[HH] DIY Christmas Tree Water Sensor Tells You When to Water

[Tom Metro]

http://lifehacker.com/5866012/hack-a-water-sensor-into-your-christmas-tree-to-tell-you-when-to-water
  ...blogger Eric Ayars created a reminder system with an Arduino.
  Using a small sensor, an Arduino Pro Mini, a relay, transformer, and
  an outlet box, he created a system that can detect the amount of water
  in the tree's tray and blink a morse code message when it's in need
  of water.

Full plans here;
http://hacks.ayars.org/2011/12/christmas-tree-water-level-alert.html

I guess you plug your tree light into it, and then it flashes them in a
pattern to let you know the water state.

I used to use a water alarm[1] taped to a length of PEX as a way to tell
when the tree reservoir was full when filling the tree, and for spot
checking it.

1.
http://www.homedepot.com/h_d1/N-5yc1v/R-100038838/h_d2/ProductDisplay?storeId=10051&catalogId=10053

I've since replaced it with some cheap gizmo that has a water sensor
built-in to a long funnel that you permanently install, obscured among
the branches of the tree. It talks when the tree is in need of water.

 -Tom