Light-emitting Diodes (LEDs)
LEDs light up when current is applied.
LEDs are composed of gallium arsenide phosphide semiconductor diodes that glow
red when current is applied. In watches and clocks these diodes, arranged in
segments and electrified in the appropriate sequence by the watch's
integrated circuit, light up to display the time.
LED technology was developed by researchers in the semi-conductor industry.
These red digits we see on electronic watches and pocket calculators
from the 1970s resulted from the work of independent groups of
researchers exploring the properties of semiconductors in the
1950s and 1960s. The first LEDs were infrared (invisible) and unsuitable for displays.
In 1962 Nick Holonyak Jr.,
while at General Electric, demonstrated the first visible-light laser, a
semiconductor he and colleague S. F. Bevacqua made of gallium arsenide
phosphide that glowed red when driven with electric current.
LED displays were available in 1964, but they were handmade and very
expensive--about $10 per digit. These early light-emitting diodes became the
basis of an important industry that complemented the integrated circuits
in consumer microelectronics. Their intense brightness, long life, and
miniature size made them attractive for use in digital watch displays. A popular novelty when they were first introduced in 1972, LED watches fell out of favor with consumers by the end of the decade.
Time Displays ~