: What's a water computer doing?

Turning the water of and off.

Sounds simple..

I'd like to water my greenhouse using my rainwater barrel. But for that I
picture I need this setup:

A water pump connected to the barrel
A water computer connected to the pump
An automatic water system connected to the water computer.

But how does the pump and water computer work together? The pump just wanna
go all the time? or maybe I can get them combined.