So i'm making this model of a house, and the basic idea is that its meant to be secure, one of the features i wanted to add was the automatic back-up power system in case someone cuts the main power supply from the outside, for that i could only think of a NOT gate, but i have no idea how to use one or how to even make one, i want the power to run through the not gate and when the power is cut off, it automatically turns on the backup current from another 9V battery which will then power the same house appliances and other security systems that i've put in the house, i also wanted to use the NOT gate in the door lock system so that no one can enter the house through the door

 

Same as your other thread, don't suggest solutions. A NOT gate is not a viable solution.

Lay out the problem first.

You have a toy house powered by AC mains to DC power supply.
You have back up power from a 9V battery.
Your solution for automatic switch over on AC power failure consists of two rectifier diodes.

Draw a circuit schematic of what you have so far showing your power, voltages, and loads.

Let's talk about the door lock later in another thread.