A common problem with projects that involve microcontrollers, sensors and servos is that when the servo runs, it alters the supply voltage to the sensors. This creates a lot of noise. If you cannot use a proper voltage regulator, you can do a few things to mitigate.
This allows some decoupling between the logic and the current-hungry parts. Buy a 3xAA battery holder and a 1xAA battery holder.
Adding one battery holder allows using this with rechargeable batteries.