One common requirement for many different digital devices is a visual numeric display. Individual LEDs can of course display the binary states of a set of latches or flip-flops. However, we're far more used to thinking and dealing with decimal numbers. To this end, we want a display of some kind that can clearly represent decimal numbers without any requirement of translating binary to decimal or any other format. 
  We've all seen seven-segment displays in a wide range of applications. Clocks, watches, digital instruments, and many household appliances already have such displays. In this experiment, we'll look at what they are and how they can display any of the ten decimal digits 0-9 on demand.
                                    

A seven-segment display is used for displaying any one of the decimal digits 0 through 9. A BCD-to-seven-segment decoder accepts a decimal digit in BCD and generates the corresponding seven-segment code. A block diagram representation of the BCD-to-seven-segment code conversion is depicted in the following figure:        
                        

Truth table:

       To construct the truth table, note that digit 0-to-9 are same in both binary and BCD so we don't need to convert them, digit10-to-15 are unused and hence they are considered as don’t care conditions.