Electronics Basics - Arduino Sketches: Tools and Techniques for Programming Wizardry - page 52

Table 3-1 ( continued )

COLOR DIGIT 1 DIGIT 2 MULTIPLIER TOLERANCE

x 10 7

Violet 7 7

Gray 8 8 x 10 8

White 9 9 x 10 9

Gold ± 5%

Silver ± 10%

The i rst two bands indicate the value in ohms, the third band is a multiplier
for scaling, and the fourth indicates how far the actual value may deviate from
the stated value. A resistor with red, violet, orange, and gold stripes has a value
of 2.7 KΩ; 2, 7, 10 3 , and 10-percent tolerance. A 100 Ω resistor is brown, black,
brown, and silver; 1, 0, 10 1 , 10 percent.

NOTE Color-blind people might be starting to worry here; don't. Whatever your
color vision problems, I can assure you, you will be able to identify resistor values. I
have acute achromatopsia, meaning that I see more or less in black and white. All col-
ors are di cult for me to see. This was a problem during my studies, where teachers
didn't know how to react, but today, this is never a problem for me. A simple ohmme-
ter or multimeter can quickly tell you the value of a resistor.

Using Resistors

The current and voltage can be regulated in an electronic circuit by resistors.
Imagine an electronic circuit powered by a 5 V DC power supply. You want
to add a Light Emitting Diode (LED) to show that the circuit is powered, in
this case a red LED. This LED has a voltage drop of 1.7 V. A voltage drop means
that the voltage of the circuit will be reduced by that amount. Therefore, if you
were to place an LED directly between the +5 V and 0 V it would be damaged
(remember, all voltage gets used up in a circuit). There must be a component to
reduce the voltage across the LED, a resistor is the ideal candidate. The schematic
of this circuit is shown in Figure 3-6.
Because you want 1.7 V across the LED, and because the circuit is powered
by 5 volts, that means there should be a voltage drop of 3.3 volts across the
resistor. Also, the LED is rated for 20 milliamps of current, but for this project
15 milliamps should be enough. Therefore, to have 15 milliamps l ow through
the LED, you will have to use a 220 Ω resistor. Another example is shown at the
end of this chapter.