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[on:]

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".

[Reply #1 on:]

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.

[Reply #2 on:]

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?

[Reply #3 on:]

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