If you want to use modulated 38KHz IR for proximity detection, no need to send rc5 codes, just send plain 38KHz IR and recieve using the TSOP1738 or similar.

But a better way would be to use a simple IR source and a IR reciever diode pointed in the same direction with a comparator connected to the reciever diodes output and using the output of the comparator for proximity detection. Do a google search of line tracers, and you will find a lot of circuits explainig the simple IR proximity detection.
By practical experience i can tell you that there is not much interference from ambient light unless the light is directly falling into the receiver diode. I would like to advise you also that the proximity detection using IR is dependent on the relative absorption/reflection of IR by the surface of the object you want to detect.