Water Light Grafitti - Nice LED art work

[mariush]

http://www.tecca.com/news/2012/08/22/water-light-graffiti-video

Video : 

The "Water Light Graffiti" is a surface made of thousands of LED illuminated by the contact of water. You can use a paintbrush, a water atomizer, your fingers or anything damp to sketch a brightness message or just to draw. Water Light Graffiti is a wall for ephemeral messages in the urban space without deterioration. A wall to communicate and share magically in the city.
More pictures and details on Digitalarti : bit.ly/O2v7aw

[T4P]

I would think a shitload of precision moisture sensors? Or capacitive sensors 

[cwalex]

There should be one in every urinal! 

[Psi]

Maybe just adjacent pcb tracks/pads.
Subtract out the resistance from the thin film of residual water that sticks on the pcb.
Any lower resistance than that = water detected.

[Astroplio]

From what I understand here:

http://youtu.be/Ch-gYF1v3ls

They use no special sensors, water comes in contact with PCB, bridges the traces around the LED so as the total resistance changes so does the LED luminosity. When water evaporates LED goes off again.

[mikeselectricstuff]

Cool but won't last long due to electrolytic corrosion. Running on AC with back-to-back LEDs may be slightly more durable.