I was thinking about doing something like that but I'm still
pondering some details. This is what I had in mind:
I would like to use AC power in normal operation and OLEDs
controlled by PIC. OLEDs (organic LEDs) would be multiplexed
and powered by PIC directly. This is possible because PIC can drive
up to 25mA sink or source. OLEDs take much less power than
standard LEDs (ca 10x lower current) so that even if all 4 LEDs
in one group are turned at the same time that would mean
2mA * 4 = 8mA. Multiplexed LEDs should be driven with higher
current so they apear as bright as if they were powered continuously.
Also note that in one group, max number of LEDs to be on at the
same time is only 3 (when displaying number '7').
So 3mA * 3 = 9mA per group which is well under 25mA limit.
This way no additional transistors are needed for multiplexing.
Whole circuit can be one PIC with Xtal and two capacitiors,
LEDs, one diode, 6 resistors, two buttons to set time,
pair of rechargable batteries and little 3V PSU. I don't think
this can get any simpler without sacrificing some functionality.
Even with lowered current requirement, PIC would be only thing
running from (trickle charged) 2x1.5V batteries if the AC power is off.
(thats where diode and two resistors are planed to be used).
While batteries are full, LEDs would be cycled one group at the time
with low duty cycle at very low frequency (optional).
For example each group of LEDs would be illuminated only one
group at the time and only for one second (first display 10s of hour,
then 1s of hour, then 10s of minute etc.). After that all LEDs would
be off. This would repeat periodically (for example every 30 second
to save power). Once the voltage drops bellow certain setpoint,
LEDs would turn off completelly so I don't have to set time when clock
is moved from one outlet to another or in case of power outage (remember Big Blackout?).
Anyway, I would appreciate comments or ideas.