Electronics 2000

I need to build a small setup which will operate a battery powered device (2 x AA batteries) on a battery powered timer. The timer needs to provide current for a few 20 minute intervals in an 8 hour period. Nothing can be mains powered or greater than 9v batteries. I considered using a product called Tethercell which is a bluetooth battery holder that can be remotely controlled from an iphone with timer software. However it's proving very hard to get hold of this item, so I thought I'd look into building my own solution. I have very little electrical expertise other than being able to solder
If anyone is able to offer any advice on what components I could use and how to construct this I'd be very grateful.

If the 20 min periods are regular within the 8 hours a simple CMOS 555 device should be easy and reliable. Search Google for "CMOS 555 circuits" - you will find a huge number and there should be many that will work for you. If need be, you could have two timers - the second being triggered by the first, which would allow you independently to adjust time between on periods, and the length of the on period.
Hope this helps

It helps a lot. Thank you very much.