Any modern battery powered device has a automatic power off function. It is easy to forget switch device off. And so it slowly drains the batteries leaving you with blank screen next time you decide to use it. If you are designing a battery driven project, think about adding auto power off. If this is a microcontroller based circuit it is easy to do by adding a transistor switch. But if you are looking for more universal approach, then check out following small circuit.
It is based on Attiny85 which takes control of power lines through couple of transistors (NPN and PNP). Once device is powered timer starts counting which after countdown cuts the power. Prototype is based on pin through components, but with SMT parts it could be squeezes in to really small piece. Transistors could also be replaced with MOSFETS for better efficiency.
We recommend EasyEDA for circuit design and PCB prototype