http://df.magmawiki.com/index.php/40d:Repeater Page is for 40d, but the principle hasn't changed in v31.
You just link a pressure plate inside the repeater to your spike trap. Another (not as reliable) possibilty is to have a 2 tile room with two pressure plates, both linked to the spikes, and drop a kitten in there. It will randomly walk over the plates and trigger the spikes.
Sadly, there's no way to build a repeater with some mechanical knowledge and potential for Fun. As I said, link the spikes to a lever and let a dwarf pull the lever, if you don't want to build something complicated.