Getting HYDRUS DN20 pulses

Projects General Guidance
1

Hello,
I have a Arduino Mega 2560 and a hydrometer HYDRUS DN20. I want to get the hydrometer pulses for mesuring purposes.
In the datasheet, it says the pulses output is a open-collector circuit. Need a protective resistence in the collector branch for current limitation and the internal resistence of the switching device must be 5x the protective resistence, and i'm assuming the switching device is the arduino mega.
Someone could help me with those resistence, and how it should be the circuit.

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Datasheet: https://www.enerkon.si/wp-content/uploads/2017/12/20220622_InstallationGuide_non_lingual_HYDRUS.pdf

2

A Pull-Up of 27k will be good for a 5 volt controller.

3

So, 27K pull-up and the pulse plugged directly to arduino mega? No other resistence required?

4

No other resistance needed, no. Connect the 24 volt system GND with controller GND.
To awoid all risks for ground loops an opto coupler can be used. Then a pullup of 2.7 kOhm is used as pullup. Install the resistor close to the controller.

5

Thank you for you reply!
Just for safety, you said in the first reply 27k reistor. 2.7 KOhm is the correct?

6

Yes. The 27k has to be used to voltage divide 24 volt to something less than 5 volts. 2.7k for an opto device limits the current to 10 mA.
2 completely aspects.

7

To be sure schematics showing the total build, 24 volt system powering as well as controller powering, is needed for safety. Using the opto couplper eliminates the risk for mains ground loops.

8

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