What's the Missing Component?

Above, you see before you a circuit for an infrared diode, except it's missing one vital component. Can you figure out what it is? Thanks to Forrest Mims and the people at Jameco electronics for this puzzle, which appears on the back cover of the latest issue of Make Magazine, which you can pick up at the CHMS Library. It's a great magazine for those of us who love to tinker, even if we don't know what we're doing. Check it out. And if you think you know the missing component, comment below! (Of course, Mrs. Anderson is pretty sure I'm missing a few components myself, but that's another matter)