How's this scheme *dons flame retardant armour*  It takes 2 micros (looks
like everything with me does ;) Use a permanent magnet on the vertical mount
of the shaft of the clock, and put two coils in the propeller; one on each
blade.  Use those to generate power.

Now, there are two ways of connecting the communications between the prop
micro and the stationary micro.  One could be done via AM modulation of a
UART signal, and using the power coils as a receiver, the other way, which I
like better, would be to put an LED on the base, aimed at the propeller, and
a photo-transistor on the propeller fairly near the shaft, so that it
doesn't move much.  Let the micro on the bottom do the tracking of the prop
position by putting a little shiny bit on the back of the prop, and have the
static micro transmit a packet (or more, depending on possible transfer
speeds vs. rotation speed) via the LEDs once per rotation then use the
interrupt generated by the UART receive to trigger the display sequence.

Rough guesstimate on feasability: Assuming that the propeller rotates at
say... 10 revs/s (I think that's enough to give a visible refresh), that's
600 RPM, then, say the photo-transistor is in receive range for 1/4 of the
cycle, that gives it 1/10/4 = 0.025s to get a burst off.  At 115200bps, on
10-bit frame (start, 8-data, stop), a single byte transmission takes
10/115200 = 87us, which should give way more than enough time to send all
the data required for all the letters you can display on your clock, and
since the timing is so small, the position error should be minimal.

And the static micro can do all the timekeeping, etc, leaving the prop micro
to do fancier letters/numbers/graphics

Now, using that, you can connect the static micro to a PC, and display
whatever you want on the propeller, not just the time ;)

On a related note,
I had a thought on how to drive a light wand to oscillate at high enough
freqency to display something (anything?) and wanted to hear if anyone
thought it would work:  put a farily light weight magnet on the bottom of
the wand, then put a coil connected to the mains--the way a cheap aquarium
air pump is--which would make the wand go back and forth across the display
range 120x per second.  This should produce a fairly nice, solid disply, I
think.  Higher refresh rate than my monitor, too ;o)

--Brendan

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