On Tue, Dec 20, 2005 at 09:45:50AM -0500, M. Adam Davis wrote:
> I suppose there's a reason you're not simply using the hour hand from
> a strong clock module?
> 
> http://www.klockit.com/products/dept-157__sku-AAAAG.html
> 
> The nice thing is that these are coil actuated, so if the timing isn't
> right, rip out the circuit, and pulse the coil yourself.  Put it on
> the hour hand.  The second hand is moved 1/60 rev each pulse, the
> minute hand goes 1/3600 rev per pulse, and hour hand 1/216,000 rev per
> pulse.
> 
> The advantage above a micro-stepping stepper motor is that you don't
> need current between pulses, and in fact you could run the whole
> assembly off a AA cell for the year.  Not to mention much higher
> resolution and accuracy than the microstepping stepper.  Not that it
> matters much since you're running the laser continuously...

Yeah, this will be wired to mains current. Though I do plan to add a
supercap to power the PIC only for short power outages. I've done that
once before for another clock powered off of a doorbell circuit and it's
been tolerating about 10 power outages a day from people ringing the
doorbell for months now with no problem. I just made it possible to have
it detect when it's running off of the capacitor by isolating the PIC's
Vcc from the rest of the circuit with a diode, then added a connection
to the normal power to an IO line, the second the PIC detects that IO
line go low it drastically reduces it's oscillator speed to go into
low-power mode.

As for why you'd power a clock from a doorbell circuit? Well, it's an
old house, and I wasn't allowed to touch anything more than a small
segment of the drywall, so no running power lines anywhere. Easiest just
to tap off of the doorbell as I was installing the device by the front
door!

Website here if anyone's interested: http://petertodd.ca/art/meter_clock.php

> Perhaps there are other constraints that this doesn't meet, but the
> high-torque clock in the link above may be worth considering.  If it
> doesn't have a spot for a second hand, it is likely that the gears
> still provide 1/216,000 rev per pulse to the hour hand so the circuit
> is the same for all the clock modules.

Hmm... That's actually a really interesting idea. One problem I see is
that I need some method of reversing the mechanism when at the end of
the 12 hour period, the laser needs to track back to the starting
position. Still that could work rather well for some other clock designs
I have... I've got metalworking equipment sure, but building gear trains
takes lots of work!

-- 
pete@petertodd.ca http://www.petertodd.ca
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