use with these BOE Shield-Bot circuits. The capacitors in your
kit store fractions of millionths of farads. A millionth of a farad is called a microfarad, and it
is abbreviated μF. This one stores one tenth of one millionth of a farad: 0.1 μF.
Common Capacitance Measurements
microfarads: (millionths of a farad), abbreviated μF 1 μF = 1×10
-6
F
nanofarads: (billionths of a farad), abbreviated nF 1 nF = 1×10
-9
F
picofarads: (trillionths of a farad), abbreviated pF 1 pF = 1×10
-12
F
The 104 on the 0.1 μF capacitor’s case is a measurement in picofarads (pF). In this labeling
system, 104 is the number 10 with four zeros added, so the capacitor is 100,000 pF, or 0.1 μF.
(100,000) × (1 × 10
-12
) F = (100 × 10
3
) × (1 × 10
-12
) F
= 100 × 10
-9
F = 0.1 × 10
-6
F
= 0.1 μF.
Building the Photosensitive Eyes
These circuits can respond independently to the light level reaching each phototransistor.
They will be pointing upward at about 45°, one forward-left and the other forward-right.
This way, a sketch monitoring the values of both phototransistors can determine which side
of the BOE Shield-Bot sees brighter light. Then, this information can be used for navigation
decisions.