Well, I'll try to buid my first low-cut filter. 
Because it is my first time I would be very grateful if someone could help me to calculate C and R.
These are my values:
digital in ( 0-5 V) (Low/High)
pulses (between 0 and 30 per second...)
I suppose I have to connectthis RC filter to the input pin (pin2 ) isnt'it?
About the suggestion of the digital low pass filter I can see how it really works...becasue I want to count pulses not voltage...so I don't know how to implemnt this in my code.
tahnks again for your support...