> The scheme probably needs three photocells to be sure that the one > in the middle is darker than the others. Might be able to mask it > with a slit and use a fine wire gnomon, in a coarse/fine servo. > Could use a variable frequency motor and precision reduction, like > a phonograph turntable only much slower.
Not sure about needing three. This is what I had in mind -- given there's a stepper/servo on the sundial base, I suggest a detection method not unlike the way a 5061A stays on the cesium resonance peak. (see http://www.leapsecond.com/pages/cspeak/) Namely, you continuously sweep the base a small amount, perhaps a fraction of a degree, at a couple of Hz rate, back and forth across the minimum gnomon shadow to determine just where the "zero crossing" is. The two photodiode *differential* is all you care about. Using a sampling ADC (free inside most uC these days) you can then infer where the center is. When the virtual sweep center is off by more than one full physical step of the geared stepper, you advance the sundial base one step. I've not done the math, but I suspect you can get quite accurate this way. Not only do you have the digital step count as a function of time of day, but you also have the digital/analog interpolation of the steps; so the resolution is quite good. Also if the photodiodes are sensitive beyond visible this digital servo'ed sundial might work in cloudy weather too -- which, being here in Seattle, is one reason I came up with the idea. /tvb http://www.LeapSecond.com/time-nuts.htm _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
