canabiath:
Thanks for replies. I try to explain better.
I would like to open an old digital camera and conect the arduino to the push button for make photos. I have some old digital cameras for use it in boxes at home.
I read in some proyects that need an 5v relay for use between camera and arduino, but im not sure why, i bought it because was cheaper for take advantage of the shopping cart and is difficult to find here and expensive buy only it later.
You want an opto-coupler or relay between the camera and Arduino to isolate the Arduino from the camera and vice versa. I tend to think an electronic switch like an opto-coupler is better than a mechanical relay when it comes to small voltages that would be in the camera button. A relay can probably handle higher voltages than an opto-coupler.
An opto-coupler logically has 4 pins. The ground and signal pins that you connect to the Arduino, connect to a LED inside of the opto-coupler. The pins you connect to the camera shutter connect to a light sensor tuned to the frequency of the LED, and when they see light will complete a circuit.
The relays that I've seen are mechanical devices, and when current is applied will use a magnet to move a mechanical switch that makes an electrical connection between a ground and power terminals, and when the power is off, the switch move to make a connection between the ground and other power terminal.
The push button of most still cameras have 3 wires, ground, focus, shoot. When the button is pressed 1/2 way, it connects the ground and focus wires, which when the circuit is completed, the camera does whatever action it does for 1/2 press (usually focus the shot). When the button is connected all of the way, it connects the ground and shoot wires to take the shot. Some cameras (like my Olympus DSLR cameras) need both circuits connected before they will shoot, other cameras only need the ground and shoot wires connected.
The more expensive cameras often times have an option for a wired shutter release, that plugs into the camera with a special cable. Typically you would use a shutter release made by the manufacturer or by a clone company to trip the shutter, but you can also have an Arduino control it. Other cameras have an infrared shutter release, and the newer ones have shutter releases that connect via bluetooth or wifi. Finally, some cameras have what is called tethered support, where you can send commands to the camera from a computer, and control all of the settings.
Terry King, who post often on these forums, has a lot of tutorials (and runs a store selling Arduino related pieces). Here is his tutorial on opto-isolators and relays: http://arduino-info.wikispaces.com/RelayIsolation
If you don't want to mod the camera, and it doesn't have the connections for an external shutter release, you can rig up a servo to fire the shutter button:
canabiath:
The project will remain in the back yard so autonomous working, and I only visit it on weekends for replace the memory card form camera
Unfortunately, you will need to think about how to power the camera. Most camera batteries last less than a day, when the camera is turned on all of the time. You can probably diddle with the settings and have the camera sleep between shots, and then you need to have the Arduino send a long enough signal on the focus wire to wake up the camera before taking a shot. However, whether the camera has power to last a whole week or so, I don't know.
A site that is dedicated to such camera hacking to capture wildlife in the field is: http://www.diytrailcams.com/. A more general camera hacking site is: http://www.diyphotography.net/.
I have a setup that I have used occasionally with my steampunk camera to fire off the camera via a telegraph key and it used opto-isolators, but I can't find the old post that described it. Unfortunately I need to finish up the current build (non-electrical, all wood working) for this weekend's event. But if I find it, I will try to post it.
