This kind of 4" digit?
I'd recommend the Common Anode parts, drive from 12V, connect the cathode segments to TPIC6B595 shift registers to drive them.
This board here can drive 12 digits, use 2 boards and wire up one TPIC6B595 for the 25th digit.
Probably want to wire the anodes together seperately and not source from the board tho. Connect grounds together.
I sell bare boards for $5 each mailed to US locations, all thru hole, easy to assemble. International, a little more for postage.
Complete kit, or assembled tested boards, I'd have to price up.
The actual counting down part is easy.
set up 25 bytes with 9 in each.
once a time period, test if the lowest digit is not 0 and decrement it.
If it is 0, set it to 9 and decrement the next digit.
if the next one is not 0, decrement it.
if it is 0, set it 9 and decrement the next digit.
Will have one big long chain checking if a digit is to rollover or not.
Longest chain will from 1000000000000000000000000 to 0999999999999999999999999
And you can add leading 0 blanking too.
