The Unpowered Valve Box allows you to automatically irrigate you garden using gravity feed from an elevated rainwater tank without uisng  a pump and without power.
 
Installation instructions:
Install drip line (subsurface or surface, PC or NPC, drip tube or drip tape).
Replace a two drippers with a subsurface clay pots. 
Connect the clay pots to the Unpowered Valve Box.
Connect the water supply to the inlet of the valve box (10 kPa to 800 kPa).
Connect the drip line irrigation system to the outlet of the valve box.
Connect two drip line drippers inside the valve box.
Fill the valve box with water until the float is partially covered.
Turn on the water supply and all your plants will be watered automatically without power.
 
The discharge from each dripper can be set to any value between 285 ml and 612 ml.
 
The interval between irrigation events adjusts automatically to ensure that the discharge from each dripper during an irrigation event is the same as the average on-demand discharge from a clay pot since the previous irrigation event.