tachometer, review of methods

Projects General Guidance
16

aster94:
in the next days i will look for a schematics for the resistors and then i will try

The blades of the propeller are 1 cm wide

If you mode the pin INPUT_PULLUP then you can safely ground it through the phototransistor.
INPUT_PULLUP supplies the pin with 5V through a 20k to 50k internal resistor, the current is small.