This action is translated to the actual pressing of the button by turning a transistor on and off, which is in turn connected via our camera wires to the switches on the real camera.
The grounds of the camera and the Arduino are linked to give the Arduino and external camera circuits a common ground, allowing the transistors to work properly.
You may have to experiment with which wire goes to the collector and which goes to the emitter in each pair; which is which will depend on the internal wiring of the specific camera you're hacking.
Finally, **and this is important**, take your ground wire from the camera and plug it into the ground pin on the Arduino.
This is necessary for your transistors to work and thus for your camera to respond to your arduino! I just completed my instructable for a motion-sensing camera and alarm that I made from my electronics class.
Following your instructions, I hacked my own keychain digital camera.
First, a bit of explanation: to control the camera fully, our code takes two digital output pins and map one to the on switch, and another to the shutter switch.
When the code wants to turn the camera on or off or switch modes, it briefly holds the camera on output pin high - and when the code wants to take pictures, it holds the camera shutter output pin high.