[sdiy] Optical Pickup

[Dave Manley]

Matthew Smith wrote:
> Hi Folks
>
> I have this thing:
>
> <http://www.flickr.com/photos/msmiffy/sets/72157609086659400/>
>
> ...which I plan to use as a tone source.
>
It looks like the disk has a low to high frequency modulation (the 
change in the ripple length).  The text on the disk: "312.5 - 3300 c/s 
at 1500 RPM" implies the output will be modulated from 312.5Hz to 3300Hz 
and back with a modulation frequency of 25Hz (1500 rev/min * 1min/60sec 
= 25 rev/sec).  I think this is some sort of 'alert' tone generator - 
perhaps when the phone has been off-hook for too long.  The min/max 
freqs match the band pass of a typical phone system.

-Dave