The one i use is a variation on one from a reefcentral thread called "DIY auto-topoff with solenoid" or something like that. The thread has at least 100 posts. It's worked very well for me.

Here's an article about auto top offs:
http://avdil.gtri.gatech.edu/RCM/RCM...illSystem.html

Keep in mind that most auto top-offs are VERY prone to failure and there are common stories about tanks getting ruined by auto top offs gone wild. I have a solenoid, float switch, and timer; for a variety of reasons, my set up is not likely to fail open. This is more important to me than if they fail closed; if that happens, the worst thing is that your sump runs dry and burns out the return pump. If it fails open, you've got a mess on the floor and a brackish water tank!