Common Anode/Cathode 7-segment displays
There are two types of LED 7-segment displays: common
cathode (CC) and common anode (CA). The difference between the two
displays is 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. Shown below is a common anode seven segment.
As shown above all the anode segments are connected together. When working
with a CA seven segment display, power must be applied externally to the
the anode connection that is common to all the segments. Then by applying
a ground to a particular segment connection (a-g), the appropriate segment
will light up. An additional resistor must be added to the circuit to limit
the amount of current flowing thru each LED segment.
The above diagram shows the instance when power is applied to the CA
connection and segments b & c are grounded causing these two segments
to light up. A typical pinout for a seven segment common anode display
is shown below.
A common cathode seven segment is different
from a common anode segment in that the cathodes of all the LEDs are connected
together. For the use of this seven segment the common cathode connection
must be grounded and power must be applied to appropriate segment in order
to illuminate that segment.