Hexadecimal keypad for PCs:

Many computers and microcontrollers are programmed in hexadecimal, which represents numerical values using the numerals 0 through 9 plus the letters A through F. Entering the numerals is quite easy with the numeric keypad of a PC keyboard, but the letters are scattered over the entire alphanumeric portion of the keyboard. This article describes the construction of a keypad with 20 buttons, which includes all the necessary characters as well as several control characters.