Milk Jug Auto Top off??

Ok, all you DIyers, heres what i need to do:

I'd like to make an auto top off system for a ten gallon tank without spending more than 15-20 dollars.

I was thinking, why not use a milk jug or other similar 1 gallon (10%, so it should be enough) container and hook it up to some kind of a sensor.

to save money..i'd kind like it to be gravity fed (think gravel vaccuuming) but controlled so i dont soak my floor.

so umm, do you know of anyone who has done something like this?

and where can i find some good system parts like the sensor (if needed) ?

thank you all, DIYers,
Black_Wolf

thanks for the responses, but this is a small ten galln tank, i dont think there would be room for the floating valve,

if you know of the valves most commonly seen in SW projects, that might suit the tank better.

Thanks.