Here is where the electronic age gets tricky. If your device (photocell, timer, motion sensor, etc.) does not have a neutral wire, it is doing one of the following:
-letting a minute amount of current flow through your load (light) all the time; if this is the case, having more than one such device in the circuit will cause problems; or,
-the device uses the ground path as a neutral. If this is the case, the device will not work properly if the ground path is poor (or non-existent, as with k&t wiring).