CTC prusa i3 pro B Auto bed leveling

Hi all has any one ever fitted this printer with a probe like this LJC18A3-H-Z/BX 1-10mm Capacitance Proximity Sensor Switch NPN NO DC 6-36V 300mAhttp://3dprintboard.com/images/icons/icon5.png
if you have can you please help a novice who is stuck i have fitted the sensor to the printer and i have wired it but on the signal wire i get 0 volts that can't be correct I thought i would need at least 5 volts i just don't know what i have done wrong.

So long as you have the polarity correct, then yes I agree, you should see a voltage when at the correct height. Are you using the resistor method to connect to the Z stop and have you connected it correctly?