Dear community,
Yeah, my first question on this forum!
I'm really lost and researching for two days now. I want to read a 10V Tachometer which also can be up to 28V. Which Optocoupler or transistor can I use in order to make it work with a Digitial IO on an Arduino Uno REV3.
Thank you in advanced!
Hi, @wesley_franken
Welcome to the forum.
A Tachometer does not necessarily OUTPUT a signal.
I inputs a signal to display speed, usually engine speed.
What do you mean 10V which can also be up to 28V.
Can you post a picture and link to specs/data of your "Tacho"?
Thanks... Tom.. 
Hi Tom,
Thank you for the pleasant welcoming.
The fan has a 10Vdc (output) pulse, but also it must be able to read a 28Vdc (output) pulse. Is there a way to read both voltages on the same Digital Input?
Yes, if you specify how to deal with overlapping pulses.
I'm sorry, I do not mean two voltages at the same time, but the possibility to read a 10Vdc pulse and if I want a fan with a 28Vdc pulse.
I have the following solution:
Does this work? 
Your setup has just one input pin & Gnd, that will see a 10V signal, or up to a 28V signal, yes? And not both at once?
A simple transistor as buffer will do that.
The transistor and optocoupler solution work almost the same, with perfect insulation by the optocoupler.
BTW are you sure that the pulse voltage varies widely, or is it the range of an external voltage on an open collector output?
Hi,
The opto-coupler is a valid option but you would have to check if the TACHO output is a pulse or to get a pulse you need to connect the output to Vcc via a resistor.
(NPN Open Circuit Collector output)
Check how much current the output can supply.
Tom...
I think I've found the right specifications:
The other fan has a plain and simple 24VDC output. Is there a way to connect the one or the other voltage on the same Digital Input? (Arduino Uno PIN3)
Just so you can use interrupts ?
Well yes, but i guess you'd want to switch between inputs ? You could use a logic-gate (CMOS or 7400 series TTL) to switch between inputs.
Have the right voltage divider on each fan output and connect any one to the digital input.
I used a optocoupler with the right resistor so I can put 10 to 28 volt in the input of the optocoupler.
Thank you all so much for the help!
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