While showing date and time is already an achievement, I wanted to make the display of my converted desk calculator look more dynamic so that it would drag the attention of bypassers. Showing hh:mm:ss is as easy as typing in the 6-digit number, press '+' '1' and keep on hitting the '+' key once a second.
While I could make it work interacting with
the real keyboard, my first attempt at it with the emulated keypresses
failed. The reason is that without a real keyboard connected some
"control" settings are missing, especially the "print/don't print" line
which defaults to "print every confirmation keypress" (like arithmetical
operations, total, ...). This causes the printer go crazy, which delays
the sum operation and ruins the effect (besides being annoyingly
So, next step will be reconnecting
the keypad in parallel with my adapter board, adding a power switch to
exclude my added circuitry and write a simple firmware to prove +1 can
be done every second.
Since the "don't
print" setting still activates the printer on fewer conditions
(basically when a total is displayed), all uC-controlled interaction will
probably have to rely on arithmetical operations: "clear screen" will
be "multiply by 0".
Fast forward to Keyboard re-connected. I cannot switch off my added circuitry because the analog multiplex influences key lines and it behaves like a key is always pressed. So, everything must stay powered (at least the adapter board), and the switch must be handled by the Arduino firmware that has to know when it is allowed to take control of the calculator.
Next: I have to write a simple firmware to map the "T" key ("totalizer one"), so that the screen can be wiped without activating the printer (a double press on CE/CA clears "buffers" and prints something).