Datasheet

Chapter 6 • Light-Sensitive Navigation with Phototransistors

186 • Robotics with the BOE Shield-Bot

If you replace the 2 kΩ resistor with a 1 kΩ resistor, V

will see smaller values for the same

currents. In fact, it will take twice as much current to get V

to the same voltage level,

which means the light will have to be twice as bright to reach the 3.5 V level, the default

voltage in HaltUnderBrightLight to make the BOE Shield-Bot stop.

So, a smaller resistor in series with the phototransistor makes the circuit less sensitive to

light. If you instead replace the 2 kΩ resistor with a 10 kΩ resistor, V

will be 5 times larger

with the same current, and it’ll only take 1/5

the light to generate 1/5

the current to get

past the 3.5 V level. So, a larger resistor makes the circuit more sensitive to light.

Connected in Series: When two or more elements are connected end-to-end, they are connected

in series. The phototransistor and resistor in this circuit are connected in series.

Ohm's Law

Two properties affect the voltage at V

: current and resistance, and Ohm’s Law explains

how it works. Ohm’s Law states that the voltage (V) across a resistor is equal to the current

(I) passing through it multiplied by its resistance (R). So, if you know two of these values,

you can use the Ohm’s Law equation to calculate the third:

V = I × R

E = I x R: In some textbooks, you will see E = I × R instead. E stands for electric potential, which is

another way to say “volts.”

Voltage (V) is measured in units of volts, which are abbreviated with an upper-case V.

Current (I) is measured in amperes, or amps, which are abbreviated A. Resistance (R) is