Q:

Using 5v device on 3.3v

I got a flowmeter that is operating at 5v (basically an enclosed hall effect sensor and magnet). It needs a pullup resistor of between 1.6k - 2.2k between sig and vcc.
( https://www.conrad.com/p/bio-tech-ek-fch-m-pom-lc-g-18-flow-meter-fch-m-pom-lc-001-35-lmin-150391 )
Now, knowing that connecting a 5v device to the pis gpio that are using 3.3v, i would probably kill the pi.
I know you can put some resistors to step down the voltage to 3.3v, but I have no clue of how what resistors to use and how to connect them.
Could someone please help me telling me what resistors to use and how to connect them?
Thanks! :)

A:

This device is and open collector sinking (see the datasheet).
That mean that you need to connect a pull-up resistor on the output.
So if you connect the output pull-up resistor to the Pi 3.3V and the output to the Pi GPIO it will work.

A:

If the device outputs a square wave (i.e. a digital signal) you should use a level converter. These are quite inexpensive and guaranteed safe. They are readily available on-line and from most retailers who sell Pi products.
https://learn.sparkfun.com/tutorials/bi-directional-logic-level-converter-hookup-guide shows one common option.
See also https://elinux.org/RPi_GPIO_Interface_Circuits
It MAY be possible to use a resistive divider, but in order to determine the values more detail is required.
If the output is open drain, then connecting the pullup to 3.3V should work, but again detail of the interface is required to be sure.

