Can you help me?
I want to use infrared LED as an electrical circuit can feel the presence of man in space 40 cm. Sorry because I do not speak English very well
Why don't you use photo sensor? it could be easier than use IR; since the distance you required is very close (only 40 Cm). Put the photo sensor (or RTS) in an enclosure and drill a hole for sensing. The size of the hole and the amplifier will allow you to adjust the sensitivity of the detector.
Search on the web for it. I am sure there are plenty of the circuits.
Go here: http://www.electrokits.com/photo/sensor
Go here for "Visible Light Photo-Detector Circuits - Basic Phototransistor Detector":http://home.cogeco.ca/~rpaisley4/PhotoDetectors.html