A 7 segment display uses 7 LEDs to display decimal numbers from 0 to 9. It has either a common anode or common cathode configuration. A 7 segment decoder takes a 4-bit binary coded decimal input and outputs a 7-bit code that illuminates the correct LED segments to display that decimal number. For example, the input 0000 would output 1111110 to display the number 0. The decoder operates based on a truth table to map each input to the proper 7-bit segment output.