Once upon a time, I urgently needed to fire some shots remotely with my new omg shiny Fujifilm X-E1. The beast has a 2.5mm audio input for microphone. The same input can be used also as a connection for a cable shutter trigger.

The ingredients:
2.5mm audio jack
a cable (preferably 3-wired)
a trigger (preferably 2-stepped, like on a camera)

I was lucky to have found a 2.5mm->3.5mm audio jack adapter. Upon having it disassembled, I plugged it into the camera's 2.5mm port and started connecting individual contacts together, watching how the camera would behave. Because the input has 3 lines, there are actually only 3 possibilities of interconnecting any 2 of them.

Having examined all 3 of them, I found what already had been found:
Focus is quite self-explanatory. Shutter means focus AND immediately shoot. This was enough for me - I didn't have to search for a complicated 2-step trigger. Later I found myself difficult to find even a simple one. I didn't want to solve it just by connecting raw wires (like in my article about calibrating the lens) so I pushed my searching abilities and fantasy to the max and I found something.

Btw, the wires in the trigger and in the jack are properly soldered by a soldering gun that I will never sell again. ;)