-- http://www.parallax.com/dl/docs/prod/audiovis/PIRSensor-V1.2.pdf (official Parallax documentation)
-- http://www.efx-tek.com/downloads/pir_docs.pdf (using the PIR with EFX-TEK controllers)
-- http://www.scary-terry.com/itw/pirsensor/pirsensor.htm (great write-up by our friend, "Scary" Terry Simmons)