You're talking about the interlock loop, right? (Pins 4 and 17)
You need to put those two pins in series with your interlock circuit. That way if the ILDA cable is unplugged from the projector, the circuit will be broken, which trips the interlock and shuts down the projector. (Remember that pins 4 and 17 are connected together internally on the QM-2000 board.)
Have a look at this drawing for an example of a basic interlock circuit.
Adam

