The circuits are designed around the LM339 comparator chip and can use a wide assortment of matched infrared - emitter / detector pairs. The basic circuit shown below has been tested with sensor gaps as wide as 12 inches but a distance of 8 inches or less is more practical.
http://home.cogeco.ca/~rpaisley4/ATDetIR.html
The following is a method to allow day and night detection using Infrared or Visible light sensitive phototransistors and a simple LM339 voltage comparator circuit.
http://home.cogeco.ca/~rpaisley4/DayNight.html
OSCILLATOR OUTPUT - A - Continuous 40 KHz. In this case the output will pulse LOW only once when the detector sees the signal. The signal will have to be removed before the output can again go LOW. If the signal is not removed the detector will in effect go blind and the output will remain HIGH.
http://home.cogeco.ca/~rpaisley4/xIrRemote.html
http://www.discovercircuits.com/PDF-FILES/40krvr1.pdf
describes PIR detector operation
http://www.glolab.com./pirparts/infrared.html
http://www.seattlerobotics.org/guide/infrared.html
http://academic1.bellevue.edu/robots/ircircuit.html
http://www.reconnsworld.com/ir_ultrasonic_basicirdetectemit.html
how they works and other documents.
http://www.misty.com/~don/irfilter.html
http://www.web-ee.com/Schematics/PIR/pir.htm