? for the electrically inclined

I need to make a battery power supply switch as a project. Essentially I need to have two seperate batteries, a primary and a backup. Power will be drawn from the primary until it gets low at which time power will be automatically switched over to the secondary, allowing me to change the primary battery but still have power supplied to the system. Anyone know how to go about doing this or if there is a commercially available item out there that will do this for me. Power will be supplied from 9V batteries if that helps at all. I talked to someone about it and they suggested a complex system of Schmitt triggers and flip-flops but I know nothing about these.

 

Listen to your friend. Just those two words is enough for me to assume he knows what he is talking about. (yes, I know what they are). There is not really an easy way to do it, and I know of no commercially available products.