We were installing a photoelectric sensor for our application, but, unfortunately we bought a PNP sensor and our process requirement was a NPN sensor. Is it possible to convert the output of a PNP sensor in to the output of a NPN sensor and vice-versa ?

 

A couple of external resistors should do the trick.

 

In a pinch, yes. It can be made to work but will not be
the complete equivalent of an NPN sensor.
You simply add a pull down resistor of a value that will
pull the input low when off. Then you invert the signal,
either at the sensor if it has that capability, or in your
logic. A 1200 Ohm .5 watt resistor from the output of the
sensor to the supply negative may suffice. Some PLC inputs
draw more current than others so you might have go lower.
Another way is to add a NPN transistor and a couple resistors.
this is better but it is usually inconvenient to mount raw
components in a PLC terminal block setup. A 10k resistor in
series with the base and a 1k from base to emitter with the
emitter to supply negative will generally work. The collector
is now an NPN output. a 10 cent 2N2222 will suffice.

Regards
cww

 

Some years back we used a plug supplied from the Sensopart Company. These small plugs could convert a PNP into NPN. If these plugs still exists I dont know. Try and call them.

 

Yes! Connect the sensor output to your process input, and from that point put an appropriately sized resistor to ground. Check the current and voltage requirements of your input, but a typical value is around 3K ohms.

When the sensor is on (conducting) it will load the resistor and pull the input up to near the supply voltage, presumably 24V, which will be off from the point of view of the process input. When the sensor is off (not conducting) the input is connected to ground through the resistor and will be on.

Not that this also inverts the sensor true/false state so you'll have to adjust your logic to compensate, or perhaps the sensor has both NO and NC outputs and you can use the opposite of the one your process needs.

Or just fix your process so you aren't using ground as an input true level.