An LED or Light Emitting Diode, is a solid state optical
PN-junction diode which emits light energy in the form of “photons”
when it is forward biased by a voltage allowing current to flow
across its junction, and in Electronics we call this process
electroluminescence.
The actual colour of the visible light emitted by an LED, ranging
from blue to red to orange, is decided by the spectral wavelength of
the emitted light which itself is dependent upon the mixture of the
various impurities added to the semiconductor materials used to
produce it.
Light
Emitting Diodes have many advantages over traditional bulbs and
lamps, with the main ones being their small size, long life, various
colours, cheapness and are readily available, as well as being easy
to interface with various other electronic components and digital
circuits.
But the main advantage of light emitting diodes is that because of
their small die size, several of them can be connected together
within one small and compact package producing what is generally
called a 7-segment Display.
The 7-segment display, also written as “seven segment
display”, consists of seven LEDs (hence its name) arranged in a
rectangular fashion as shown. Each of the seven LEDs is called a
segment because when illuminated the segment forms part of a
numerical digit (both Decimal and Hex) to be displayed. An additional
8th LED is sometimes used within the same package thus allowing the
indication of a decimal point, (DP) when two or more 7-segment
displays are connected together to display numbers greater than ten.
Each one of the seven LEDs in the display is given a positional
segment with one of its connection pins being brought straight out of
the rectangular plastic package. These individually LED pins are
labelled from a through to g representing each individual LED. The
other LED pins are connected together and wired to form a common pin.
So by forward biasing the appropriate pins of the LED segments in
a particular order, some segments will be light and others will be
dark allowing the desired character pattern of the number to be
generated on the display. This then allows us to display each of the
ten decimal digits 0 through to 9 on the same 7-segment display.
The displays common pin is generally used to identify which type
of 7-segment display it is. As each LED has two connecting pins, one
called the “Anode” and the other called the “Cathode”, there
are therefore two types of LED 7-segment display called: Common
Cathode (CC) and Common Anode (CA).
The difference between the two displays, as their name suggests,
is that the common cathode has all the cathodes of the 7-segments
connected directly together and the common anode has all the anodes
of the 7-segments connected together and is illuminated as follows.
1. The Common Cathode (CC) – In the common cathode
display, all the cathode connections of the LED segments are joined
together to logic “0” or ground. The individual segments are
illuminated by application of a “HIGH”, or logic “1” signal
via a current limiting resistor to forward bias the individual Anode
terminals (a-g).
Common Cathode 7-segment Display
2. The Common Anode (CA) – In the common anode
display, all the anode connections of the LED segments are joined
together to logic “1”. The individual segments are illuminated by
applying a ground, logic “0” or “LOW” signal via a suitable
current limiting resistor to the Cathode of the particular segment
(a-g).
Common Anode 7-segment Display
In general, common anode displays are more popular as
