K Quartz watches with Light Emitting Diode display [LED]
Two types of quartz watches without moving parts will be described. In this paragraph:
Watches with a light emitting diode or LED display. In the next paragraph: Watches with a
liquid crystal display or LCD.
All these watches use a quartz crystal as a time-base, a binary frequency-divider, no stepping
motor and no hands but an integrated circuit (the display driver), which composes a visible
time indication on the display. In 1976, Hughes Aircraft Co. was the world's largest producer
of LED modules with 1.5 million pieces.
A light emitting diode is a semiconductor diode. The dimensions are from 0.3 to 1 mm.
The time is displayed by pushing the crown of the watch. The electrical current will start to
flow through the diode and while the electrons stream from the N-area into the P-area,
some of these electrons will 'fall' into the holes of the P-area. This process is called
recombination. The energy which is released this way, will be converted into warmth or
light, depending on the composure of the diode.
For instance a combination of 40 % gallium phosphide and 60 % gallium arsenide on a
gallium arsenide substrate will result in a bright red light. Normally the display will contain
four digits of seven segments each. Each segment consists of four or five light emitting diodes.
Because the electrical current consumption of these watches ranges from 10 mA, without
display, to 10 mA in the dark and 100 mA in full light, the two batteries need to be
replaced twice a year. This is the main reason the production stopped within a few
years after first release. From that time the Liquid Crystal Display watch (LCD) with an
energy consumption during display a few thousand times lower than that of a LED
watch, started to dominate the market for at least the following 15 years.
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