I recommend that you watch the measured Irrigation video:
DIY Universal Measured Irrigation Controller Kit
 
The Kit includes everything that you need except for the evaporator. The components in the kit are as follows:
Valve assembly
Cylindrical float
6 float rings
Outlet pipe (15mm BSP)
2 round plastic nuts (15mm BSP)
Inlet adaptor 15mm x 20mm
Adjustable control dripper assembly
 
Key features
Completely automatic
No electricity is needed (no batteries, no solar panels, no computers, and no electronics)
Smart irrigation controller – the irrigation is controlled by the prevailing weather conditions rather than a program
Use for gravity feed or pressurised irrigation (water supply pressure at least 10 kPa)
Use for sprinkler or drip irrigation
Use for pressure compensating drippers or non pressure compensating drippers
You can adjust the water usage by adjusting the control dripper
You can adjust the irrigation frequency by adjusting the float
The water usage is directly proportional to the net evaporation rate (this is a unique feature of measured irrigation)
Responds appropriately to an unexpected heat wave
When it rains, water enters the evaporator and delays the start of the next irrigation
The water usage is independent of the water supply pressure (this is a unique feature of measured irrigation)
Uses much less water without affecting the yield
Simple and low tech and so easy to assemble and fewer things to go wrong
You can leave your garden unattended for weeks on end