"Hacking" the remote control
Below is how the remote control for this camera (Nikon D700/D800) works. I have no idea how common this design is, so you'll have to open yours to see how it works. There are three metal plates which can be bended by pressing the button.
Situation I: The control is at rest and none of the metal plates make contact with each other.
Situation II: The button is pressed down halfway and the first two contacts touch each other; the camera measures the exposure and focuses.
Situation III: The button is pressed down completely and all three plates make contact with each other; the camera takes the picture.
Situation IV: This is what my "hacked" remote control looks like. I soldered wires to all three plates and the connections will be activated electronically by using a relay (see below).
The sensor
This system is based on a photoresistor together with a resistor connected in serial. When light hits the photoresistor, its decreasing resistance changes the voltage drop over the photoresistor, and this voltage is compared by a comparator to a second voltage which is regulated with a trimpot. The varying voltage from the trimpot is used to compensate for the change in light intensity of the background of the scene. The output from the comparator is then used to activate a double relay, which in turn activates the camera. The LED on this system is activated simultaneously with the camera, making it possible to adjust the trimpot with the camera turned off.
The breadboard with the electronics
[ dark || light ] ⇛ The electronics and the hacked remote control in a plastic box. With only background light falling on the sensor, the trimpot is adjusted so that the LED is just off. Now, when more light hits the sensor (simulated in this case by shining with a light on the sensor), the system is activated and the LED lights up.
The set-up mounted on a camera. I added a conical reflector around the sensor to increase the sensitivity of the system by 2-3 times.
The set-up mounted on a camera