Thanks everyone for the answers. It is a really small item, an airplane
model propeller. Basically I just wanted to make a simple rotation speed
measurement device and thought I just can put something together with my
existing components I have at home at the moment -- and also to learn
something from it.

So this prop rotating at 16k - 18k depending the size and sometimes the goal
is to keep the speed low enough to not overspeed the outer part of the
blades of the sound as it is not good to the performance and for my ear :-).
But mostly to set a proper speed that is healthy and worthy. There are quite
a few commercial products, however, wanted to build one by myself for my
entertainment basically and thought it could be something unique.

And no, there is no beam, just the measurements of the daylight (or normal
room lights) when the prop color differs from the background and where the
human eye could not even see it! (It is sometimes very dangerous when you
forget how big is your prop and move your fingers too close to it). The size
of the blade is few centimeters (1-2-3 inches) and if you have 2 blades you
can expect a 32k - 36k changes a minute which is just around 600Hz so not
really fast if you compare it with the PICs speed. And not need to measure
when the blade exactly passed the sensor, however, if there is a delay is
should be constant of course :-)

Also thought that later on I could play with other LED based sensors like a
a LED display that is brighter on sunlight (switch on for a while, then off
and put the pin to input, measure... etc.. or maybe I could use the
currently unused segments for that purpose). But this one is only because of
fun :-)

Tamas


On 11/22/06, Jinx <joecolquitt@clear.net.nz> wrote:
>
> Pity the schematic text in this pdf is so blurry
>
> http://www.edn.com/contents/images/92602di.pdf
>
> Quite a few others found with Google - LED "light detector"
>
> eg
>
> http://laser.physics.sunysb.edu/~tanya/report2/<http://laser.physics.sunysb.edu/%7Etanya/report2/>
>
> This one mentions half-duplex data transmission
>
> http://www.fiber-optics.info/articles/detector.htm
>
> and this one talks of response times of a few ns
>
> http://link.aip.org/link/?RSINAK/69/3751/1
>
> --
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