I saw in Argentina an instrument that worked this way: It's made out a
vertical line of lights, that flashes at different frequencies, and
when looking at it and noding your head from right to left and back,
you can see shapes. The one that I saw was blue and made the shape of
a camel.
Where can I find this item, how is it called, how does it work, can
build one of my own, how?

Hi royr1-ga,

These items are sometimes called "Message Wands", but it seems that
each manufacturer uses a different name for their product.

These devices have a row of light emitting diodes that are repeatedly
switched on and off in a pattern, so quickly that our eyes can't see
the individual flashes, but if we move our head (or move the wand) we
will see bright spots in the correct places to form an image in our
mind.

The result is surprisingly effective because of our persistence of
vision: we see the image of a light for a time after the light has
been switched off (or moved to a different part of the overall image),
and our mind combines all the different flashes of light to perceive a
coherent image.

You could make one of these yourself using a digital logic circuit to
drive a row of light emitting diodes. Here are instructions to build a
motor-driven rotating message wand (although you could just leave out
the motor and move your head instead):

"3D Scanning LED Display"
http://www.twyman.org.uk/3D-Display/index.htm

The above page also has a simple animation that shows the principle of operation.

It would probably be cheaper to buy a mass-produced message wand than
to build your own. I found several that can be ordered from Think
Geek:

1. The Wavy Wand ($30) can display messages and pre-loaded images, or
you can connect it to a computer with a USB cable and download your
own images. You can view the images by waving the wand, or by keeping
the wand still and moving your head or eyes.
http://www.thinkgeek.com/gadgets/lights/80ee/

2. The Light Talk 2 ($40) works like the Wavy Wand but includes a
scanner so that you can easily load your own designs into the wand.
http://www.thinkgeek.com/gadgets/lights/81ff/

3. The Spacewriter ($25) also works like the Wavy Wand but is a much
more compact message wand:
http://www.thinkgeek.com/gadgets/lights/81ac/

4. The XP3 ($50) is a message wand that forms part of a clock, and can
display either the time or a message.
http://www.thinkgeek.com/homeoffice/lights/59ac/

I trust this provides the information that you are seeking.

Regards,
eiffel-ga

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Request for Answer Clarification by royr1-ga on 07 May 2006 14:57 PDT

Hello,

Thank you for your answer.

Just one thing: I was looking for one of these that is big, and used
for advertising or "gimik" outside pubs and clubs. It should be at
least 1/2 meter long, and not the small ones with the size of a hand.
Didn't find this kind of item in any of the posted links.

Thank you.

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Clarification of Answer by eiffel-ga on 08 May 2006 06:56 PDT

Hi royr1-ga,

The SignLine "kinetic space imager" is 560mm high and is made for
premises advertising:
http://www.spacewriter.com/SignLine.aspx?menuproduct=4

However, it costs about $500.

Regards,
eiffel-ga

Thank you, pretty good answer. I am not sure this is the excact item I
looked for, but its close enough.