Self-regulating Watering System

In short: this method uses the technology of some bird water bottles (like this). Additionally, you'll also use some communicating vessels. 

The basic idea is drawn in pic #1.

To make it, you'll need:

- plastic bottle with cap
- pot plate
- tubing (about 3 to 5 mm of internal diameter)

How to do it
1. Drill a hole (about 1 cm wide) at the bottom of the bottle (pic #2). The hole must be below the rim of the plate. 
2. Fill the bottle through the hole. Then put the bottle inside the plate (pic #3).
3. Fill the tube with water and use it to connect this plate to other plates (pic#4; more on this later).

How it works
A1. Water will fall off the bottle until its level (outside) reach the top of the hole.
A2. When the level of water (outside) goes down, some air will enter the bottle through the hole, and more water will fall off. 

B. The tubing will transform the two (or more) plates in communicating vessels. The water level will be the same in all plates (relative to the ground).

Notes
1. You can connect several plates to the main plate.
2. Make sure all the plates are in adequate level. You can easily level them with chopsticks or folded newspaper under any plate, for example.
3. The pot size and soil type may interfere in the success of this method. Also, be aware of watering needs of your plants.
4. Make sure there are no air bubbles inside the tubing. Due to the small diameter of the tubing, air will probably clog it.

Advantages
1. It is really self-regulating. (Some other methods depends on climate, watering needs and dripping rate.)
2. No need for electrical devices. No powering needs.

Drawbacks
1. It only works with pot plates or something like it (i.e. good for indoor use, but almost useless outdoor).
2. May not work with large pots (if soil capillarity is not sufficient to pull water from plate to the plant roots).


(P.S.: English is not my first language. Please, correct me if there is something wrong. Also... this is my first Instructable, so... please let me know if it lacks something.)

Step 1: Self-regulating Watering System: Alternatives

Alternatives

1. You can use a large tray to put the water bottle and a lot of pots. (See pic #1 in this step.)

2. If you don't have tubings, and you can place the bottle near the plants, then you can connect the plates using some cotton string or cloth. (See pic #2.)

3. You can adjust this idea in many ways. (See pic #3.)