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://members.home.nl/b.vandam/lonely/pagina33.html
Measure the distance to an object within 25 cm to 8 meters using standard 40 kHz transducers. Takes around 20 samples per second.
http://www.techdesign.be/projects/008/008.htm
http://www.discovercircuits.com/PDF-FILES/40krvr1.pdf
This ten channel Auto Roll RF Remote Control is virtually impossible to crack. Ten channels that are selectable as either latching or momentary. Each relay has led indicators. Receiver can support up to 15 transmitters. This page has a document that has c
http://www.electronickits.com/kit/complete/elec/ck1618.htm
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