Single drum trigger question

I found a website that tells you how to make a homemade electronic drum trigger using a practice pad and a piezoelectric transducer from Radioshack. The whole thing would cost less than 20 dollars to make, and since that's pretty much my whole budget for drum gear at the moment, I wouldn't have enough money to buy a drum brain. Would I have to have a drum brain to run my trigger or could I hook it up to a computer to run it through GarageBand on my Mac which is what I wanted to do so it could trigger percussion or a hand clap? If the GarageBand plan wouldn't work, is there anything I can do for cheap to trigger sounds?