prox sensor with NPN open collector output or a dry contact
It means you need to pull down the input to instrument ground with a transistor or switch. 

Read the specifications to determine what voltage it pulls up to, or just add an NPN transistor and base resistor (eg. 2N4401/4.7K) to your logic circuit. Grounds must be common. 







simulate this circuit &ndash; Schematic created using CircuitLab

You can probably use a transistor or MOSFET-output optoisolator rather than a transistor if you need isolation, but you may need a driver for the optoisolator LED.