Wiring 24v NPN Proximity Switch to 5v BOB - Why does'nt it work

[MikeyC38]

Hi Guys

I am using NPN Proximity switches from here and a 5v BOB from Zapp Automation . I fitted a 2.4k ohm resistor between the black signal wire and the blue (-ve line) and connected up to a 24v dc power supply. When the switch is open, the voltage between the signal wire and the blue line is 4.5v ok I think for the input to the BOB. When the switch is closed, it drops to 0.7v - measured using a DMM and the LED on the back lights up. I connect the black wire to pin 10 of the BOB and the blue wire to the GND. When I activate the switch, the LED does'nt light and nothing is detected via the diagnostic screen of MACH3. And yes the port & pins is set for the Home to pin 10.
The 24v power supply buzzes and the indicator LED on it goes out. What have I done wrong?

Thanks
Mike