Bob Blick wrote:

> On 5 Sep 2005 at 0:10, Stef Mientki wrote:
> 
>>We want (well better saying my son wants) to build a propellor clock
>>and make a projection of the actual star map,
>>or in other words, "build a desktop planetarium".
>>
>>Although I like the idea,
>>I've the feeling that are quiet a lot large problems to overcome.
>>The first problem I see (or better said I don't expect to see anything ;-)
>>is how to get such a small spot like "a star at the sky" projected ?
> 
> 
> Hi Stef,
> 
> You will use lots of LEDs. Also, have two strips of them so you have 
> an interlaced display. You can put the two strips 180 degrees apart 
> or next to each other as long as the timing is right.

Or 4 strips, at 90 degrees, to get better resolution yet.
Use the smallest LED's you can get with adequate brightness.
White or yellow to look more like stars. RGB LEDS would be cool but
much more costly.

Use more than one pulse per revolution in your
speed sensor to reduce jitter (assuming perfect construction
symmetry). Be sure to filter the trigger pulses to remove
mechanical jitter jumpyness in the display. Software PLL works
well for this, hence the desire for multiple pulses per rev.

If you are building a 'planetarium' type display, you
probably want a hemispherical arc instead of a straight one.
Radio Shaft (shack) sells a propeller style clock which
is hemispherical and would give you an idea of how fuzzy
T1 LEDs are. Around $40.

How big a dome were you thinking of building?

Robert

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