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I am totally new in this field. I want to use a 5 V / TTL signal from a National Instrument to switch on/off a Red lion PXU series temperature controller. But this controller required a dry contact input or a prox sensor with NPN open collector. What does it mean?

How will the circuit look like?

Thank you in advance!

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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.

schematic

simulate this circuit – 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.

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