Matt Mets has written a nice Nikon remote control library for Arduino. I used it to make a motion trigger and intervalometer. It looks like this:
Here’s a Fritzing image of the circuit (I finished it in Illustrator, because there’s no PIR part in Fritzing yet, and I have trouble with the part editor.)
And here’s the schematic:
And finally, the code:
[include file="../../code/Arduino/intervalometer_0001/intervalometer_0001.pde" clean="true"]