Yup, tons of info on DIY ATOs.
http://www.autotopoff.com/
http://www.melevsreef.com/plumbing/auto_topoff.html
Use the ol' Googletron to find a lot more. For the love god, please be careful when using line voltages when you if you want to make one. And remember: You + Aquarium/Water + Electricity = Extra Crispy.