No resistor in optocoupler collector side?

Hey everyone!
I am doing a project, where i will reset a scale, when there is no product on the scale. I will use 24v industrial light sensors as an input. The resetting of the scale is fairly simple- i just need to connect two pins on the scale board (“reset” and “ground”). I am planning to use 4N32 optocouplers for the input (from sensors) and also for the output (resetting).

I measured the voltage between reset pin and ground pin. It was 4,3V and i measured current (with a multimeter, directly from reset to gnd)- it was 5,4 mA. Which parameters I should be looking, to figure out if I need to put a resistor before the collecter. I can see from the datasheet, that max Ic= 100 mA and max collector- emitter breakdown voltage is 30V. So it seems to me, I wouldnt need to put a resistor before collector. As the scale itself limits the current. Or should I? What else should i keep in mind, as I have always used relays, but i want to use optos this time.

I added the input and output drawings.

Furthermore, I see that 4N32 has 3 pins on the output side- B,C and E. Should i leave B, the base, unconnected?

4N32 datasheet http://www.vishay.com/docs/81865/4n32.pdf

scale_reset.JPG

If you have measured the short circuit current and its only 5.4mA, which is well within the range of the opto isolator, I can't see why you need a resistor.

The "scales" must have a equivalent resistor of 4.3 / 0.0053 ohms inside them already i.e around 760 ohms, in order to pass only 5.4mA when the open circuit voltage of 4.3V.

You will get even less current than that through the opto as it has multiple PN junctions in it.

Re: 3rd connection. I think you can normally use this to adjust the sensitivity of the opto by a small amount, but in the opto's I've used, it can safely be left disconnected

Now while testing it on breadboard more questions came up. Looking at the arduino INPUT side. Which way is the right/best way: a) throw 10 kOhm resistor away and enable internal pull-up. I measured the current through pull-up enabled pin-> GND and got 0,1 mA. So it should be ok? b) enable internal pull-up, from that pin through 10kOhm resistor to collecter ( no external +5v wire like in the schematic) c) do it like in the pic, which was in the first post. And write

/pinMode(2, INPUT);
 digitalWrite(2,LOW);
if (sensor_value== HIGH)
etc

Go with a