On 19 Mar 2005 13:16:49 -0800, (Sean Hall) put
finger to keyboard and composed:

Hello all,
My son has recently asked me to sort out his "Leappad" electronic
reading aid. Its a bit of kit which looks like a large green and blue
hard back book. You open it and place a printed paper book inside it.
As you open the pages it allows you to place a stylus on the printed
book and it will read words, sentences or phrases out of the book.

Anyway to the point. The stylus started erratic operation and I found
that by holding the stylus in a particular place the stylus worked OK.
So I deduced a fractured wire. The stylus can be removed and unplugged
from the main unit. I had to force open the stylus to replace the thin
coaxial cable connecting it to the main unit. All that was at the end
of the stylus was a 3-4mm length of the coax with a small brass
"nipple" on the end. Nothing fancy at all.

Needless to say it does not recognise what part of the page you are on
or even that you have placed the stylus on the paper. Under the book
is a large plastic membrane with like an XY matrix on it.

Anyone got any ideas how it works. I suspect the matrix is scanned
quickly and wherever the stylus rests tells the unit where the stylus
is. One idea......

I've seen several methods used in digitizing tablets. One method
involves pulsing the H&V wires and detecting these pulses via a coil
in the stylus or puck. The firmware in the tablet then determines the
precise location of the stylus by assessing the signal strengths from
adjacent wires and performing interpolations. Another method (used by
Summagraphics) involves sending pulses down H&V magnetic delay lines
and timing their arrival at points in the H&V wire matrix. Still a
third method (used by Kurta) pulses the stylus and detects the induced
signal in the matrix. The latter has problems with pencil drawings and
such like. All three methods use a micro-switch in the stylus to
indicate to the tablet when data are valid.


- Franc Zabkar
--
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