24v prox switch wired into pxi-6221

[Deana]

I have a 3 wire (brown, black, blue) 24V magnetic proximity sensor (similar to MAFM1-A0-1H) which I need to wire into a DI on a PXI-6221 card.

I realize that the input card can only handle 5V, so I have used a bread board to create a voltage divider circuit.

I wired the 24V power supply across R1 (39k Ohms) and R2 (10k Ohms), in series. Then I connect the brown and blue wires across the +24V and -24V, respectively.

Then I connected the black wire to the junction between the R1 & R2, in essence, the input for the DI is across R2. This, in theory, should give me 5V across R2, and I should be able to connect the PXI-6221 card slot 52 and 50 across the black and blue wires, respectively.

When I attach an oscilloscope across R2 (the black and blue wires), and constantly turn the prox sensor on and off, I see the voltage drop across R2 goes from 0-15V, instead of 0-5V.

What am I doing wrong?

Thanks!
Deana

[Deana]

Is there a minimum current for the input for a PXI-6221. Maybe that is my problem?

[aputman]

I am by no means an expert with this so take my comments with a grain of salt.  

I think you want the output of the prox switch (black wire?) to be wired in series to the resistors.  Brown and blue are typically power wires with black being the output.  It is the output of the switch that you want to divide.  and you may also need a pull-up resistor, depending on the operation of your switch (NPN vs PNP). 

Again....grain of salt.