This is an old post, preserved for reference.
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I am working on a more complicated version of the
'Simple LED Clock' in my previous post, however this is currently delayed as CPC has messed up my order again and I'm still waiting for some of the parts I ordered. So the prototype is currently using 2 MCP23008's to drive one digit each:
Whilst I'm waiting, here's a look inside the original 30 year old clock.
Inside there's a lot of space where the radio used to live.
It's a three board construction
With one for the top buttons, one the logic board with a single IC and finally the display
The chip is a TMS3450NL, and the date code (32nd week of 1983) seems to fit with the 30 year old estimate.
I have managed to find a scanned datasheet for the chip:
After all this time of putting up with a 12 hour clock, I find that not only did the display have sufficient segments to display 1 and 2 (but not 8), the chip supported 24 hour mode, it just needed a pin shorting to ground.
On to part 3 - more on display multiplexing.
