I am trying to connect PIR motion sensor to a nano. The motion sensor is part of an existing alarm system which uses a 6 volt signal for logical 0 (no motion detected) and a 12 volt signal for logical 1 (Motion detected).

So the question is how can I shift these voltage levels down to something that I can connected to the Arduino Nano. I have tried a voltage divider but couldn't get a wide enough range to get reliable detection of high or low signals. I have also tried a logic level shiftier. The problem with these is that they still require the input signal to go to zero to pull down the output signal.

Any advise or suggestions would be most welcome.