>It should be the small device containing the small keypad (eg. 16 keys),

The other trick to get up to is when the user comes to the keypad, they have to
activate it, e.g. by a swipe card, or maybe the first keypress. Up to this point
the keypad has no legends for the keys. The operator initiation then causes the
micro to display the key legends in random order, so anybody watching cannot
determine what the number sequence is, because the next time it is activated the
legends will be different. It would require a little extra code in the micro to
match random legends to the key actually pressed to get the correct number
entered, but it would stop "pattern matching" (top left, centre ,mid right,
bottom left) by an observer, as the operator has to enter the code corresponding
to the random legends.