Want To shrink down your arduino project, you can do so by using any one of these integrated circuits these can be programmed to perform any function which an arduino performs, ATtiny 45/85 can be used in those projects in which no of pins required is less, if your project includes much more pins then you could use ATtiny2131.
Benefit Of using ATtiny45/85/2131:
1.Makes the project small and portable.
2.Very low cost as compared to arduino uno and it is cheaper than arduino nano.
Digi spark has made a complete functional board with these integrated circuits check it below:
Buy These Board In Less Than 1 $ From Here:
For further information chech the description after clicking above link.