I used the nocturnals on both my crossbow bolts and my regular arrows. (different ones but same brand) Its really nice watching your projectile hit its mark and knowing that you have a short tracking job ahead of you. And helps a great deal in locating it in low light.On the off chance you don't hit where you want these things will show just where you hit and how long you should wait. I won't hunt without them.
MRBB - that is a cool idea. I'm sure it can be done. My hearing aids beep when the battery gets low.