3-51. OPTOELECTRONIC devices either produce light or use light in their operation.
The first of these, the LIGHT EMITTING DIODE, was developed to replace the fragile,
short-life incandescent light bulbs used to indicate on/off conditions on panels. A LED is a
diode which, when forward biased, produces visible light. The light may be red, green, or
amber, depending upon the material used to make the diode.
3-52. Figure 3-27 shows an LED and its schematic symbol. The LED is designated by a
standard diode symbol with two arrows pointing away from the cathode. The arrows
indicate light leaving the diode. The circuit symbols for all optoelectronic devices have
arrows pointing either towards them, if they use light, or away from them, if they produce
light. The LED operating voltage is small (about 1.6 volts forward bias and generally about
10 milliamperes). The life expectancy of the LED is very long, over 100,000 hours of
Figure 3-27. Light Emitting Diode
3-53. LEDs are widely used as "power on" indicators of current and as displays for
similar devices, LEDs are typically placed together in seven-segment displays. Figure
3-28, view (A) shows the display with the seven LED segments labeled A through G.
Figure 3-28, view (B) shows a schematic for a common-anode display. All anodes in a
display are internally connected. The segments can be lit in different combinations to form
any number from "0" through "9." When a negative voltage is applied to the proper
segments, a number is formed. For example, if negative voltage is applied to all segments
except that of LED "E," the number "9" is produced (see Figure 3-29, view (A)). If the
negative voltage is changed and applied to all segments except LED "B," the number "9"
changes to "6" (see Figure 3-29, view (B)).
3-54. Seven-segment displays are also available in common-cathode form, in which all
cathodes are at the same potential. When replacing LED displays, you must ensure the
replacement display is of the same type as the faulty display. Since both types look alike,
you should always check the manufacturer's number.
23 June 2005