A 7-segment display is an 8-shaped component which packages 7 LEDs. Each LED is called a segment – when energized, one segment forms part of a numeral (both decimal and hexadecimal) 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 of the LEDs in the display is given a positional segment with one of its connection pins led out from the rectangular plastic package. These LED pins are labeled from “a” through to “g” representing each individual LED. The other LED pins are connected together forming a common pin. So by forward biasing the appropriate pins of the LED segments in a particular order, some segments will brighten and others stay dim, thus showing the corresponding character on the display.
The common pin of the display generally tells its type. There are two types of pin connection: a pin of connected cathodes and one of connected anodes, indicating Common Cathode (CC) and Common Anode (CA). As the name suggests, a CC display has all the cathodes of the 7 LEDs connected when a CA display has all the anodes of the 7 segments connected.
Common Cathode 7-Segment Display
In a common cathode display, the cathodes of all the LED segments are connected to the logic “0” or ground. Then an individual segment (a-imgg) is energized by a “HIGH”, or logic “1” signal via a current limiting resistor to forward bias the anode of the segment.
Common Anode 7-Segment Display
In a common anode display, the anodes of all the LED segments are connected to the logic “1”. Then an individual segment (a-g) is energized by a ground, logic “0” or “LOW” signal via a current limiting resistor to the cathode of the segment.