waking up arduino mini with a external signal
Arduino Mini has only two interrupt capable pins: 2 and 3. You have to move your sensor to other pin.

From the AtMega328p datasheet:


  If a level triggered interrupt is used for wake-up from Power-down,
  the required level must be held long enough for the MCU to complete
  the wake-up to trigger the level interrupt. If the level disappears
  before the end of the Start-up Time, the MCU will still wake up, but
  no interrupt will be generated. The start-up time is defined by the
  SUT and CKSEL Fuses.