Step #1 ... find and procure a 20-key hexadecimal keypad arranged in a matrix of 4x5 switches.

Step #2 ... find and procure 20 common cathode 7-segment LED displays and wire them into a multiplexed array with 20 digit selects and 7 (8 if you want a decimal point) segments.

Step #3 ... select a microcontroller (based on its datasheet) with which you can (a) scan and encode/interpret the keypad switches, (b) display as many as 20 digits on the LED's one segment at a time and (c) select an appropriate cathode driver for this LED array. Be sure (run the numbers!) the MCU can operate fast enough to do both the LED display task and the segment and digit multiplexing adequately.

Step #4 ... design, devise, and implement a power supply including a high-quality regulator capable of providing a raw input and holding Vcc regulation such that the operating voltage (Vcc) at the regulator output has no more than 5 mV of random noise (without the MCU and ancillary circuitry, of course). Now fine-tune that regulator circuit such that the Vcc noise with the MCU operating all the circuitry will still be as low as possible, yet the Vcc rise time when power is turned on is well under 10 milliseconds.

Step #5 ... design a code set that accomplishes the keypad scanning including switch debouncing, and displays the results on the LED's.

Step #6 ... Have some FUN doing all this!

RE