Random Nerd Tutorials

Circuits – 7 Segment Displays


I’ve decided to write this post to support a bit of the theory behind my new project (click here to see) in which I’ve used a 7 segment 4 digit display (Common anode).

How they work:


A 7-segment display is an assembly of light emitting diode-bars (LED segments) each which can be powered individually. The display show the number 8 when all the segments are powered on, an additional 8th the ‘dot’ or DP, represents the decimal point, usually it’s on right side of the display.

Each one of the seven LEDs in the display is given a positional segment with one of its connection pins, these individually LED pins are labeled from a through to g representing each individual LED. The other LED pins are connected together and wired to form a common pin.

So when we use those displays 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 allows us to display each of the ten decimal digits 0 through to 9 on the same 7-segment display.

The are two types of 7-segment displays the common-cathode and the common-anode.

  • 1. The Common Cathode (CC): In the common cathode, all the cathode connections of the LED segments are joined COMMON_ANODE_CATHODE ogether to logic “0″ or GND. 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).


  • 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).

The Truth Table:


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