The blow sensor is not only for blowing, it will generally
register any bending of the piezo disk. I posted it because if
it can register a bit of air, then it can probably also register
the movement of an eyebrow when attached close to it on the
skin. when you are referring to the picture here
http://www.arduino.cc/en/Tutorial/KnockSensor without the
resistor the voltages created by the piezo will be very high,
since there is no path to ground. the input pin consumes
virtually no current at all. all electrons will "queue up" in
front of that input pin. leaving the resistor out *might* damage
the chip (the chip has some protection built in, and when one
pin is damaged there are often lots more left functioning
anyway) But if you take too low a resistor, the piezo will be
sort of shorted to ground and you will not see a signal. be sure
to use a resistor of several megaohm, as the example says. also,
the piezo requires a 5 volts voltage difference between it's two
wires, that difference will not be there when o