I have to work on a circuit that basically goes like this. I have two voltage sources, a 9V battery and AC. The AC is the main source, however, when this is unplugged, the 9V battery must come to the rescue after 5 seconds. For this work, I truly have no idea of where to start with, so it would be great if your could guide me. I'm not asking for anyone to solve my problem, but if you could help me with a starting point that would be very welcome.