Introduction: CO2 Drop Checker

Another piece of equipment in planted tank is drop checker. Drop checker will tell you about carbon dioxide level in the water, which is essential information if you've got live stock. The right optimal level which is good for plants and still acceptable by fish is 30 ppm (parts per milion) (actually the level might be slightly different from 20-40 or even 45 but 30 is fair enough). The drop checker is basically pH level indicator.
How its working? Between indicator liquid (which is dkH 4 - water plus few drops of pH indicator) and water in tank is trapped air bubble (see the second picture). The colour of the indicator solution is changing when CO2 is evaporating from tank water and dissolving into solution water, making it more acidic.
So initialy the color of the solution is blue (not enough CO2) and is changing to green when is ~30ppm, changing to green-yelowish if the level is too high.
The shape of drop checker might be different, but idea is always the same.

Step 1: Parts and Assembling

For my drop checker I've used bits and pieces lying around at home which was:
small black cup (don't remember exactly where it came from, shampoo or something?)
small vial (my is made from plastic)
small sucker (unnecessary from electronic thermometer , black will be better)
piece of neoprene (my was from mouse pad)
some glue or double sided tape
scisors, scalpel, etc

First of all I've adjusted the black cup height. Then lenght of the transparent vial has been adjusted to slightly longer than black cup, to make the indocator inside visible. (see schematic cross section)
Next stage was stick the neoprene blocks from both sides of the transparent vial. The blocks was used to keep vial in place inside the black cup. (still has to be a gap so the air inside has got contact with indicator and water inside tank)
Than hole for sucker has been drilled, and that basically was end of the story.

Step 2: Indicator Solution

You can actually get already prepared indicator solution from local aquatic shop, ebay etc.. alternatively you can get 4dkH water from shop, pH indicator mix together and make your own solution.
Anyway, I'm going to describe how to DIY.
At first of all you have to get deionised water. You'll need at least 3l. and pH indicator of course, also You'll need some of soda bicarbonate (should be somewhere in the kitchen ;-) if not get it from you local food store).
Also you have to borrow or ask someone for a favour and measure exactly 1.2g of soda.
Now if you have 1.2g soda bicarbonate let's dissolve it in exactly 2l of deionised water, that will give you 2l water exactly 40dkH.
Now measure 90ml of deionised water and mix with 10ml 40dkH water, that mix will give 100ml 4dkH water which is the goal.
You need actually couple of drops ~1 ml, so store in some glass jar rest of the 4dkH water for the future.
Ok put some of that water inside transparent bottom section of the drop checker and put 2-3 drops of pH indicator. The water should be blue because pH is exactly 7. Put everything together and place in the tank about two inches below water level.

Step 3: Testing

Finally you have to check how it's working. Obviously the drop checker got quite big delay in showing result. After placing drop checker its about 2h delay after the solution will become green from blue (of course if you got right CO2 level).
Small hint at the end. If its hard to read colour of the solution inside the vial put piece of white plastic inside.
Good luck.