Datasheet

Light-Sensitive Navigation with Phototransistors • Chapter 6
Robotics with the BOE Shield-Bot • 187
measured in ohms which is abbreviated with the Greek letter omega (Ω). The current levels
you are likely to see through this circuit are in milliamps (mA). The lower-case m indicates
that it’s a measurement of thousandths of amps. Similarly, the lower-case k in kΩ indicates
that the measurement is in thousands of ohms.
Let’s use Ohm’s Law to calculate V
A3
in with the phototransistor, letting two different
amounts of current flow through the circuit:
• 1.75 mA, which might happen as a result of fairly bright light
• 0.25 mA, which would happen with less bright light
The examples below show the conditions and their solutions. When you try these
calculations, remember that milli (m) is thousandths and kilo (k) is thousands when you
substitute the numbers into Ohm’s Law.
Example 1: I = 1.75 mA and R = 2 kΩ
V
A
A
A
k mA
R I V
A
5 . 3
5 . 3
2 75 . 1
2000
1000
75 . 1
2 75 . 1
3
=
Ω =
Ω × =
Ω × =
Ω × =
× =