Step 5: Schematic
I drew this from looking at the finished sniffer that I built a couple years ago. I hope it's right.
C1 - .01uF (sets pitch, larger value = lower pitch)
R1 - 100k (also sets pitch, larger value = lower pitch)
R2 - 10k or so
R3 - 22k (or 2 10k's in series)
R4 - 10k
C2 & C3 0.1uF but not critical, probably anything 0.01uF to 1uF would work.