I did some minor research into this last night actually. The easiest (althought not the best) solution is to use an IR LED (as bright as possible) to shine through the finger and on the opposite side have an LDR (light dependent resistor) to measure how much of the infrared that passes through. The density of the blood in the finger changes with the heartbeats, so this is what you're supposed to pick up. I've seen some pages describing both the theory + how to build one, but I don't have the bookmarks here at the moment. Let me know if you need them and I'll post them for you. Also, let me know if you manage to build something that works nicely 