> Although you could low-pass filter the output of the sensor to reduce the
> noise, you can also take another approach, if your PIC is "idling around"
> waiting to take a reading every few seconds - or whenever.  Instead of
> using an op-amp to implement a low pass filter before the A/D conversion,
> if the noise is random, you can implement a low-pass filter in software!
> Simply take a large number of A/D samples in a given time period (say, for
> the sake of arugment, in 5 seconds) and average them.  If the noise is
> randomly distributed (in amplitude), then this will have the same effect as
> low-pass filtering.  This works when the actual signal is a slowly varying
> one w.r.t time, as is the case of the humidty sensor.

Averaging is in many cases a less-than-ideal filtering method.  There are
some easy FIR and IIR methods that can work better.

Alternatively, you could generate a simple median filter by running the
following procedure many times a second:

  read sample
  if sample > myvalue
    myvalue = myvalue+1
  else
    myvalue = myvalue-1

This method will not respond terribly quickly to changes in the input signal
(it's sharply slew-rate limitted) but it has the advantage that a one-cycle
noise glitch will affect the output by at most one count.