The good thing about this project is that you can start easy and grow it.
For example by adding a few LEDs you can have a countdown bar after which it times out and maybe sounds a different sort of sounder.
Getting more complex you could extend this to a 7 segment display count down.
The important thing is to start simple and look at other designs and tutorials, as Tom Lehrer said plagiarise, plagiarise , plagiarise but always please to call it research.