To open the cam - turn the lens as much as you can CCW, it then comes out, then a little bit back.

Remove the screw on the bottom.

You may now put the two parts of the housing apart and remove the switch assembly. 

Try to unhook the clamp you can see to the left - after this hurdle the cam is open.

Usual warning:

Electronic devices are delicate and may be damaged by electrical charges. Use appropriate equipment as soon as you want to open the housing - and you are on your own, no warranty left whatsoever (I assume)