Design a Decoder using Logic Gates to display letters on a 7-Segment Display
Start from truth table. You have 26 alphabets so 26 input combinations(respective asci values). And in the output side 7 leds. 

For example to display A (0110 0101) 



from this picture you can conclude that when input is 0110 0101 then LEDs A,B, G, F, E, C must glow and LEDs D must be OFF. 

Similarly find LED condition for all 26 combinations. 

Draw K map for each LED, by that you will get 7 Boolean equations. If you implement these you will get the circuit.  

Hope this will help you. If you need any clarification in any step let me know.