Shimano 105 ST-5500 gear sensor mechanism I have a pair of Shimano 105 levers and would like to know what sort of mechanism is inside the shifter to track what gear it's on?



Is it a potentiometer? Or maybe small switches? 

Also, does anyone know the pin-out of the 4-pin cable that connects the shifter to the Shimano's computer?
Here's a screenshot of part of the instructions (Shimano pdf) that appear to match your levers. 



You're right, there is a 4 pin cable in there, but I have no idea where you'd find out what the pins do. I strongly suspect it's a classic +power, +signal, -signal, -power layout but whether it's analogue or digital I don't know (it might just be a potentiometer). You would need to take it apart yourself and have a look, sorry.