To J. Guy:

I won't learn anything building the eraser myself... it's just for the money.. I paid for the lamp and a fluorescent lanter about US$22, for a "real" eraser it would cost US$100 or more. It's all for a college project that will surpass it's budget if I don't take care... and since my pocket is financing it I can't afford it happens :-)

To all others:
Thanks for all the info and sugestions! the shorter time was just for curiosity. during the development I'm using RAMs. The eprom is just for the presentations day, since using it I won't need a computer to load the program to my project. By the way... I won't get a catharat in any of my eyes :-) The "eraser" will work inside a box (opaque, of course) so no harm will be done. Blindness doesn't pay an extra credit :-)