The Canon 450D supports external shutter release mechanisms using a simple 2.5mm TRS stereo plug - closing Tip or Ring to Sleeve will cause the camera to either take a picture or autofocus:
I created a quick circuit to test out the shutter (I've left out autofocus for now) and take a picture every few seconds using my Arduino, with the idea that I'd put together a nice sunrise timelapse. More on this later. The circuit itself is pretty simple, but the tough part for me was splicing my cable in a way that I could connect it to the breadboard.
In practice my setup is actually a little more complex as I wanted to use my SSD1306 display to show a little indicator of how many photos I'd taken without having to fiddle with the camera - note the very sketchy wiring :-)
Timelapse #1 - overcast/snow
Timelapse #2 - sun/sunset
The second timelapse is a little more interesting. I used the same basic timing and setup but used a lower ISO since the evening pictures ended up a little darker and blurrier anyway, and this time pointed directly towards the sunset (SSW/220°). The main thing I learned from this video is that my window is filthy
Again I used ffmpeg to produce an h264 video:
Code for the Arduino sketch is available HERE but really it's just writing a 1 to a GPIO every few seconds so not particularly tough.