Infrared Detector

Hello Everyone 
I'm making a coil gun and not sure how to make a infrared detector, so when my projectile is going down the barrel an infreared sensor will detect the projectile and activate the next coil.
Thanks Everyone 

Why a detector?

The speeds you need to fire accuratly would need a high speed processor so the timing is not messed up.

Also when waiting for a signal to activate the next coil you will loose too much time, especially to the end of the barrel.

Have you considered a design like the nono-rail-trains?

You don't really ned to fire the coils one after the other, a contious field like in the train should be easier to realize.

In that case you can use the feedback that is generated by firing the first coil to activate the second.

E.g.: First coil activated and there will be a certain voltage induced into the second coil.

This voltage will raise while the projectile travels, when it enters the second coil it should maximize - this signal is used to turn off the first coil and fire the second.

Same store for the next coil packs.

Only thing you need is the right electronics and a fast processor.