There is numerous information about the GoPro cameras available elsewhere so it won't be repeated here. The Hero 2 version is used since it meets all the requirements for recording at normal speed (30 fps) so upgrading seemed unnecessary.
The method used to turn on the camera is simple and straightforward. At the time the Hero 2 was new, wireless methods to control this camera was not available. However, a simple interface circuit is all that is needed to turn on and off the camera.
As you can see from the schematic, the GoPro Trigger Circuit is very straight forward. The cameras ON/OFF button has contacts at the connector on the back of the camera. By connecting these contacts to a solid state relay, only a small signal is need to turn the camera on and off. Since it is a solid state relay, a current limiting resistor is needed. In addition, a full wave bridge rectifier is added to make the circuit polarity independent.
With this design, only a very small current is required to turn the camera on and off. Typically this is only 2 mA. The camera must be set up to start recording automatically (see owners manual). The connector that goes into the back of the camera can be difficult to locate but there are a number of sources to purchase them from.
Other interesting bits of information about controlling the GoPro cameras, the time required to turn them on is typically only 1/2 of a second, perhaps less. However, to turn the camera off, the circuit must be triggered for around 3 to 4 seconds. It is possible to turn the camera on with the same 3 to 4 second signal if necessary. Finally, for the GoPro Hero 3 cameras, this circuit turns the camera on but will not turn the camera off.