BCD to 7-Segment decoder
To display any digit on 7-segment display the digit needs to be converted from BCD to 7-segment code. This is also must because any single digit BCD can be represented in 4-bits, but to display any single digit on "7-segment display" we need 7 bits. These 7 bits respectively glows 7 LEDs on the display, which together looks like a digit.
For example to display 3 on the display, LEDs a,b,c,d and g should glow and others should be off. Thus the seven bits code will be "1111001". Hence we need some kind of decoder which will convert BCD to appropriate 7 bit code that can be directly fed to the 7-segment-display.
A typical arrangement is shown below....
Truth table ..
One can very well design this decoder from scratch, but there are already some BCD-to-seven segment decoder ICs (IC 7447) available, and which can reduce lot of efforts. Note that, IC 7447 gives active zero output, so please refer datasheet for using it.