> "what kind of device" that would close a switch on my shutter release.
I don't know what brand of camera you're using.
I have a Canon EOS 450D, to take the picture you just short the two pins on the remote control connector, which is a standard Jack.
Use Manual focus to get rid of forus lag...
If you have another kind of camera, you might have to do otherwise, (like buy an IR remote from ebay and hack that).
Simpler solution and easier to get working for an electronics noob : find a cheap IR sensor ding-dong, hack. Note you need active IR (light barrier) not passive IR (like patio lights) because the passive sensor need something big (like a robber) and has no precision, whereas you can put the light barrier on your focus plane for instance.
To connect to the camera, you could use an optocoupler. Isolation isn't strictly needed, but when connecting a $1000 camera to a cheap hack, you never know !
If you want to DIY, google "light barrier schematics". Most will probably use IR remote control receiver chips which have good tolerance to ambient light, because they use a pulsed signal.
You could do even simpler : a laser pointer, and a photo transistor at the other end. But since this doesn't use pulsed light, you will need to put the detector inside a black tube to shield it, and lasers will use a lot more battery than a dumb IR LED.