How i read low voltage digital signal from arduino with digital input ?

Hello!I want read low voltage 1,8 volt signal (f=40KHz) with digital input pins.What hardware must connect with Arduino pin for amplifier the signal?Transistor?The arduino pin need 4-5 volt to understand high level "1".

Yes, use a transistor, see attached schematic. I assume you don’t need to measure the amplitude of the signal, just see whether it is there - otherwise you can’t use a digital pin. The diode D is only needed if the input signal is coupled through a capacitor. The transistor can be any small signal NPN type. The resistor values are not critical, I suggest 10K for both of them. You can omit R2 if you enable the internal pullup resistor for that pin.

Thanks.I have BC 547 is ok?I make the similar circuit with Mosfet but no work....the Vgs(min)=2volt i think....The signal inverting with this circuit?

Yes, BC547 is OK. This circuit does invert the signal.