I have not used Nikon so the below should be checked.
The Nikon D750 uses the MC-DC2 connector and looks to work by bringing the focus and shutter lines to ground the same as the Canon I have.
Since the MC-DC2 connector is unique to Nikon it would probably be better to buy a cheap shutter release cable and repurpose it. This will also let you check how it works to confirm it is basic contact switches.
I have built the drop controller to the exact plan from Martyn and use it with a Nikon d5300. You do not use the dropcontroller to focus it would be impossible. The drop would be long gone before the camera had even started to focus. One must manually focus on the point where the drop is actually falling (use a pen pencil or something similar) then set your lens and camera body to manual focus and let the Dropcontroller do the triggering works perfectly with my D5300