How do I connect multiple sensors to same power source without crosstalk?

Thread Starter

MeenAg15

Joined Jul 7, 2017
8
Morning All,

I need help. I apologize for my severe lack of knowledge. As an engineer I feel I should know how to do this, however, I'm lost. Forgive my ignorance. Here's the problem.

I have three sensors in "NPN Mode". These sensors are fiber optics amps, that trigger when the reflected light is above a certain threshold. Their output wires are shown below. Each sensor has four wires as shown. These are Keyence FS-N43N if that is helpful. In my case, I only need Output 1. Output 2 is not connected.

1669821903715.png

The measurement equipment we are using is a National Instruments 9232 (3 Channel +/- 30V) card. It's easy enough to use one of the sensors. Power supply +24VDC to power and ground. NI 9232 Channel 1 (-) to sensor wire 4 (black) and NI 9232 Channel 1(+) to Power (+). This results in an on/off signal equal to the input power. I have used a voltage divider circuit to reduce output to ~5VDC to be able to utilize our NI +/-5VDC cards, but this is not necessary for final design.

My problem is when I wire up the second sensor, using the same power supply, a trigger for either sensor results in a trigger of both sensors, by channels 1 and 2 of the NI9232. I'm assuming some sort of isolation is needed between the 2. I have tried different methods of using diodes, resistors to ground, with no luck. I have leaned towards buying an opto-isolated board to run the signal OUT through, but unsure if that would work.

What am I doing wrong? How can I wire three of these sensors, using the same power supply, without cross talk?

Some side notes:
On/Off frequency when in use will be >50kHz at times. If analog circuitry is needed, such that the output signal would be filtered, this low pass filter frequency would need to be very high.
 

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Dodgydave

Joined Jun 22, 2012
10,591
If your psu is not smoothing out the DC and giving enough current you may have problems, i would use a diode in series with each sensor and a100 to 470uF capacitor across the sensor supply to filter out these things.
 
Last edited:

dl324

Joined Mar 30, 2015
15,453
I have three sensors in "NPN Mode". These sensors are fiber optics amps, that trigger when the reflected light is above a certain threshold.
What are the NPN outputs driving? Do you have typical power supply decoupling? Have you tried putting 0.1uF ceramic caps across the power connections of each sensor and any other ICs in your circuit?
 

Thread Starter

MeenAg15

Joined Jul 7, 2017
8
Response to Dodgy Dave -- Thanks for the advice. I tried it out. Same results, when one triggers, both go to +24VDC. Ideas?1669827863282.png

Will look through other replys now
 

Thread Starter

MeenAg15

Joined Jul 7, 2017
8
I would try connecting the sensor like this:

View attachment 281837
This seems to have worked. Either sensor can trigger and it doesn't effect the signal of the other. Fantastic! Only thing is when fiber amp is triggered - signal is low, and when fiber amp is not triggered -- signal is high. This isn't an issue, just opposite of the way i normally think.

Is there a simple way to say what i was doing wrong to begin with? I really appreciate your help
 

sghioto

Joined Dec 31, 2017
3,698
Is there a simple way to say what i was doing wrong to begin with? I really appreciate your help
Not sure exactly but I normally would read the output with reference to ground or the negative side of the 24 vdc.
Did you have a load resistor connected on pin 4 to 24volts?
 
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