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LIGHT- EMITTING DIODES (LEDS) 397

even color. And like a diode, all LEDs exhibit a forward voltage drop— only it’s often much
higher than that of a standard diode. Depending on the LED (and often related to its color),
expect a voltage drop of between 1.5 and 3.5 volts. Some specialty high- brightness LEDs
have even higher drops.
And, as with a standard diode, the terminals on an LED are its anode (+) and cathode (–).
Rather than a white or colored stripe, most LEDs distinguish the two using other methods, as
shown in Figure 31- 15. Not all LEDs follow the same marking conventions, so you may need
to experiment. (Usually nothing bad happens if you connect an LED backward— that is, if you
switch the anode and the cathode— but the LED will not light.)

POWERING LEDS
LEDs are most often used in low- power DC circuits and are powered with 12 volts or less.
Always remember that the component can be ruthlessly damaged if you expose it to cur-
rents exceeding its maximum rating. So, unless the LED has a built- in resistor, you always
need to add one to limit the amount of current that flows through the LED. See the applica-
tion examples for the resistor, mentioned previously in the chapter, which demonstrate this
process.

SHAPE, SIZE, AND LIGHT OUTPUT
Light- emitting diodes come in all kinds of shapes and sizes. The most common are cylindrical
and shaped with a domed top. Popular sizes are:
• T1, or miniature: 3mm in diameter
• T1- 3/4, or standard: 5mm
• Jumbo: 10mm

While the most common LED is round, there are also square, rectangular, even triangular
LEDs. The shapes are handy for different kinds of applications— the triangles look like arrows,
so they can be used to show direction.

Multi- LED Displays
LEDs can come one in a package or as part of a larger package with other LEDs. Each indi-
vidual LED in the package can be individually lit. Three common variations on the multi- LED
theme are:

7- segment display: Has seven individual LEDs in special shapes to form a large numeral.
By controlling which LEDs are on and off, the display can show numbers 0 through 9.
Bar graph display: Typically contains 10 miniature rectangular LEDs.
Dot matrix display: Contains rows and columns of dots; any number, letter, or special
character can be created by lighting up certain dots.

LED COLORS

Most LEDs emit a single color, but others are designed to produce two or three colors. You
can control which color is shown by applying current to various terminals on the LED (Fig-
ure 31- 16).

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