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

[cropwell]

For those who mentioned the possible 10K resistor in the switch - the BOB appears to have a current-limiting resistor in series with the input to the buffer inverter which also includes clamp diodes to restrict input voltage at the input pin. So the 10K resistor, if it does exist, is just going to act as another pull-up when the switch is open and can be ignored when the switch is closed. If it doesn't exist, it doesn't matter as the BOB has an onboard pull-up anyway.

Don't know the zener voltage (zen, zener, zenest), but here's the essentials of the internal circuit.

Now I am going to get ANOTHER Brandy !!!!