The best way to set this up depends largely on what kind of camera you have. So I am going to cover a variety of methods that you can use with different cameras.
Step 1: Connecting to a Camera with a Remote Shutter Switch Terminal
For example, a Canon EOS Rebel T3i uses a 2.5mm stereo plug (3/32" can also work). As shown in the picture, the sleeve is connected to ground. The center ring activates the focus function. The tip activates the shutter. To activate the focus and the shuttle, you just need to connect that pin to ground (the sleeve). You can do this with either a mechanical button or an electrical circuit.
Step 2: Connecting to a Camera without a Remote Shutter Switch Terminal
First remove all the screws from your camera and remove the outer housing. Keep in mind that there may be screws hidden behind stickers.
As you are opening up the camera, be careful to avoid the camera flash capacitor. If you aren't familiar with what a capacitor looks like, it is a large cylinder with two wires coming out of it. This capacitor can hold a charge of several hundred volts even after the batteries have been removed. Do not touch it!
Locate the camera's shutter switch. This will typically have one terminal that connects to ground to activate the focus and one terminal that connects to ground to activate the shutter. Most cameras will auto focus when you activate the shutter. If this is the case, you only need to make a connection to the shutter terminal. Solder one wire to the shutter terminal and one wire to the ground terminal. Touch the two wires together. If all the connections are made properly, this should activate the camera's shutter.
Now you can activate the shutter with an external circuit. All you need to do is connect the ground wire from the camera to the ground of the circuit. Then have your circuit connect the shutter terminal to the common ground. The easiest way to do this is with a simple momentary switch or a transistor. With some circuits you can activate the shutter by outputting a LOW signal to the camera's shutter terminal.
Here are two other instructables that do a great job of illustrating this process:
Step 3: Make an External Button Actuator
One method is to use a servo to press the button. The servo is mounted immediately next to the shutter button. When the servo turns, the arm (sometimes called the horn) presses the button.
It is also possible to use a regular motor. Just mount it above the shutter switch. Then attach a bracket to the shaft that is capable of pressing the button. You can use something as simple as an offset circle. This method works best for taking pictures at regular intervals such as with a time lapse setup.
Here are a couple of other instructables that do a good job of illustrating these methods.
Step 4: Activate the Camera Shutter with an External Switch
Step 5: Activate the Camera Shutter with a Microcontroller
Insert the cathode of the diode (the side with the stripe) into one of the digital pins of the microcontroller. Then insert the jumper wire into one of the GND pins on the microcontroller. Now you need to connect these to the shutter switch cable. The jumper wire should connect to the ground of the camera. The diode should connect to the shutter function on the camera.
When the microcontroller sends a LOW signal to the digital pin, the camera shutter should activate. But when a HIGH signal is sent to the digital pin, the diode becomes reverse biased and prevents the electricity from flowing into the camera. This protects the circuitry of the camera from being potentially damaged.
Using the microcontroler, you can easily incorporate your camera into any kind of automated system.
Step 6: Activate the Camera Shutter with a 555 timer
This circuit is a little different from regular 555 timer circuits. Pin 3 is used to charge and discharge the capacitor and pin 7 is the output. I chose this layout because during the discharge cycle pin 7 is shorted to ground through an internal transistor. This is the ideal way to activate the shutter of the camera.
You can power the circuit with anything from 4.5V to 18V. The timing is controlled by the resistor and the capacitor according to the formula:
Time interval (secs) = 1.386 * R * C
I used a 680µF capacitor and a 100k variable resistor. This let me adjust the picture interval from every few seconds to every 94 seconds.
To connect it to your camera, connect the ground line to the ground on the camera. Then connect pin 7 to the shutter function.