Reading 10v Fan Tachometer safely with Uno?

[tobychaloner]

I am not from an electonics background, so reading the Tachometer from a fan is something I would like to do without frying my arduino input with a 10v signal.

I have a Cloudline AC Infinity AI-CLS4 fan, without a datasheet. It provides a 10v Powersupply and has a 10v PWM input and a 'FG' Tach output.

My issue is with safely reading the Tach on Pin 2 of the Arduino. The software is easy for me, but am not sure how a Hall effect device works. I read that it will output 10v, but with various components I can have a 5v output.

I've attached 3 circuit diagrams:
an overkill circuit using an IRF520 MOSFET to level convert from 10v to 5v.

A circuit from a recommendation to use a single pullup resistor.
This looks simple, I'm uncomfortable whether it will actually be safe for the Arduino input voltage range for high, being 3v to 5.5v.

Pullup resistor resource link

A circuit from a recommendation using a voltage divider
Voltage Divider resource link
Using a 10v supply, at 10k and 4.7k resistor, yields about 3.1v at the arduino input. This seems sufficient on paper. Any thoughts?

[Railroader]

Applying 5 volt to an UNO Vin will not work. Apply the 5 volt to the 5 volt pin.
You need to find out what jind of out out the Tach pin provides. If it's an open collector type a 10k pullup to +5 is good.
If it's an active 10 volt a 50/50 voltage divider is needed. Measure the voltage there, turning the fan slowly.

[tobychaloner]

Thanks @Railroader, there is no voltage between the Tach pin and ground when I turn the fan slowly by hand or have if running at 0% through to 100% controlled via PWN. So I guess its Open Collector. Fixing the Vin->5V typo and adding the 10k pullup, the circuit now looks like...

I found a good explanation of Open Collector here thanks