How about drawing a thick pencil line around the perimeter, with
one dot in one corner, two dots in next corner, three dots in third
corner, and ticks along the sides for a grid.  use a whisker that
detects resitance of pencil line.  locate wall, feel around till you find
a corner, count dots to identify corner, and go back to where you
were when you got lost, and pick up what you were doing.

alice

> Ahaaaa!  LEDS doesn't work.
>
> If the board can be made by some translucid material as for example
> white acrylic, and if you have enough ambient light, then you can instal
> one cheap LDR below each object position.  Connect the LDR's in a matrix
> to form 16 x 32 or two matrix 16 x 16.  You can produce it in such way
> that only an object would be sensed, not shadows or things like that.
> You probably would need two regular 40W fluorescent lamps above the
> board to have enough light.
>
> Your board is 4 x 3 ft, with 16 x 32 cells, so probably the 32 side
> would be at the 4 ft side, leaving cells of 1.5" x 2.25".  This row and
> column width is small to use LED matrix, since the 1.5" width row is 36
> inches long, you would need to use focused lenses LEDs and Receivers,
> since the angle opening is less than 2.4¡.
>
> Another problem for LED matrix, is when you have multiple objects, you
> can not identify where they are. See the example, LEDs emitting left to
> right, top to bottom, sensors at right and at bottom:
>
>
> Columns   A    B    C    D
>           |    |    |    |
> Row 1 --------------------- * Light
>           |    |    |    |
> Row 2 --------B/2           Dark
>           |         |    |
> Row 3 -------------C/3      Dark
>           |              |
> Row 4 --------------------- * Light
>           |              |
>           *              *
>           ON  OFF  OFF   ON
>          Light          Light
>
> When you do it with keys, it is different, since keys "deviate, switch"
> electric current, than a scanning system works..
>
> At the matrix above you can "see" where the objects are, at B2 and C3,
> but if the objects were at C2 and B3, the situation would be the same at
> the sensors... right?  or wrong?
>
> How can you tell where are the *only 2* objects at the matrix below,
> based on the light/dark sensors, C3 and D4 or C4 and D3? (you can use
> scanning, turn on one LED at time, etc). Of course there is an object in
> Column C and another in column D, intersection with rows 3 and 4, but
> where?:
>
> Columns   A    B    C    D
>           |    |
> Row 1->-------------------------* Light
>           |    |
> Row 2->-------------------------* Light
>           |    |
> Row 3               ??   ??      Dark
>           |    |
> Row 4               ??   ??      Dark
>           |    |
>           v    v
>        Light Light Dark Dark
>
> The LDR solution (or even photodiodes below the board) would work, since
> they would switch electric signals in the matrix.
>
> Wagner