What you need:
1 x PNP transistor (bc547)
2 x 2K2 Ohm resistance
4 x hole female pins
Step 1: The Schematic
Attach your camera cable to Camera +/-
Attach your controller circuit output digital to Control +/-
When Control goes HIGH, Camera pins will be closed, with only a voltage drop of 0.2V, so it will take a photo.
When Control goes LOW, the resistance will be pulling-down to 0V and Camera pins will be open.
This time i've connected both grounds, because the application requires it. Both must be 0V in order to work. If they are not the same, you can have different voltages on each ground and strange things can happen.
Note: This circuit can be greatly improved, but i wanted it to be very cheap/easy/small, and the matter is it just works with my camera (canon 350D).
Those cameras usualy have 2 buttons, 1 for focussing and 1 for taking the shot. I short-circuited both in my cable.
Step 2: That's All
I hope you liked it.